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  • richardmitnick 12:28 pm on July 20, 2019 Permalink | Reply
    Tags: "Ask Ethan: What Does ‘Truth’ Mean To A Scientist?", , , , , Ethan Siegel   

    From Ethan Siegel: “Ask Ethan: What Does ‘Truth’ Mean To A Scientist?” 

    From Ethan Siegel
    July 20 2019

    1
    If you look farther and farther away, you also look farther and farther into the past. The farthest we can see back in time is 13.8 billion years: our estimate for the age of the Universe. It’s the extrapolation back to the earliest times that led to the idea of the Big Bang. While everything we observe is consistent with the Big Bang framework, it’s not something that can ever be proven. (NASA / STSCI / A. FELID)

    NASA/ESA Hubble Telescope

    It’s very different from the colloquial meanings of “true-and-false” or “right-and-wrong.”

    In many ways, the human endeavor of science is the ultimate pursuit of truth. By asking the natural world and Universe questions about itself, we seek to gain an understanding of what the Universe is like, what the rules that govern it are, and how things came to be the way they are today. Science is the full suite of knowledge that we gain from observing, measuring, and performing experiments that test the Universe, but it’s also the process through which we perform those investigations. It might be easy to see how we gain knowledge from that endeavor, but how do scientists arrive at the idea of a scientific truth? That’s Curtis Brand’s question, as he asks:

    I was speaking to a friend [who’s] an economic analyst, and his personal definition of a truth was when something’s 51%+ likely to happen… In science, do you ever truly accept anything as a truth, and if so, on what grounds do you typically decide its worthy of being called “true”?

    When we’re speaking scientifically, “truth” is something very different than how we colloquially use it. Here’s how.

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    One of the great puzzles of the 1500s was how planets moved in an apparently retrograde fashion. This could either be explained through Ptolemy’s geocentric model (L), or Copernicus’ heliocentric one (R). However, getting the details right to arbitrary precision was something that would require theoretical advances in our understanding of the rules underlying the observed phenomena, which led to Kepler’s laws and eventually Newton’s theory of universal gravitation. (ETHAN SIEGEL / BEYOND THE GALAXY)

    Let’s consider the following statement: “the Earth is round.” If you’re not a scientist (and also not a flat-Earther), you might think that this statement is unimpeachable. You might think of this as being scientifically true. In fact, stating that the Earth is round is a valid scientific conclusion and a scientific fact, at least if you contrast a round Earth with a flat Earth.

    But there’s always an additional nuance and caveat at play. If you were to measure the diameter of the Earth across our equator, you’d get a value: 7,926 miles (12,756 km). If you measured the diameter from the north pole to the south pole, you’d get a slightly different value: 7,900 miles (12,712 km). The Earth is not a perfect sphere, but rather a near-spherical shape that bulges at the equator and is compressed at the poles.

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    Planet Earth, viewed in its entirety (as much as one can see at once) from the GOES-13 satellite. In this image, the planet may appear to be perfectly spherical, but its equatorial diameter is slightly larger than its polar diameter: Earth is more accurately approximated by an oblate spheroid than by a perfectly round sphere. (NASA / GODDARD SPACE FLIGHT CENTER / GOES-13 / NOAA)

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    NOAA GOES R satellite with Earth

    To a scientist, this illustrates extremely well the caveats associated with a term like scientific truth. Sure, it’s more true that the Earth is a sphere than that the Earth is a disc or a circle. But it isn’t an absolute truth that the Earth is a sphere, because it’s more correct to call it an oblate spheroid than a sphere. And even if you do, calling it an oblate spheroid isn’t the absolute truth, either.

    There are surface features on Earth that demonstrate significant departures from a smooth shape like either a sphere or an oblate spheroid. There are mountain ranges, rivers, valleys, plateaus, deep oceans, trenches, ridges, volcanoes and more. There are locations where the land extends more than 29,000 feet (nearly 9,000 meters) above sea level, and places where you won’t touch the Earth’s surface until you’re 36,000 feet (11,000 meters) beneath the ocean’s surface.

    There are a few important ways of thinking scientifically that differ from how we think colloquially.

    There are no absolute truths in science; there are only approximate truths.
    Whether a statement, theory, or framework is true or not depends on quantitative factors and how closely you examine or measure the results.
    Every scientific theory has a finite range of validity: inside that range, the theory is indistinguishable from true, outside of that range, the theory is no longer true.

    This represents an enormous difference from how we commonly think about fact vs. fiction, truth vs. falsehood, or even right vs. wrong.

    5
    According to legend, the first experiment to show that all objects fell at the same rate, irrespective of mass, was performed by Galileo Galilei atop the Leaning Tower of Pisa. Any two objects dropped in a gravitational field, in the absence of (or neglecting) air resistance, will accelerate down to the ground at the same rate. This was later codified as part of Newton’s investigations into the matter, which superseded the earlier notions of a constant downward acceleration, which apply only to the surface of the Earth. (GETTY IMAGES)

    For example, if you drop a ball on Earth, you can ask the quantitative, scientific question of how it will behave. Like everything on Earth’s surface, it will accelerate downwards at 9.8 m/s² (32 ft/s²). And this is a great answer, because it’s approximately true.

    In science, though, you can start looking more deeply, and seeing where this approximation is no longer true. If you perform this experiment at sea level, at a variety of latitudes, you’ll find that this answer actually varies: from 9.79 m/s² at the equator to 9.83 m/s² at the poles. If you go to higher altitudes, you’ll find that the acceleration starts to slowly decrease. And if you leave the Earth’s gravitational pull, you’ll find that this rule isn’t universal at all, but is rather superseded by a more general rule: the law of Universal gravitation.

    5
    The Apollo mission trajectories, made possible by the Moon’s close proximity to us. Newton’s law of universal gravitation, despite the fact that it’s been superseded by Einstein’s General Relativity, is still so good at being approximately true on most Solar System scales that it encapsulates all the physics we need to travel from Earth to the Moon and land on its surface, and return. (NASA’S OFFICE OF MANNED SPACE FLIGHT, APOLLO MISSIONS)

    This laws is even more generally true. Newton’s law of universal gravitation can explain all the successes of modeling Earth’s acceleration as a constant, but it can also do much more. It can describe the orbital motion of the moons, planets, asteroids and comets of the solar system, as well as how much you’d weigh on any of the planets. It describes how the stars move around inside galaxies, and even allowed us to predict how to send a rocket to land humans on the Moon, with extraordinarily accurate trajectories.

    But even Newton’s law has its limits. When you move close to the speed of light, or get very close to an extremely large mass, or want to know what’s occurring on cosmic scales (such as in the case of the expanding Universe), Newton won’t help you. For that, you have to supersede Newton and move on to Einstein’s General Relativity.

    6
    An illustration of gravitational lensing showcases how background galaxies — or any light path — is distorted by the presence of an intervening mass, but it also shows how space itself is bent and distorted by the presence of the foreground mass itself. Before Einstein put forth his theory of General Relativity, he understood that this bending must occur, even though many remained skeptical until (and even after) the solar eclipse of 1919 confirmed his predictions. There is a significant difference between Einstein’s and Newton’s predictions for the amount of bending that should occur, due to the fact that space and time are both affected by mass in General Relativity. (NASA/ESA)

    Gravitational Lensing NASA/ESA

    For the trajectories of particles moving close to the speed of light, or to obtain very accurate predictions for the orbit of Mercury (the Solar System’s closest and fastest planet), or to explain the gravitational bending of starlight by the Sun (during an eclipse) or by a large collection of mass (such as in the case of gravitational lensing, above), Einstein’s theory gets it right where Newton’s fails. In fact, for every observational or experimental test we’ve thrown at General Relativity, from gravitational waves to the frame-dragging of space itself, it’s passed with flying colors.

    Does that mean that Einstein’s theory of General Relativity can be taken as a scientific truth?

    When you apply it to these specific scenarios, absolutely. But there are other scenarios we can apply it to, all of which are not yet sufficiently tested, where we fully expect that it won’t give quantitatively accurate predictions.

    8
    Even two merging black holes, one of the strongest sources of a gravitational signal in the Universe, doesn’t leave an observable signature that could probe quantum gravity. For that, we’ll have to create experiments that probe either the strong-field regime of relativity, i.e., near the singularity, or that take advantage of clever laboratory setups. (SXS, THE SIMULATING EXTREME SPACETIMES (SXS) PROJECT (BLACK-HOLES.ORG))
    The SXS project is a collaborative research effort involving multiple institutions. Our goal is the simulation of black holes and other extreme spacetimes to gain a better understanding of Relativity, and the physics of exotic objects in the distant cosmos.
    The SXS project is supported by Canada Research Chairs, CFI, CIfAR, Compute Canada, Max Planck Society, NASA, NSERC, the NSF, Ontario MEDI, the Sherman Fairchild Foundation, and XSEDE.

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    There are many questions we can ask about reality that require us to understand what’s happening where gravity is important or where the curvature of spacetime is extremely strong: just where you’d want Einstein’s theory. But when the distance scales you’re thinking about are also very small, you expect quantum effects to be important as well, and General Relativity cannot account for those. These include questions such as the following:

    What happens to the gravitational field of an electron when it passes through a double slit?
    What happens to the information of the particles that form a black hole, if the black hole’s eventual state is to decay into thermal radiation?
    And what is the behavior of a gravitational field/force at and around a singularity?

    Einstein’s theory won’t just get these answers wrong, it won’t have sensible answers to offer. In these regimes, we know we require a more advanced theory, such as a valid quantum gravitational theory, to tell us what’s going to happen under these circumstances.

    20
    Encoded on the surface of the black hole can be bits of information, proportional to the event horizon’s surface area. When the black hole decays, it decays to a state of thermal radiation. Whether that information survives and is encoded in the radiation or not, and if so, how, is not a question that our current theories can provide the answer to. (T.B. BAKKER / DR. J.P. VAN DER SCHAAR, UNIVERSITEIT VAN AMSTERDAM)

    Yes, masses at the surface of Earth accelerate downwards at 9.8 m/s², but if we ask the right questions or perform the right observations or experiments, we can find where and how this description of reality is no longer a good approximation of the truth. Newton’s laws can explain that phenomenon and many others, but we can find observations and experiments that show us where Newton, too, is insufficient.

    Even replacing Newton’s laws with Einstein’s General Relativity leads to the same story: Einstein’s theory can successfully explain everything that Newton’s can, plus additional phenomena. Some of those phenomena were already known when Einstein was constructing his theory; others had not yet been tested. But we can be certain that even Einstein’s greatest accomplishment will someday be superseded. When it does, we fully expect it will happen in exactly the same way.

    21
    Quantum gravity tries to combine Einstein’s General theory of Relativity with quantum mechanics. Quantum corrections to classical gravity are visualized as loop diagrams, as the one shown here in white. Whether space (or time) itself is discrete or continuous is not yet decided, as is the question of whether gravity is quantized at all, or particles, as we know them today, are fundamental or not. But if we hope for a fundamental theory of everything, it must include quantized fields, which General Relativity does not do on its own. (SLAC NATIONAL ACCELERATOR LAB)

    Science is not about finding the absolute truth of the Universe. No matter how much we’d like to know what the fundamental nature of reality is, from the smallest subatomic scales to the largest cosmic ones and beyond, this is not something science can deliver. All of our scientific truths are provisional, and we must recognize that they are only models or approximation of reality.

    Even the most successful scientific theories imaginable will, by their very nature, have a limited range of validity. But we can theorize whatever we like, and when a new theory meets the following three criteria:

    it achieves all of the successes of the prevailing, pre-existing theory,
    it succeeds where the current theory is known to fail,
    and it makes novel predictions for hitherto unmeasured phenomena, distinct from the prior theory, that pass the critical observational or experimental tests,

    it will supersede the current one as our best approximation of a scientific truth.

    Inflationary Universe. NASA/WMAP

    All of our currently held scientific truths, from the Standard Model of elementary particles to the Big Bang to dark matter and dark energy to cosmic inflation and beyond, are only provisional.

    Standard Model of Particle Physics (LATHAM BOYLE AND MARDUS OF WIKIMEDIA COMMONS)

    Coma cluster via NASA/ESA Hubble, the original example of Dark Matter discovered during observations by Fritz Zwicky and confirmed by Vera Rubin

    Dark Energy Camera Enables Astronomers a Glimpse at the Cosmic Dawn. CREDIT National Astronomical Observatory of Japan



    Dark Energy Camera Enables Astronomers a Glimpse at the Cosmic Dawn. CREDIT National Astronomical Observatory of Japan

    Lambda-Cold Dark Matter, Accelerated Expansion of the Universe, Big Bang-Inflation (timeline of the universe) Date 2010 Credit: Alex Mittelmann Cold creation

    They describe the Universe extremely accurately, succeeding in regimes where all prior frameworks have failed. Yet they all have limitations to how far we can take their implications before we arrive at a place where their predictions are no longer sensible, or no longer describe reality. They are not absolute truths, but approximate, provisional ones.

    No experiment can ever prove that a scientific theory is true; we can only demonstrate that its validity either extends or fails to extend to whatever regime we test it in. The failure of a theory is actually the ultimate scientific success: an opportunity to find an even better scientific truth to approximate reality. It’s being wrong in the best way imaginable.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:08 am on July 20, 2019 Permalink | Reply
    Tags: "Yes Virtual Particles Can Have Real Observable Effects", A neutron star despite being mostly made of neutral particles produces the strongest magnetic fields in the Universe., As particle-antiparticle pairs pop in-and-out of existence they can interact with real particles, , , , , Ethan Siegel, In 2016 scientists were able to locate a neutron star that was close enough and possessed a strong enough magnetic field to make these observations possible., On scales of both the very large and the very small we do far better by applying our best scientific theories extracting physical predictions and then observing and measuring the critical phenomena., , Real particles like electrons or photons leaving signatures imprinted on the real particles that are potentially observable., The light around RX J1856.5–3754 is just perfect., The nature of our quantum Universe is puzzling counterintuitive and testable. The results don’t lie., Vacuum birefringence, When you apply a strong magnetic field particles and antiparticles have opposite charges from one another., When you have particle/antiparticle pairs present in empty space you might think they simply pop into existence live for a little while and then re-annihilate and go back into nothingness.   

    From Ethan Siegel: “Yes, Virtual Particles Can Have Real, Observable Effects” 

    From Ethan Siegel
    July 19, 2019

    1
    As electromagnetic waves propagate away from a source that’s surrounded by a strong magnetic field, the polarization direction will be affected due to the magnetic field’s effect on the vacuum of empty space: vacuum birefringence. By measuring the wavelength-dependent effects of polarization around neutron stars with the right properties, we can confirm the predictions of virtual particles in the quantum vacuum. (N. J. SHAVIV / SCIENCEBITS)

    The nature of our quantum Universe is puzzling, counterintuitive, and testable. The results don’t lie.

    Although our intuition is an incredibly useful tool for navigating daily life, developed from a lifetime of experience in our own bodies on Earth, it’s often horrid for providing guidance outside of that realm. On scales of both the very large and the very small, we do far better by applying our best scientific theories, extracting physical predictions, and then observing and measuring the critical phenomena.

    Without this approach, we never would have come to understood the basic building blocks of matter, the relativistic behavior of matter and energy, or the fundamental nature of space and time themselves. But nothing matches the counterintuitive nature of quantum vacuum. Empty space isn’t completely empty, but consists of an indeterminate state of fluctuating fields and particles. It’s not science fiction; it’s a theoretical framework with testable, observable predictions. 80 years after Heisenberg first postulated an observational test, humanity has confirmed it. Here’s what we’ve learned.

    2
    An illustration between the inherent uncertainty between position and momentum at the quantum level. There is a limit to how well you can measure these two quantities simultaneously, and uncertainty shows up in places where people often least expect it. (E. SIEGEL / WIKIMEDIA COMMONS USER MASCHEN)

    Discovering that our Universe was quantum in nature brought with it a lot of unintuitive consequences. The better you measured a particle’s position, the more fundamentally indeterminate its momentum was. The shorter an unstable particle lived, the less well-known its mass fundamentally was. Material objects that appear to be solid on macroscopic scales can exhibit wave-like properties under the right experimental conditions.

    But empty space holds perhaps the top spot when it comes to a phenomenon that defies our intuition. Even if you remove all the particles and radiation from a region of space — i.e., all the sources of quantum fields — space still won’t be empty. It will consist of virtual pairs of particles and antiparticles, whose existence and energy spectra can be calculated. Sending the right physical signal through that empty space should have consequences that are observable.

    3
    An illustration of the early Universe as consisting of quantum foam, where quantum fluctuations are large, varied, and important on the smallest of scales. (NASA/CXC/M.WEISS)

    The particles that temporarily exist in the quantum vacuum themselves might be virtual, but their effect on matter or radiation is very real. When you have a region of space that particles pass through, the properties of that space can very much have real, physical effects that be predicted and tested.

    One of those effects is this: when light propagates through a vacuum, if space is perfectly empty, it should move through that space unimpeded: without bending, slowing, or breaking into multiple wavelengths. Applying an external magnetic field doesn’t change this, as photons, with their oscillatory electric and magnetic fields, don’t bend in a magnetic field. Even when your space is filled with particle/antiparticle pairs, this effect doesn’t change. But if you apply a strong magnetic field to a space filled with particle/antiparticle pairs, suddenly a real, observable effect arises.

    4
    Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. (Specifically, for the strong interactions.) Even in empty space, this vacuum energy is non-zero. As particle-antiparticle pairs pop in-and-out of existence, they can interact with real particles like electrons or photons, leaving signatures imprinted on the real particles that are potentially observable. (DEREK LEINWEBER)

    When you have particle/antiparticle pairs present in empty space, you might think they simply pop into existence, live for a little while, and then re-annihilate and go back into nothingness. In empty space with no external fields, this is true: Heisenberg’s energy-time uncertainty principle applies, and so long as all the relevant conservation laws are still obeyed, this is all that happens.

    But when you apply a strong magnetic field, particles and antiparticles have opposite charges from one another. Particles with the same velocities but opposite charges will bend in opposite directions in the presence of a magnetic field, and light that passes through a region of space with charged particles that move in this particular fashion should exhibit an effect: it should get polarized. If the magnetic field is strong enough, this should lead to an observably large polarization, by an amount that’s dependent on the strength of the magnetic field.

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    There have been many attempts to measure the effect of vacuum birefringence in a laboratory setting, such as with a direct laser pulse setup as shown here. However, they have been unsuccessful so far, as the effects have been too small to be seen with terrestrial magnetic fields, even with gamma rays at the GeV scale.(YOSHIHIDE NAKAMIYA, KENSUKE HOMMA, TOSEO MORITAKA, AND KEITA SETO, VIA ARXIV.ORG/ABS/1512.00636)

    This effect is known as vacuum birefringence, occurring when charged particles get yanked in opposite directions by strong magnetic field lines. Even in the absence of particles, the magnetic field will induce this effect on the quantum vacuum (i.e., empty space) alone. The effect of this vacuum birefringence gets stronger very quickly as the magnetic field strength increases: as the square of the field strength. Even though the effect is small, we have places in the Universe where the magnetic field strengths get large enough to make these effects relevant.

    Earth’s natural magnetic field might only be ~100 microtesla, and the strongest human-made fields are still only about 100 T. But neutron stars give us the opportunity for particularly extreme conditions, giving us large volumes of space where the field strength exceeds 10⁸ (100 million) T, ideal conditions for measuring vacuum birefringence.

    6
    A neutron star, despite being mostly made of neutral particles, produces the strongest magnetic fields in the Universe, a quadrillion times stronger than the fields at the surface of Earth. When neutron stars merge, they should produce both gravitational waves and also electromagnetic signatures, and when they cross a threshold of about 2.5 to 3 solar masses (depending on spin), they can become black holes in under a second. (NASA / CASEY REED — PENN STATE UNIVERSITY)

    How do neutron stars make such large magnetic fields? The answer may not be what you think. Although it might be tempting to take the name ‘neutron star’ quite literally, it isn’t made exclusively out of neutrons. The outer 10% of a neutron star consists mostly of protons, light nuclei, and electrons, which can stably exist without being crushed at the neutron star’s surface.

    Neutron stars rotate extremely rapidly, frequently in excess of 10% the speed of light, meaning that these charged particles on the outskirts of the neutron star are always in motion, which necessitated the production of both electric currents and induced magnetic fields. These are the fields we should be looking for if we want to observe vacuum birefringence, and its effect on the polarization of light.

    7
    Light coming from the surface of a neutron star can be polarized by the strong magnetic field it passes through, thanks to the phenomenon of vacuum birefringence. Detectors here on Earth can measure the effective rotation of the polarized light. (ESO/L. CALÇADA)

    It’s a challenge to measure the light from neutron stars: although they’re quite hot, hotter even than normal stars, they’re tiny, with diameters of just a few dozen kilometers. A neutron star is like a glowing Sun-like star, at perhaps two or three times the temperature of the Sun, compressed into a volume the size of Washington, D.C.

    Neutron stars are very faint, but they do emit light from all across the spectrum, including all the way down into the radio part of the spectrum. Depending on where we choose to look, we can observe the wavelength-dependent effects that the effect of vacuum birefringence has on the light’s polarization.

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    VLT image of the area around the very faint neutron star RX J1856.5–3754. The blue circle, added by E. Siegel, shows the location of the neutron star. Note that despite appearing very faint and red in this image, there is enough light reaching our detectors for us, with the proper instrumentation, to search for this vacuum birefringence effect. (ESO)

    ESO VLT at Cerro Paranal in the Atacama Desert, •ANTU (UT1; The Sun ),
    •KUEYEN (UT2; The Moon ),
    •MELIPAL (UT3; The Southern Cross ), and
    •YEPUN (UT4; Venus – as evening star).
    elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo,


    Glistening against the awesome backdrop of the night sky above ESO_s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 UT4 of the VLT, a major asset of the Adaptive Optics system

    All of the light that’s emitted must pass through the strong magnetic field around the neutron star on its way to our eyes, telescopes, and detectors. If the magnetized space that it passes through exhibits the expected vacuum birefringence effect, that light should all be polarized, with a common direction of polarization for all the photons.

    In 2016, scientists were able to locate a neutron star that was close enough and possessed a strong enough magnetic field to make these observations possible. Working with the Very Large Telescope (VLT) in Chile, which can take fantastic optical and infrared observations, including polarization, a team led by Roberto Mignani was able to measure the polarization effect from the neutron star RX J1856.5–3754.

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    A contour plot of the phase-averaged linear polarization degree in two models (left and right): for an isotropic blackbody and for a model with a gaseous atmosphere. At top, you can see the observational data, while at the bottom, you can see what you get if you subtract out the theoretical effect of vacuum birefringence from the data. The effects match partically perfectly. (R.P. MIGNANI ET AL., MNRAS 465, 492 (2016))

    The authors were able to extract, from the data, a large effect: a polarization degree of around 15%. They also calculated what the theoretical effect from vacuum birefringence ought to be, and subtracted it out from the actual, measured data. What they found was spectacular: the theoretical effect of vacuum birefringence accounted for practically all of the observed polarization. In other words, the data and the predictions matched almost perfectly.

    You might think that a closer, younger pulsar (like the one in the Crab Nebula) might be better suited to making such a measurement, but there’s a reason that RX J1856.5–3754 is special: its surface is not obscured by a dense, plasma-filled magnetosphere.

    If you watch a pulsar like the one in the Crab Nebula, you can see the effects of opacity in the region surrounding it; it’s simply not transparent to the light we’d want to measure.

    Supernova remnant Crab nebula. NASA/ESA Hubble

    But the light around RX J1856.5–3754 is just perfect. With the polarization measurements in this portion of the electromagnetic spectrum from this pulsar, we have confirmation that light is, in fact, polarized in the same direction as the predictions arising from vacuum birefringence in quantum electrodynamics. This is the confirmation of an effect predicted so long ago — in 1936 — by Werner Heisenberg and Hans Euler that, decades after the death of both men, we can now add “theoretical astrophysicist” to each of their resumes.

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    The future X-ray observatory by the ESA, Athena, will include the capability of measuring the polarization of X-ray light from space, something that none of our leading observatories today, such as Chandra and XMM-Newton, can do. (ESA / ATHENA COLLABORATION)

    NASA/Chandra X-ray Telescope

    ESA/XMM Newton

    Now that the effect of vacuum birefringence has been observed — and by association, the physical impact of the virtual particles in the quantum vacuum — we can attempt to confirm it even further with more precise quantitative measurements. The way to do that is to measure RX J1856.5–3754 in the X-rays, and measuring the polarization of X-ray light.

    While we don’t have a space telescope capable of measuring X-ray polarization right now, one of them is in the works: the ESA’s Athena mission. Unlike the ~15% polarization observed by the VLT in the wavelengths it probes, X-rays should be fully polarized, displaying right around an 100% effect. Athena is currently slated for launch in 2028, and could deliver this confirmation for not just one but many neutron stars. It’s another victory for the unintuitive, but undeniably fascinating, quantum Universe.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 7:34 am on July 17, 2019 Permalink | Reply
    Tags: "This Is Everything That’s Wrong With Our Definition Of ‘Planet’", , , , , , Ethan Siegel   

    From Ethan Siegel: “This Is Everything That’s Wrong With Our Definition Of ‘Planet’” 

    From Ethan Siegel
    July 16, 2019

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    When we place the known objects in the Solar System in order, four inner, rocky worlds and four, outer, giant worlds stand out. Yet it’s 2019, and astronomers (and planetary scientists) are more divided than ever over the definition of planet. (NASA’S THE SPACE PLACE)

    Not only can’t astronomers and planetary scientists agree, but the IAU made it worse for everyone.

    If you were alive in 2006, you likely remember a momentous event in astronomy: the International Astronomical Union (IAU) took it upon themselves to redefine what it meant to be a planet. While eight of the nine classical planets in our Solar System were still in, from Mercury to Neptune, the smallest and most distant among them — Pluto — was out. Its demotion to the status of ‘dwarf planet’ was met with worldwide dismay, much to the chagrin of plutophiles everywhere.

    What most people don’t realize is that until this resolution was made 13 years ago, there was no universally agreed-upon definition of a planet at all. In an interesting perspective over at Scientific American, Chris Impey discusses the history of how this fateful decision was made at the time. But in many ways, the definition created more problems than it solved. Here’s the story behind what it truly means to be a planet.

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    The largest galaxy in the Local Group, Andromeda, appears small and insignificant next to the Milky Way, but that’s because of its distance: some 2.5 million light years away. The Moon, the stars and planets, the Milky Way, and various nebulae are all distinctly identifiable in Earth’s night sky. (SCIENCETV ON YOUTUBE / SCREENSHOT)

    Local Group. Andrew Z. Colvin 3 March 2011

    Andromeda Galaxy Adam Evans

    Milky Way NASA/JPL-Caltech /ESO R. Hurt. The bar is visible in this image

    Milkdromeda -Andromeda on the left-Earth’s night sky in 3.75 billion years-NASA

    When you look out at the points of light in the night sky, it’s pretty easy to see that there are multiple classes of object out there. There’s the Moon, clearly unique among the astronomical objects. There are the nebulae: faint, extended objects that look akin to clouds, only they never move or change in appearance. There’s the Milky Way, an enormous silhouette of light and dark bands extending across the entire sky. And, occasionally, there are comets and other transient sights that come and go in relatively short order.

    But most ubiquitous of all are the pinpoints of light dotting the night sky: stars and planets. Recognized to be different from one another thousands of years ago, stars twinkle and remain in the same relative position night after night, while planets don’t twinkle and wander through the sky from night-to-night. This wandering behavior — πλανήτης in Greek — is where the term ‘planet’ originates.

    3
    One of the great puzzles of the 1500s was how planets moved in an apparently retrograde fashion. This could either be explained through Ptolemy’s geocentric model (L), or Copernicus’ heliocentric one (R). However, getting the details right to arbitrary precision was something that would require theoretical advances in our understanding of the rules underlying the observed phenomena, which led to Kepler’s laws and eventually Newton’s theory of universal gravitation. (ETHAN SIEGEL / BEYOND THE GALAXY)

    For generations, there was no need to codify anything further. There were only a handful of planets: Mercury, Venus, Mars, Jupiter, and Saturn. Even after Copernicus, Kepler, and Galileo came along, demonstrating the validity of heliocentrism, the phases of Venus, and the moons of Jupiter, that only served to demonstrate that Earth was no more significant — at least in astronomical terms — than any of the other planets.

    The science of astronomy continued to develop, with larger, more advanced telescopes, the application of photography, and eventually the rise of modern computer systems, CCDs, and adaptive optics all increasing our knowledge and what we were capable of observing. The discovery of Uranus brought with it a 7th planet. Temporarily, Ceres became the 8th, although a deluge of small objects between Mars and Jupiter led to the general recognition that these objects were a new class unto themselves: the asteroids. Neptune became the permanent 8th planet, followed by Pluto in the 20th century becoming the 9th.

    4
    Clyde Tombaugh’s original images identifying Pluto in 1930. The tiny, faint dot moves very slightly relative to the background stars, but sufficiently so that we’ve been able to successfully reconstruct its orbit. (LOWELL OBSERVATORY ARCHIVES)

    For nearly all of the 20th century, that was the story of our Solar System. We had nine planets, with Pluto being the outlier: smaller, farther, and very different from the rest. With astronomical advances, though, the need to revise how we thought about things would become an inevitability. Some of the unanswered questions about the Universe from 30 years ago would have to point the way to a superior classification scheme. Consider the following mysteries:

    Do stars other than the Sun have worlds that orbit them, and should they be considered planets, too?
    If our Solar System previously had planets that orbited the Sun but were ejected by gravitational interactions, should those orphaned worlds be considered planets?
    Were there additional objects out there in our own Solar System beyond Neptune, and was Pluto typical of them?

    Fast forward from 1989 to 2019, and most of these questions — along with many others we might have asked — now have definitive, scientific answers.

    5
    The orbit of 2015 RR245, compared with the gas giants and the other known Kuiper Belt Objects. Note the relative insignificance of Pluto compared to the 8 major planets in the Solar System, as well as its insignificance compared to the other objects of the Kuiper Belt. (ALEX PARKER AND THE OSSOS TEAM)

    We’ve surveyed huge swaths of the outer Solar System, where we’ve discovered hundreds upon hundreds of trans-Neptunian objects out there. They have different colors from one another (with some redder and others bluer), a wide variety of orbital properties, and they appear to cluster into a disk-like configuration: the Kuiper belt.

    Kuiper Belt. Minor Planet Center

    Many of the largest objects are massive enough to pull themselves into hydrostatic equilibrium: the spheroidal shape a massive body takes on owing to its mass, angular momentum, and the presence of any satellites. One of them — now known as Eris — is even more massive than Pluto, while a former Kuiper belt object, Triton, is both more massive and larger than Pluto, but was captured by Neptune back in pre-Cambrian times.

    6
    The large moons of the solar system as compared with Earth in size. Mars is approximately the same size as Jupiter’s Ganymede. Note that pretty much all of these worlds would become planets under the geophysical definition alone, but that only Earth’s moon is comparable in size to its parent planet; the large moons of the gas giants pale in comparison. (NASA, VIA WIKIMEDIA COMMONS USER BRICKTOP; EDITED BY WIKIMEDIA COMMONS USERS DEUAR, KFP, TOTOBAGGINS)

    Meanwhile, our understanding of planet formation has advanced tremendously. We’ve been able to directly image newly-forming solar systems, discovering protoplanetary disks complete with gaps, hot spots, and other evidence for planets in the process of forming. At the same time, our simulation power has increased accordingly, enabling us to understand the presence of soot lines, frost lines, and how planets and moons form.

    The cores of planets form first, followed by material from the outer portions of early solar systems falling onto those cores, creating the mantles of planets. Finally, if a protoplanet has the right properties, it can hold onto a volatile atmosphere of mostly hydrogen and helium, leading to the formation of a gas giant world. Early planets merge, migrate, or gravitationally interact. When we look at a solar system today, all we see are the survivors.

    7
    Today, we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Kepler data.

    NASA/Kepler Telescope, and K2 March 7, 2009 until November 15, 2018


    These planets range in size from larger than Jupiter to smaller than Earth. Yet because of the limitations on the size of Kepler and the duration of the mission, the majority of planets are very hot and close to their star, at small angular separations. TESS has the same issue with the first planets it’s discovering: they’re preferentially hot and in close orbits.

    NASA/MIT TESS replaced Kepler in search for exoplanets


    Only through dedicates, long-period observations (or direct imaging) will we be able to detect planets with longer period (i.e., multi-year) orbits. (NASA/AMES RESEARCH CENTER/JESSIE DOTSON AND WENDY STENZEL; MISSING EARTH-LIKE WORLDS BY E. SIEGEL)

    In addition, our understanding of exoplanetary systems has literally exploded. We have now identified and confirmed thousands of worlds around stars other than the Sun, owing to a variety of techniques but most prolifically to the Kepler mission and its work on transiting planets.

    Today, we can look at this enormous suite of data and recognize that, of all the worlds we’ve discovered, the vast majority of them are also the easiest to discover: close-orbiting planets, mostly around low-mass stars. Even with that, we’ve come to understand that there are four categories of planet:

    the low-mass worlds that have either no atmospheres or thin atmospheres, including Earth-like worlds,
    the intermediate-mass worlds that can hold onto thicker atmospheres, from super-Earths up to Saturn-like worlds,
    the high-mass worlds that begin to experience gravitational self-compression, including Jupiter-like worlds,
    and the worlds that can begin fusing heavy isotopes of hydrogen in their core: brown dwarfs, which are also known as failed stars to astronomers.

    8
    The classification scheme of planets as either rocky, Neptune-like, Jupiter-like or stellar-like. The border between Earth-like and Neptune-like is murky, but direct imaging of candidate super-Earth worlds should enable us to determine whether there’s a gas envelope around each planet in question or not. Note that there are four main classifications of ‘world’ here, and that the cutoff for hydrostatic equilibrium is mass-dependent, but only around a few percent the physical size of planet Earth. (CHEN AND KIPPING, 2016, VIA ARXIV.ORG/PDF/1603.08614V2.PDF)

    Armed with all of this knowledge, what should we do? Where should we draw the line between planet and non-planet?

    It’s a complicated question with no easy answer.

    Some claim that any object massive enough to pull itself into hydrostatic equilibrium should be a planet. Although this is a common position among planetary scientists, it would add 107 additional planets to our Solar System, including 19 moons and 87 trans-Neptunian objects.

    Some claim that any object that formed similarly to our eight planets should remain a planet, regardless of its present location. But orbiting a star is a meaningful, important criteria, as is (potentially) orbiting with a certain set of physical parameters. Scientists are not unified.

    9
    Under a size cutoff of 10,000 kilometers, there are two planets, 18 or 19 moons, 1 or 2 asteroids, and 87 trans-Neptunian objects, most of which do not yet have names. All are shown to scale, keeping in mind that for most of the trans-Neptunian objects, their sizes are only approximately known. Pluto, to the best of our knowledge, would be the 10th largest of these worlds. (MONTAGE BY EMILY LAKDAWALLA. DATA FROM NASA / JPL, JHUAPL/SWRI, SSI, AND UCLA / MPS / DLR / IDA, PROCESSED BY GORDAN UGARKOVIC, TED STRYK, BJORN JONSSON, ROMAN TKACHENKO, AND EMILY LAKDAWALLA)



    9

    What the IAU decided back in 2006, however, may offer the worst of all worlds. The resolution they adopted held that if a body met the following three criteria, it was a planet.

    It needs to be in hydrostatic equilibrium, or have enough gravity to pull it into an ellipsoidal shape.
    It needs to orbit the Sun and not any other body.
    And it needs to clear its orbit of any planetesimals or planetary competitors.

    In other words, only the Sun can have planets; exoplanets would be excluded. “Clearing its orbit” is ambiguous and is extraordinarily difficult to assess for even our own Solar System. But there is a definition that would make sense, based on astronomically measurable parameters alone.

    10
    The scientific line between planetary (above) and non-planetary (below) status, for three potential definitions of an orbit-clearing phenomenon and a star equal to the mass of our Sun. This definition could be extended to every exoplanetary system we can imagine to determine whether a candidate body meets the criteria, as we’ve defined them, for being classified as a true planet or not. (MARGOT (2015), VIA ARXIV.ORG/ABS/1507.06300)

    Sure, pulling yourself into hydrostatic equilibrium is something most scientists can agree is necessary to be granted planetary status, but it’s hardly sufficient. Planetary scientists may be content with looking at the geophysical properties of a world in determining its planetary status, but astronomers demand more. A relatively recent study by Jean-Luc Margot put forth a definition that any object should be considered a planet if it meets the following requirements.

    They orbit their parent star.
    They dominate their orbits in terms of mass and orbital distance.
    They would clear out any debris in their orbit in well under 10 billion years.
    And their orbits, barring any outside influences, will be stable as long as their star exists.

    For our Solar System, this would yield 8 planets, would not be dependent on unobservable properties, and could be easily extended to exoplanetary systems.

    13
    Pluto’s atmosphere, as imaged by New Horizons when it flew into the distant world’s eclipse shadow. The atmospheric hazes are clearly visible, and these clouds lead to periodic snow on this outer, cold world. Pluto’s atmosphere changes as it moves from perihelion to aphelion, and can continue to be monitored through periodic occultations. It may be as geologically interesting a world as Mars. (NASA / JHUAPL / NEW HORIZONS / LORRI)

    NASA/New Horizons spacecraft

    There are many people who would love to see Pluto regain its planetary status, and there’s a part of me that grew up with planetary Pluto that’s extraordinarily sympathetic to that perspective. But including Pluto as a planet necessarily results in a Solar System with far more than nine planets. Pluto is only the 8th largest non-planet in our Solar System, and is clearly a larger-than-average but otherwise typical member of the Kuiper belt. It will never be the 9th planet again.

    But that’s not necessarily a bad thing. We may be headed towards a world where astronomers and planetary scientists work with very different definitions of what attains planethood, but we all study the same objects in the same Universe. Whatever we call objects — however we choose to classify them — makes them no less interesting or worthy of study. The cosmos simply exists as it is. It’s up to the very human endeavor of science to make sense of it all.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 10:23 am on July 16, 2019 Permalink | Reply
    Tags: "New Method For Tracing Dark Matter Reveals Its Location, Abundance As Never Before", , , , , Ethan Siegel   

    From Ethan Siegel: “New Method For Tracing Dark Matter Reveals Its Location, Abundance As Never Before” 

    From Ethan Siegel
    July 15, 2019

    1
    This image showcases the massive, distant galaxy cluster Abell S1063. As part of the Hubble Frontier Fields program, this is one of six galaxy clusters to be imaged for a long time in many wavelengths at high resolution. The diffuse, bluish-white light shown here is actual intracluster starlight, captured for the first time. It traces out the location and density of dark matter more precisely than any other visual observation to date. (NASA, ESA, AND M. MONTES (UNIVERSITY OF NEW SOUTH WALES))

    NASA/ESA Hubble Telescope

    When stars get ejected from galaxies within massive clusters, they go where the dark matter is.

    Dark matter is one of the greatest mysteries in the Universe, displaying its effects in every massive, large-scale cosmic structure.

    2
    In theory, the majority of dark matter in any galaxy exists in a vast halo engulfing the normal matter, but occupying a much larger volume. While large galaxies, clusters of galaxies, and even larger structures can have their dark matter content determined indirectly, it’s challenging to trace out the dark matter distribution accurately. (ESO / L. CALÇADA)

    It neither emits nor absorbs light like normal matter does, but its gravitational impact is undeniable.

    3
    The galaxy cluster MACS 0416 from the Hubble Frontier Fields, with the mass shown in cyan and the magnification from lensing shown in magenta. That magenta-colored area is where the lensing magnification will be maximized. Mapping out the cluster mass allows us to identify which locations should be probed for the greatest magnifications and ultra-distant candidates of all. (STSCI/NASA/CATS TEAM/R. LIVERMORE (UT AUSTIN))

    Forming large halos around individual galaxies, it hold galaxy clusters and the great cosmic web together.

    4
    Large scale projection through the Illustris volume at z=0, centered on the most massive cluster, 15 Mpc/h deep. Shows dark matter density (left) transitioning to gas density (right). The large-scale structure of the Universe cannot be explained without dark matter. The full suite of what’s present in the Universe dictates that structure forms on small scales first, eventually leading to progressively larger and larger ones.(ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

    The Illustris Simulation

    By measuring the distorted light from distant galaxies behind a galaxy cluster, scientists can reconstruct the total cluster mass.

    5
    Any configuration of background points of light, whether they be stars, galaxies or galaxy clusters, will be distorted due to the effects of foreground mass via weak gravitational lensing. Even with random shape noise, the signature is unmistakable. By examining the difference between foreground (undistorted) and background (distorted) galaxies, we can reconstruct the mass distribution of massive extended objects, like galaxy clusters, in our Universe. (WIKIMEDIA COMMONS USER TALLJIMBO)

    In every galaxy cluster, the majority of the mass is outside of the galaxies: there is a huge dark matter halo.

    Dark matter halo. Image credit: Virgo consortium / A. Amblard / ESA


    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    6
    A galaxy cluster can have its mass reconstructed from the gravitational lensing data available. Most of the mass is found not inside the individual galaxies, shown as peaks here, but from the intergalactic medium within the cluster, where dark matter appears to reside. The time-delay observations of the Refsdal supernova, for example, cannot be explained without the presence of dark matter. (A. E. EVRARD. NATURE 394, 122–123 (09 JULY 1998))

    The intracluster gas, however, may be distributed differently, as normal matter can collide and heat up, emitting X-rays.

    7
    Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative or substitute will do. However, mapping out the X-ray light (pink) is not necessarily a very good indication of the dark matter distribution (blue). (X-RAY: NASA/CXC/UVIC./A.MAHDAVI ET AL. OPTICAL/LENSING: CFHT/UVIC./A. MAHDAVI ET AL. (TOP LEFT); X-RAY: NASA/CXC/UCDAVIS/W.DAWSON ET AL.; OPTICAL: NASA/ STSCI/UCDAVIS/ W.DAWSON ET AL. (TOP RIGHT); ESA/XMM-NEWTON/F. GASTALDELLO (INAF/ IASF, MILANO, ITALY)/CFHTLS (BOTTOM LEFT); X-RAY: NASA, ESA, CXC, M. BRADAC (UNIVERSITY OF CALIFORNIA, SANTA BARBARA), AND S. ALLEN (STANFORD UNIVERSITY) (BOTTOM RIGHT))

    NASA/Chandra X-ray Telescope


    CFHT Telescope, Maunakea, Hawaii, USA, at Maunakea, Hawaii, USA,4,207 m (13,802 ft) above sea level

    ESA/XMM Newton

    But individual stars, ejected from galaxies, should trace the same path as the dark matter.

    8
    A merging galaxy cluster in MACS J0416.1–2403 exhibits a different, smaller separation of X-ray gas from the gravitational signal, but this is expected, as this cluster is in a different stage of its merger, and there is still an offset between where the normal matter (in X-rays) and the total mass (from lensing; in blue) is located. (X-RAY: NASA/CXC/SAO/G.OGREAN ET AL.; OPTICAL: NASA/STSCI; RADIO: NRAO/AUI/NSF)

    NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

    In a cosmic first, scientists measured this intracluster light, and found it traces out the dark matter perfectly.

    9
    This is the same galaxy cluster, MACS J0416.1–2403, except with the intracluster light showcased in bluish/white color. This light is a far superior tracer of the dark matter than the X-rays or galaxies are, and offers an exciting new way to probe/measure the dark matter in the Universe. (NASA, ESA, AND M. MONTES (UNIVERSITY OF NEW SOUTH WALES))

    Their locations are identical because both “are free-floating on the gravitational potential of the cluster itself,” elucidates co-author Mireia Montes.

    10
    Six ultra-distant galaxy clusters in a variety of post-collisional stages were imaged by the Hubble Space Telescope as part of its Frontier Fields program. The survey, which went fainter on these relatively wide-angle scales than any before, was able to reveal intracluster light as well in two of them. Moving forward, this new method may provide a fast, accurate, and revolutionary way to infer the existence, distribution, and density of dark matter in these massive cosmic structures. (NASA, ESA, D. HARVEY (ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE, SWITZERLAND), R. MASSEY (DURHAM UNIVERSITY, UK), THE HUBBLE SM4 ERO TEAM, ST-ECF, ESO, D. COE (STSCI), J. MERTEN (HEIDELBERG/BOLOGNA), HST FRONTIER FIELDS, HARALD EBELING(UNIVERSITY OF HAWAII AT MANOA), JEAN-PAUL KNEIB (LAM)AND JOHAN RICHARD (CALTECH, USA))

    Because both the stars and the dark matter follow the same gravitational paths, this diffuse starlight matches the reconstructed cluster lensing profiles [MNRAS].

    11
    The galaxy cluster Abell 370, shown here, was one of the six massive galaxy clusters imaged in the Hubble Frontier Fields program. Since other great observatories were also used to image this region of sky, thousands of ultra-distant galaxies were revealed. By observing them again with a new scientific goal, Hubble’s BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) program will obtain distances to these galaxies, enabling us to better understand how galaxies formed, evolved, and grew up in our Universe. When combined with intracluster light measurements, we could gain an even greater understanding, via multiple lines of evidence of the same structure, of the dark matter inside. (NASA, ESA, A. KOEKEMOER (STSCI), M. JAUZAC (DURHAM UNIVERSITY), C. STEINHARDT (NIELS BOHR INSTITUTE), AND THE BUFFALO TEAM)

    This is the fastest, most accurate visual signature ever used to successfully identify dark matter.

    12
    The technique of measuring intracluster light to infer the presence of dark matter has been successfully demonstrated for these two galaxy clusters in the distant Universe. Many astronomers believe that this will be a powerful tool in their arsenal with larger-scale, next-generation space and ground-based telescopes to explore the nature of dark matter. (NASA, ESA, AND M. MONTES (UNIVERSITY OF NEW SOUTH WALES))

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 12:39 pm on July 13, 2019 Permalink | Reply
    Tags: , , , , , , Ethan Siegel, , ,   

    From Ethan Siegel: “Ask Ethan: Why Do Gravitational Waves Travel Exactly At The Speed Of Light?” 

    From Ethan Siegel
    July 13, 2019

    1
    Ripples in spacetime are what gravitational waves are, and they travel through space at the speed of light in all directions. Although the constants of electromagnetism never appear in the equations for Einstein’s General Relativity, gravitational waves undoubtedly move at the speed of light. Here’s why. (EUROPEAN GRAVITATIONAL OBSERVATORY, LIONEL BRET/EUROLIOS)

    2

    General Relativity has nothing to do with light or electromagnetism at all. So how to gravitational waves know to travel at the speed of light?

    There are two fundamental classes of theories required to describe the entirety of the Universe. On the one hand, there’s quantum field theory, which describes electromagnetism and the nuclear forces, and accounts for all the particles in the Universe and the quantum interactions that govern them. On the other hand, there’s General Relativity, which explains the relationship between matter/energy and space/time, and describes what we experience as gravitation. Within the context of General Relativity, there’s a new type of radiation that arises: gravitational waves. Yet, despite having nothing to do with light, these gravitational waves must travel at the speed of light. Why is that? Roger Reynolds wants to know, asking:

    We know that the speed of electromagnetic radiation can be derived from Maxwell’s equation[s] in a vacuum. What equations (similar to Maxwell’s — perhaps?) offer a mathematical proof that Gravity Waves must travel [at the] speed of light?

    It’s a deep, deep question. Let’s dive into the details.

    3
    It’s possible to write down a variety of equations, like Maxwell’s equations, to describe some aspect of the Universe. We can write them down in a variety of ways, as they are shown in both differential form (left) and integral form (right). It’s only by comparing their predictions with physical observations can we draw any conclusion about their validity. (EHSAN KAMALINEJAD OF UNIVERSITY OF TORONTO)

    It’s not apparent, at first glance, that Maxwell’s equations necessarily predict the existence of radiation that travels at the speed of light. What those equations ⁠ — which govern classical electromagnetism ⁠ — clearly tell us are about the behavior of:

    stationary electric charges,
    electric charges in motion (electric currents),
    static (unchanging) electric and magnetic fields,
    and how those fields and charges move, accelerate, and change in response to one another.

    Now, using the laws of electromagnetism alone, we can set up a physically relevant system: that of a low-mass, negatively charged particle orbiting a high-mass, positively charged one. This was the original model of the Rutherford atom, and it came along with a big, existential crisis. As the negative charge moves through space, it experiences a changing electric field, and accelerates as a result. But when a charged particle accelerates, it has to radiate power away, and the only way to do so is through electromagnetic radiation: i.e., light.

    4
    In the Rutherford model of the atom, electrons orbited the positively charged nucleus, but would emit electromagnetic radiation and see that orbit decay. It required the development of quantum mechanics, and the improvements of the Bohr model, to make sense of this apparent paradox. (JAMES HEDBERG / CCNY / CUNY)

    This has two effects that are calculable within the framework of classical electrodynamics. The first effect is that the negative charge will spiral into the nucleus, as if you’re radiating power away, you have to get that energy from somewhere, and the only place to take it from is the kinetic energy of the particle in motion. If you lose that kinetic energy, you inevitably will spiral towards the central, attracting object.

    The second effect that you can calculate is what’s going on with the emitted radiation. There are two constants of nature that show up in Maxwell’s equations:

    ε_0, the permittivity of free space, which is the fundamental constant describing the electric force between two electric charges in a vacuum.
    μ_0, the permeability of free space, which you can think of as the constant that defines the magnetic force produced by two parallel conducting wires in a vacuum with a constant current running through them.

    When you calculate the properties of the electromagnetic radiation produced, it behaves as a wave whose propagation speed equals (ε_0 · μ_0)^(-1/2), which just happens to equal the speed of light.

    5
    Relativistic electrons and positrons can be accelerated to very high speeds, but will emit synchrotron radiation (blue) at high enough energies, preventing them from moving faster. This synchrotron radiation is the relativistic analog of the radiation predicted by Rutherford so many years ago, and has a gravitational analogy if you replace the electromagnetic fields and charges with gravitational ones.(CHUNG-LI DONG, JINGHUA GUO, YANG-YUAN CHEN, AND CHANG CHING-LIN, ‘SOFT-X-RAY SPECTROSCOPY PROBES NANOMATERIAL-BASED DEVICES’)

    In electromagnetism, even if the details are quite the exercise to work out, the overall effect is straightforward. Moving electric charges that experience a changing external electromagnetic field will emit radiation, and that radiation both carries energy away and itself moves at a specific propagation speed: the speed of light. This is a classical effect, which can be derived with no references to quantum physics at all.

    Now, General Relativity is also a classical theory of gravity, with no references to quantum effects at all. In fact, we can imagine a system very analogous to the one we set up in electromagnetism: a mass in motion, orbiting around another mass. The moving mass will experience a changing external gravitational field (i.e., it will experience a change in spatial curvature) which causes it to emit radiation that carries energy away. This is the conceptual origin of gravitational radiation, or gravitational waves.

    6
    There is, perhaps, no better analogy for the radiation-reaction in electromagnetism than the planets orbiting the Sun in gravitational theories. The Sun is the largest source of mass, and curves space as a result. As a massive planet moves through this space, it accelerates, and by necessity that implies it must emit some type of radiation to conserve energy: gravitational waves. (NASA/JPL-CALTECH, FOR THE CASSINI MISSION)

    NASA/ESA/ASI Cassini-Huygens Spacecraft

    But why ⁠ — as one would be inclined to ask ⁠ — do these gravitational waves have to travel at the speed of light? Why does the speed of gravity, which you might imagine could take on any value at all, have to exactly equal the speed of light? And, perhaps most importantly, how do we know?

    Imagine what might happen if you were to suddenly pull the ultimate cosmic magic trick, and made the Sun simply disappear. If you did this, you wouldn’t see the skies go dark for 8 minutes and 20 seconds, which is the amount of time it takes light to travel the ~150 million km from the Sun to Earth. But gravitation doesn’t necessarily need to be the same way. It’s possible, as Newton’s theory predicted, that the gravitational force would be an instantaneous phenomenon, felt by all objects with mass in the Universe across the vast cosmic distances all at once.

    7
    An accurate model of how the planets orbit the Sun, which then moves through the galaxy in a different direction-of-motion. If the Sun were to simply wink out of existence, Newton’s theory predicts that they would all instantaneously fly off in straight lines, while Einstein’s predicts that the inner planets would continue orbiting for shorter periods of time than the outer planets. (RHYS TAYLOR)

    What would happen under this hypothetical scenario? If the Sun were to somehow disappear at one particular instant, would the Earth fly off in a straight line immediately? Or would the Earth continue to move in its elliptical orbit for another 8 minutes and 20 seconds, only deviating once that changing gravitational signal, propagating at the speed of light, reached our world?

    If you ask General Relativity, the answer is much closer to the latter, because it isn’t mass that determines gravitation, but rather the curvature of space, which is determined by the sum of all the matter and energy in it. If you were to take the Sun away, space would go from being curved to being flat, but only in the location where the Sun physically was. The effect of that transition would then propagate radially outwards, sending very large ripples — i.e., gravitational waves — propagating through the Universe like ripples in a 3D pond.

    8
    Whether through a medium or in vacuum, every ripple that propagates has a propagation speed. In no cases is the propagation speed infinite, and in theory, the speed at which gravitational ripples propagate should be the same as the maximum speed in the Universe: the speed of light. (SERGIU BACIOIU/FLICKR)

    In the context of relativity, whether that’s Special Relativity (in flat space) or General Relativity (in any generalized space), the speed of anything in motion is determined by the same things: its energy, momentum, and rest mass. Gravitational waves, like any form of radiation, have zero rest mass and yet have finite energies and momenta, meaning that they have no option: they must always move at the speed of light.

    This has a few fascinating consequences.

    Any observer in any inertial (non-accelerating) reference frame would see gravitational waves moving at exactly the speed of light.
    Different observers would see gravitational waves redshifting and blueshifting due to all the effects — such as source/observer motion, gravitational redshift/blueshift, and the expansion of the Universe — that electromagnetic waves also experience.
    The Earth, therefore, is not gravitationally attracted to where the Sun is right now, but rather where the Sun was 8 minutes and 20 seconds ago.

    The simple fact that space and time are related by the speed of light means that all of these statements must be true.

    9
    Gravitational radiation gets emitted whenever a mass orbits another one, which means that over long enough timescales, orbits will decay. Before the first black hole ever evaporates, the Earth will spiral into whatever’s left of the Sun, assuming nothing else has ejected it previously. Earth is attracted to where the Sun was approximately 8 minutes ago, not to where it is today. (AMERICAN PHYSICAL SOCIETY)

    This last statement, about the Earth being attracted to the Sun’s position from 8 minutes and 20 seconds ago, was a truly revolutionary difference between Newton’s theory of gravity and Einstein’s General Relativity. The reason it’s revolutionary is for this simple fact: if gravity simply attracted the planets to the Sun’s prior location at the speed of light, the planets’ predicted locations would mismatch severely with where they actually were observed to be.

    It’s a stroke of brilliance to realize that Newton’s laws require an instantaneous speed of gravity to such precision that if that were the only constraint, the speed of gravity must have been more than 20 billion times faster than the speed of light! [ScienceDirect] But in General Relativity, there’s another effect: the orbiting planet is in motion as it moves around the Sun. When a planet moves, you can think of it riding over a gravitational ripple, coming down in a different location from where it went up.

    10
    When a mass moves through a region of curved space, it will experience an acceleration owing to the curved space it inhabits. It also experiences an additional effect due to its velocity as it moves through a region where the spatial curvature is constantly changing. These two effects, when combined, result in a slight, tiny difference from the predictions of Newton’s gravity. (DAVID CHAMPION, MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY)


    Max Planck Institute for Radio Astronomy Bonn Germany

    In General Relativity, as opposed to Newton’s gravity, there are two big differences that are important. Sure, any two objects will exert a gravitational influence on the other, by either curving space or exerting a long-range force. But in General Relativity, these two extra pieces are at play: each object’s velocity affects how it experiences gravity, and so do the changes that occur in gravitational fields.

    The finite speed of gravity causes a change in the gravitational field that departs significantly from Newton’s predictions, and so do the effects of velocity-dependent interactions. Amazingly, these two effects cancel almost exactly. It’s the tiny inexactness of this cancellation that allowed us to first test whether Newton’s “infinite speed” or Einstein’s “speed of gravity equals the speed of light” model matched the physics of our Universe.

    To test out what the speed of gravity is, observationally, we’d want a system where the curvature of space is large, where gravitational fields are strong, and where there’s lots of acceleration taking place. Ideally, we’d choose a system with a large, massive object moving with a changing velocity through a changing gravitational field. In other words, we’d want a system with a close pair of orbiting, observable, high-mass objects in a tiny region of space.

    Nature is cooperative with this, as binary neutron star and binary black hole systems both exist. In fact, any system with a neutron star has the ability to be measured extraordinarily precisely if one serendipitous thing occurs: if our perspective is exactly aligned with the radiation emitted from the pole of a neutron star. If the path of this radiation intersects us, we can observe a pulse every time the neutron star rotates.

    11
    The rate of orbital decay of a binary pulsar is highly dependent on the speed of gravity and the orbital parameters of the binary system. We have used binary pulsar data to constrain the speed of gravity to be equal to the speed of light to a precision of 99.8%, and to infer the existence of gravitational waves decades before LIGO and Virgo detected them. However, the direct detection of gravitational waves was a vital part of the scientific process, and the existence of gravitational waves would still be in doubt without it. (NASA (L), MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY / MICHAEL KRAMER (R))

    As the neutron stars orbit, the pulsing one — known as a pulsar — carries extraordinary amounts of information about the masses and orbital periods of both components. If you observe this pulsar in a binary system for a long period of time, because it’s such a perfectly regular emitter of pulses, you should be able to detect whether the orbit is decaying or not. If it is, you can even extract a measurement for the emitted radiation: how quickly does it propagate?

    The predictions from Einstein’s theory of gravity are incredibly sensitive to the speed of light, so much so that even from the very first binary pulsar system discovered in the 1980s, PSR 1913+16 (or the Hulse-Taylor binary), we have constrained the speed of gravity to be equal to the speed of light with a measurement error of only 0.2%!

    13
    The quasar QSO J0842+1835, whose path was gravitationally altered by Jupiter in 2002, allowing an indirect confirmation that the speed of gravity equals the speed of light. (FOMALONT ET AL. (2000), APJS 131, 95–183)

    14

    That’s an indirect measurement, of course. We performed a second type of indirect measurement in 2002, when a chance coincidence lined up the Earth, Jupiter, and a very strong radio quasar (QSO J0842+1835) all along the same line-of-sight. As Jupiter moved between Earth and the quasar, the gravitational bending of Jupiter allowed us to indirectly measure the speed of gravity.

    The results were definitive: they absolutely ruled out an infinite speed for the propagation of gravitational effects. Through these observations alone, scientists determined that the speed of gravity was between 2.55 × 10⁸ m/s and 3.81 × 10⁸ m/s, completely consistent with Einstein’s predictions of 299,792,458 m/s.

    15
    Artist’s now iconic illustration of two merging neutron stars. The rippling spacetime grid represents gravitational waves emitted from the collision, while the narrow beams are the jets of gamma rays that shoot out just seconds after the gravitational waves (detected as a gamma-ray burst by astronomers). The gravitational waves and the radiation must travel at the same speed to a precision of 15 significant digits. (NSF / LIGO / SONOMA STATE UNIVERSITY / A. SIMONNET)

    But the greatest confirmation that the speed of gravity equals the speed of light comes from the 2017 observation of a kilonova: the inspiral and merger of two neutron stars. A spectacular example of multi-messenger astronomy, a gravitational wave signal arrived first, recorded in both the LIGO and Virgo detectors. Then, 1.7 seconds later, the first electromagnetic (light) signal arrived: the high-energy gamma rays from the explosive cataclysm.

    UC Santa Cruz

    UC Santa Cruz

    UCSC All the Gold in the Universe

    A UC Santa Cruz special report

    Tim Stephens

    Astronomer Ryan Foley says “observing the explosion of two colliding neutron stars” –the first visible event ever linked to gravitational waves–is probably the biggest discovery he’ll make in his lifetime. That’s saying a lot for a young assistant professor who presumably has a long career still ahead of him.

    The first optical image of a gravitational wave source was taken by a team led by Ryan Foley of UC Santa Cruz using the Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile

    A neutron star forms when a massive star runs out of fuel and explodes as a supernova, throwing off its outer layers and leaving behind a collapsed core composed almost entirely of neutrons. Neutrons are the uncharged particles in the nucleus of an atom, where they are bound together with positively charged protons. In a neutron star, they are packed together just as densely as in the nucleus of an atom, resulting in an object with one to three times the mass of our sun but only about 12 miles wide.

    “Basically, a neutron star is a gigantic atom with the mass of the sun and the size of a city like San Francisco or Manhattan,” said Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz.

    These objects are so dense, a cup of neutron star material would weigh as much as Mount Everest, and a teaspoon would weigh a billion tons. It’s as dense as matter can get without collapsing into a black hole.

    THE MERGER

    Like other stars, neutron stars sometimes occur in pairs, orbiting each other and gradually spiraling inward. Eventually, they come together in a catastrophic merger that distorts space and time (creating gravitational waves) and emits a brilliant flare of electromagnetic radiation, including visible, infrared, and ultraviolet light, x-rays, gamma rays, and radio waves. Merging black holes also create gravitational waves, but there’s nothing to be seen because no light can escape from a black hole.

    Foley’s team was the first to observe the light from a neutron star merger that took place on August 17, 2017, and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).


    VIRGO Gravitational Wave interferometer, near Pisa, Italy

    Caltech/MIT Advanced aLigo Hanford, WA, USA installation


    Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA

    Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project

    Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib

    ESA/eLISA the future of gravitational wave research

    1
    Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)

    Now, for the first time, scientists can study both the gravitational waves (ripples in the fabric of space-time), and the radiation emitted from the violent merger of the densest objects in the universe.

    The UC Santa Cruz team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). The arrows indicate where SSS17a was absent from the Hubble image and visible in the new image from the Swope Telescope. (Image credits: Left, Hubble/STScI; Right, 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)

    It’s that combination of data, and all that can be learned from it, that has astronomers and physicists so excited. The observations of this one event are keeping hundreds of scientists busy exploring its implications for everything from fundamental physics and cosmology to the origins of gold and other heavy elements.


    A small team of UC Santa Cruz astronomers were the first team to observe light from two neutron stars merging in August. The implications are huge.

    ALL THE GOLD IN THE UNIVERSE

    It turns out that the origins of the heaviest elements, such as gold, platinum, uranium—pretty much everything heavier than iron—has been an enduring conundrum. All the lighter elements have well-explained origins in the nuclear fusion reactions that make stars shine or in the explosions of stars (supernovae). Initially, astrophysicists thought supernovae could account for the heavy elements, too, but there have always been problems with that theory, says Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz.

    The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. (Image credit: Murguia-Berthier et al., Science)

    A theoretical astrophysicist, Ramirez-Ruiz has been a leading proponent of the idea that neutron star mergers are the source of the heavy elements. Building a heavy atomic nucleus means adding a lot of neutrons to it. This process is called rapid neutron capture, or the r-process, and it requires some of the most extreme conditions in the universe: extreme temperatures, extreme densities, and a massive flow of neutrons. A neutron star merger fits the bill.

    Ramirez-Ruiz and other theoretical astrophysicists use supercomputers to simulate the physics of extreme events like supernovae and neutron star mergers. This work always goes hand in hand with observational astronomy. Theoretical predictions tell observers what signatures to look for to identify these events, and observations tell theorists if they got the physics right or if they need to tweak their models. The observations by Foley and others of the neutron star merger now known as SSS17a are giving theorists, for the first time, a full set of observational data to compare with their theoretical models.

    According to Ramirez-Ruiz, the observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.

    RIPPLES IN THE FABRIC OF SPACE-TIME

    Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.

    Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.

    The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.

    LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.

    LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.

    “This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”

    IN THIS REPORT

    Neutron stars
    A team from UC Santa Cruz was the first to observe the light from a neutron star merger that took place on August 17, 2017 and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)

    Graduate students and post-doctoral scholars at UC Santa Cruz played key roles in the dramatic discovery and analysis of colliding neutron stars.Astronomer Ryan Foley leads a team of young graduate students and postdoctoral scholars who have pulled off an extraordinary coup. Following up on the detection of gravitational waves from the violent merger of two neutron stars, Foley’s team was the first to find the source with a telescope and take images of the light from this cataclysmic event. In so doing, they beat much larger and more senior teams with much more powerful telescopes at their disposal.

    “We’re sort of the scrappy young upstarts who worked hard and got the job done,” said Foley, an untenured assistant professor of astronomy and astrophysics at UC Santa Cruz.

    David Coulter, graduate student

    The discovery on August 17, 2017, has been a scientific bonanza, yielding over 100 scientific papers from numerous teams investigating the new observations. Foley’s team is publishing seven papers, each of which has a graduate student or postdoc as the first author.

    “I think it speaks to Ryan’s generosity and how seriously he takes his role as a mentor that he is not putting himself front and center, but has gone out of his way to highlight the roles played by his students and postdocs,” said Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and the most senior member of Foley’s team.

    “Our team is by far the youngest and most diverse of all of the teams involved in the follow-up observations of this neutron star merger,” Ramirez-Ruiz added.

    Charles Kilpatrick, postdoctoral scholar

    Charles Kilpatrick, a 29-year-old postdoctoral scholar, was the first person in the world to see an image of the light from colliding neutron stars. He was sitting in an office at UC Santa Cruz, working with first-year graduate student Cesar Rojas-Bravo to process image data as it came in from the Swope Telescope in Chile. To see if the Swope images showed anything new, he had also downloaded “template” images taken in the past of the same galaxies the team was searching.

    Ariadna Murguia-Berthier, graduate student

    “In one image I saw something there that was not in the template image,” Kilpatrick said. “It took me a while to realize the ramifications of what I was seeing. This opens up so much new science, it really marks the beginning of something that will continue to be studied for years down the road.”

    At the time, Foley and most of the others in his team were at a meeting in Copenhagen. When they found out about the gravitational wave detection, they quickly got together to plan their search strategy. From Copenhagen, the team sent instructions to the telescope operators in Chile telling them where to point the telescope. Graduate student David Coulter played a key role in prioritizing the galaxies they would search to find the source, and he is the first author of the discovery paper published in Science.

    Matthew Siebert, graduate student

    “It’s still a little unreal when I think about what we’ve accomplished,” Coulter said. “For me, despite the euphoria of recognizing what we were seeing at the moment, we were all incredibly focused on the task at hand. Only afterward did the significance really sink in.”

    Just as Coulter finished writing his paper about the discovery, his wife went into labor, giving birth to a baby girl on September 30. “I was doing revisions to the paper at the hospital,” he said.

    It’s been a wild ride for the whole team, first in the rush to find the source, and then under pressure to quickly analyze the data and write up their findings for publication. “It was really an all-hands-on-deck moment when we all had to pull together and work quickly to exploit this opportunity,” said Kilpatrick, who is first author of a paper comparing the observations with theoretical models.

    César Rojas Bravo, graduate student

    Graduate student Matthew Siebert led a paper analyzing the unusual properties of the light emitted by the merger. Astronomers have observed thousands of supernovae (exploding stars) and other “transients” that appear suddenly in the sky and then fade away, but never before have they observed anything that looks like this neutron star merger. Siebert’s paper concluded that there is only a one in 100,000 chance that the transient they observed is not related to the gravitational waves.

    Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, is first author of a paper synthesizing data from a range of sources to provide a coherent theoretical framework for understanding the observations.

    Another aspect of the discovery of great interest to astronomers is the nature of the galaxy and the galactic environment in which the merger occurred. Postdoctoral scholar Yen-Chen Pan led a paper analyzing the properties of the host galaxy. Enia Xhakaj, a new graduate student who had just joined the group in August, got the opportunity to help with the analysis and be a coauthor on the paper.

    Yen-Chen Pan, postdoctoral scholar

    “There are so many interesting things to learn from this,” Foley said. “It’s a great experience for all of us to be part of such an important discovery.”

    Enia Xhakaj, graduate student

    IN THIS REPORT

    Scientific Papers from the 1M2H Collaboration

    Coulter et al., Science, Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source

    Drout et al., Science, Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis

    Shappee et al., Science, Early Spectra of the Gravitational Wave Source GW170817: Evolution of a Neutron Star Merger

    Kilpatrick et al., Science, Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger

    Siebert et al., ApJL, The Unprecedented Properties of the First Electromagnetic Counterpart to a Gravitational-wave Source

    Pan et al., ApJL, The Old Host-galaxy Environment of SSS17a, the First Electromagnetic Counterpart to a Gravitational-wave Source

    Murguia-Berthier et al., ApJL, A Neutron Star Binary Merger Model for GW170817/GRB170817a/SSS17a

    Kasen et al., Nature, Origin of the heavy elements in binary neutron star mergers from a gravitational wave event

    Abbott et al., Nature, A gravitational-wave standard siren measurement of the Hubble constant (The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration)

    Abbott et al., ApJL, Multi-messenger Observations of a Binary Neutron Star Merger

    PRESS RELEASES AND MEDIA COVERAGE


    Watch Ryan Foley tell the story of how his team found the neutron star merger in the video below. 2.5 HOURS.

    Press releases:

    UC Santa Cruz Press Release

    UC Berkeley Press Release

    Carnegie Institution of Science Press Release

    LIGO Collaboration Press Release

    National Science Foundation Press Release

    Media coverage:

    The Atlantic – The Slack Chat That Changed Astronomy

    Washington Post – Scientists detect gravitational waves from a new kind of nova, sparking a new era in astronomy

    New York Times – LIGO Detects Fierce Collision of Neutron Stars for the First Time

    Science – Merging neutron stars generate gravitational waves and a celestial light show

    CBS News – Gravitational waves – and light – seen in neutron star collision

    CBC News – Astronomers see source of gravitational waves for 1st time

    San Jose Mercury News – A bright light seen across the universe, proving Einstein right

    Popular Science – Gravitational waves just showed us something even cooler than black holes

    Scientific American – Gravitational Wave Astronomers Hit Mother Lode

    Nature – Colliding stars spark rush to solve cosmic mysteries

    National Geographic – In a First, Gravitational Waves Linked to Neutron Star Crash

    Associated Press – Astronomers witness huge cosmic crash, find origins of gold

    Science News – Neutron star collision showers the universe with a wealth of discoveries

    UCSC press release
    First observations of merging neutron stars mark a new era in astronomy

    Credits

    Writing: Tim Stephens
    Video: Nick Gonzales
    Photos: Carolyn Lagattuta
    Header image: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science
    Design and development: Rob Knight
    Project managers: Sherry Main, Scott Hernandez-Jason, Tim Stephens

    Dark Energy Survey


    Dark Energy Camera [DECam], built at FNAL


    NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

    Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet

    Noted in the video but not in the article:

    NASA/Chandra Telescope

    NASA/SWIFT Telescope

    NRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USA

    Prompt telescope CTIO Chile

    NASA NuSTAR X-ray telescope

    See the full article here

    Because this event took place some 130 million light-years away, and the gravitational and light signals arrived with less than a two second difference between them, we can constrain the possible departure of the speed of gravity from the speed of light. We now know, based on this, that they differ by less than 1 part in 10¹⁵, or less than one quadrillionth of the actual speed of light.

    15
    Illustration of a fast gamma-ray burst, long thought to occur from the merger of neutron stars. The gas-rich environment surrounding them could delay the arrival of the signal, explaining the observed 1.7 second difference between the arrivals of the gravitational and electromagnetic signatures. (ESO)

    Of course, we think that these two speeds are exactly identical. The speed of gravity should equal the speed of light so long as both gravitational waves and photons have no rest mass associated with them. The 1.7 second delay is very likely explained by the fact that gravitational waves pass through matter unperturbed, while light interacts electromagnetically, potentially slowing it down as it passes through the medium of space by just the smallest amount.

    The speed of gravity really does equal the speed of light, although we don’t derive it in the same fashion. Whereas Maxwell brought together electricity and magnetism — two phenomena that were previously independent and distinct — Einstein simply extended his theory of Special Relativity to apply to all spacetimes in general. While the theoretical motivation for the speed of gravity equaling the speed of light was there from the start, it’s only with observational confirmation that we could know for certain. Gravitational waves really do travel at the speed of light!

    See the full article here .

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    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 12:07 pm on July 7, 2019 Permalink | Reply
    Tags: "What Does It Mean That Quantum Gravity Has No Symmetry?", , Ethan Siegel,   

    From Ethan Siegel: “What Does It Mean That Quantum Gravity Has No Symmetry?” 

    From Ethan Siegel
    July 6, 2019

    1
    A diagram used to prove that quantum gravity cannot have any global symmetry. Symmetry, if existed, could act only on the shaded regions in the diagram and causes no change around the black spot in the middle. The shaded regions can be made as small as we like by dividing the boundary circle more and more. Thus, the alleged symmetry would not act anywhere inside of the circle. (DANIEL HARLOW AND HIROSI OOGURI, PRL, 122, 191601 (2019))

    The quest for a quantum theory of gravity is the holy grail of physics. Here’s why it’s murkier than anyone expected.

    If you want to fully describe how the Universe works at a fundamental level, you have to look at it in two different — and incompatible — ways. To describe the particles and their electromagnetic and nuclear interactions, you need to use the framework of quantum field theory (QFT), where quantum fields permeate the Universe and their excitations give rise to the particles we know of. To describe how every quantum of matter and energy moves through the Universe, we need the framework of General Relativity (GR), where matter and energy define how spacetime is curved, and curved spacetime tells matter and energy how to move.

    Yet these two theories are mutually incompatible; to make them work together, we’d need to develop a working theory of quantum gravity. Yet a new paper, just published (Physical Review Letters), has Alex Knapp puzzled, leading him to ask:

    “What does it mean that quantum gravity doesn’t have symmetry?”

    It’s a fascinating find with big implications. Let’s find out what it means.

    2
    Feynman diagrams (top) are based off of point particles and their interactions. Converting them into their string theory analogues (bottom) gives rise to surfaces which can have non-trivial curvature. In string theory, all particles are simply different vibrating modes of an underlying, more fundamental structure: strings. But does a quantum theory of gravity, which string theory aspires to be, have symmetries, and by association, conservation laws? (PHYS. TODAY 68, 11, 38 (2015))

    When you hear the word “symmetry,” there are probably all sorts of images that pop into your mind. Some letters of the alphabet — like “A” or “T” — display a symmetry where if you drew a vertical line down their centers, the left sides and the right sides are symmetric. Other letters — like “B” or “E” — have a similar symmetry but in a different direction: horizontally, where the tops and bottoms are symmetric. Still others — such as “O” — have rotational symmetry, where no matter how many degrees you rotate it, its appearance is unchanged.

    These are some examples of symmetry that are easy to visualize, but they’re not exhaustive. Sure, some systems have no differences from their mirror reflections, known as a parity symmetry. Others demonstrate rotational symmetries, where it doesn’t matter what angle you view it from. But there are many others, all of vital importance.

    3
    There are many letters of the alphabet that exhibit particular symmetries. Note that the capital letters shown here have one and only one line of symmetry; letters like “I” or “O” have more than one. (MATH-ONLY-MATH.COM)

    Some systems are the same for matter as they are for antimatter: they exhibit charge conjugation symmetry. Some systems obey the same laws if you evolve them forwards in time as they do if you evolve them backwards in time: time-reversal symmetry. Still others don’t depend on your physical location (translational symmetry) or on when you’re viewing your system (time-translational symmetry) or on which non-accelerating reference frame you occupy (Lorentz symmetry).

    Some physical systems have these symmetries; others don’t. Dropping a ball off of a cliff obeys time-reversal symmetry; cooking scrambled eggs does not. Flying through space with your engines turned off obeys Lorentz symmetry; accelerating, with your engines firing at full power, does not.

    4
    The DEEP laser-sail concept relies on a large laser array striking and accelerating a relatively large-area, low-mass spacecraft. This has the potential to accelerate non-living objects to speeds approaching the speed of light, making an interstellar journey possible within a single human lifetime. The work done by the laser, applying a force as an object moves a certain distance, is an example of energy transfer from one form into another. An accelerating reference frame is an example of a non-inertial system; for these systems, the Lorentz symmetry does not strictly hold. (© 2016 UCSB EXPERIMENTAL COSMOLOGY GROUP)

    It isn’t just physical systems that can obey (or disobey) symmetries. Whenever you have an equation (or a quantitative theory in general), you can test them to see which symmetries they obey and which ones they don’t.

    Within various QFTs, for example, particles experiencing the electromagnetic force obey parity, charge conjugation, and time-reversal symmetries all independently of one another. Electromagnetism is the same for particles regardless of their direction of motion; the same for particles and antiparticles; the same forwards in time as backwards in time.

    Particles experiencing the weak nuclear force, on the other hand, violate parity, charge conjugation, and time-reversal individually. Left-handed muons decay differently from right-handed muons. Neutral kaons and neutral anti-kaons have different properties. And the decays of B-mesons have time-asymmetric transformation rates [Physical Review Letters]. But even the weak interactions obey the combination of all three symmetries: if you perform an experiment on a particle in motion that moves forward in time and an antiparticle with its motion reflected moving backwards in time, you get the same physical results.

    5
    Changing particles for antiparticles and reflecting them in a mirror simultaneously represents CP symmetry. If the anti-mirror decays are different from the normal decays, CP is violated. Time reversal symmetry, known as T, is violated if CP is violated. The combined symmetries of C, P, and T, all together, must be conserved under our present laws of physics, with implications for the types of interactions that are and aren’t allowed. (E. SIEGEL / BEYOND THE GALAXY)

    Within GR, various spacetimes obey different sets of symmetry. The (Schwarzschild) spacetime describing a non-rotating black hole exhibits time-translation, mirror, and full rotational symmetries. The (Kerr) spacetime describing a rotating black hole exhibits time-translation symmetry, but only has rotational symmetries about one axis.

    The (Friedmann-Lemaitre-Robertson-Walker) spacetime describing the expanding Universe, on the other hand, has a slew of symmetries it does obey, but time-translation isn’t one of them: an expanding Universe is different from one moment in time to the next.

    6
    If you had a static spacetime that weren’t changing, energy conservation would be guaranteed. But if the fabric of space changes as the objects you’re interested in move through them, there is no longer an energy conservation law under the laws of General Relativity. (DAVID CHAMPION, MAX PLANCK INSTITUTE FOR RADIO ASTRONOMY)

    In general, these symmetries are profoundly important to our understanding of the Universe, and have enormous additional implications for reality. You see, there’s a brilliant theorem at the intersection of physics and mathematics that states the following: every unique mathematical symmetry exhibited by a physical theory necessarily implies an associated conserved quantity. This theorem — known as Noether’s theorem after its discoverer, the incomparable mathematician Emmy Noether — is the root of why certain quantities are or aren’t conserved.

    A time-translation symmetry leads to the conservation of energy, which explains why energy is not conserved in an expanding Universe. Spatial translation symmetry leads to the conservation of momentum; rotational symmetry leads to the conservation of angular momentum. Even CPT conservation — where charge conjugation, parity, and time-reversal symmetry are all combined — is a consequence of Lorentz symmetry.

    7
    Quantum gravity tries to combine Einstein’s General theory of Relativity with quantum mechanics. Quantum corrections to classical gravity are visualized as loop diagrams, as the one shown here in white. Whether space (or time) itself is discrete or continuous is not yet decided, as is the question of whether gravity is quantized at all, or particles, as we know them today, are fundamental or not. But if we hope for a fundamental theory of everything, it must include quantized fields.(SLAC NATIONAL ACCELERATOR LAB)

    Some symmetries are inherent to specific QFTs or to QFTs in general; some symmetries are inherent to specific solutions in GR or to GR in general. But these two descriptions of the Universe are both incomplete. There are many questions we can ask about reality that require us to understand what’s happening where gravity is important or where the curvature of spacetime is extremely strong (where we need GR), but also when distance scales are very small or where individual quantum effects are at play (where we need QFT).

    These include questions such as the following:

    What happens to the gravitational field of an electron when it passes through a double slit?
    What happens to the information of the particles that form a black hole, if the black hole’s eventual state is thermal radiation?
    And what is the behavior of a gravitational field/force at and around a singularity?

    To address them, GR and QFT individually are both insufficient. We need something more: an understanding of gravity at the quantum level.

    We don’t have a working theory of quantum gravity, of course, or we’d be able to understand what symmetries it does (and doesn’t) exhibit. But even without a full theory, we have a tremendous clue: the holographic principle. Just as a two-dimensional hologram encodes three-dimensional information on its surface, the holographic principle allows physicists to relate what happens in a spacetime with Ndimensions to a conformal field theory with N-1 dimensions: the AdS/CFT correspondence.

    The AdS stands for anti-de Sitter space, which is frequently used to describe quantum gravity in the context of string theory, while the CFT stands for conformal field theory, such as the QFTs we use to describe three of the four fundamental interactions. While no one is certain whether this is applicable to our Universe, there are many good reasons to think it does.

    8
    In the Standard Model, the neutron’s electric dipole moment is predicted to be a factor of ten billion larger than our observational limits show. The only explanation is that somehow, something beyond the Standard Model is protecting this CP symmetry in the strong interactions. We can demonstrate a lot of things in science, but proving that CP is conserved in the strong interactions can never be done. Which is too bad; we need more CP-violation to explain the matter-antimatter asymmetry present in our Universe. There can be no global symmetries if the AdS/CFT correspondence is correct. (PUBLIC DOMAIN WORK FROM ANDREAS KNECHT)

    The new result, which is very far-reaching in its implications, is this: within the framework of AdS/CFT, there are no global symmetries. The paper itself, published on May 17, 2019 [above], is titled Constraints on Symmetries from Holography and was written by Daniel Harlow and Hirosi Ooguri. In particular, it showed that — again, within the context of AdS/CFT — that the following three conjectures are true.

    Quantum gravity does not allow global symmetries of any type.
    Quantum gravity requires that any internal gauge symmetry (which implies conservation laws like electric charge, color charge, or weak isospin) is mathematically compact.
    Quantum gravity requires that any internal gauge symmetry necessarily comes along with dynamical objects that transform in all irreducible representations.

    Each of these deserve elaboration, but the first one is the most powerful and profound.

    9
    Different frames of reference, including different positions and motions, would see different laws of physics (and would disagree on reality) if a theory is not relativistically invariant. The fact that we have a symmetry under ‘boosts,’ or velocity transformations, tells us we have a conserved quantity: linear momentum. This is much more difficult to comprehend when momentum isn’t simply a quantity associated with a particle, but is rather a quantum mechanical operator. This symmetry, if the holographic principle is correct, cannot exist globally. (WIKIMEDIA COMMONS USER KREA)

    All three of these conjectures have been around for a long time, and none of them are strictly true in either QFT or GR (or any form of classical physics) on their own. The classic arguments for all of them, in fact, are rooted in black hole physics and are known to require certain assumptions that, if violated, admit various loopholes. But if the AdS/CFT correspondence is true, and the holographic principle is applicable to quantum gravity in our Universe, all three of these conjectures are valid.

    The first one means that there are no conservation laws that always necessarily hold. There might be good approximate conservation laws that are still valid, but nothing — not energy, not angular momentum, not linear momentum — is explicitly or strictly conserved under all conditions. Even CPT and Lorentz invariance can be violated. The other two are more subtle, but help extend global symmetries to local conditions: they held prevent things like the instantaneous teleportation of electric charge in one location to another, disconnected location, and require the existence of all possible charges allowed by the theory, such as magnetic monopoles.

    10
    In 1982, an experiment running under the leadership of Blas Cabrera, one with eight turns of wire, detected a flux change of eight magnetons: indications of a magnetic monopole. Unfortunately, no one was present at the time of detection, and no one has ever reproduced this result or found a second monopole. Still, if string theory and this new result are correct, magnetic monopoles, being not forbidden by any law, must exist at some level. (CABRERA B. (1982). FIRST RESULTS FROM A SUPERCONDUCTIVE DETECTOR FOR MOVING MAGNETIC MONOPOLES, PHYSICAL REVIEW LETTERS, 48 (20) 1378–1381)

    The three quantum gravity conjectures that are demonstrated to hold for a holographic Universe have been around, in some form, since 1957 [Annals of Physics], but they were only conjectures until now [Physical Review D]. If the holographic principle (and AdS/CFT, and possibly string theory, by extension) is correct, all of these conjectures are necessarily true. There are no global symmetries; nothing in the Universe is always conserved under all imaginable circumstances (even if you need to reach the Planck scale to see violations), and all non-forbidden charges must exist. It would be revolutionary for our understanding of the quantum Universe.

    Despite the results and implications of this study, it’s still limited. We don’t know whether the holographic principle is true or not, or whether these assumptions about quantum gravity are correct. If it’s right, however, it means that once you include gravity, many of the symmetries that we hold so dear in the physics we know today are not global and fundamental. Paradoxically, if string theory is right, our expectations about hidden symmetries revealing themselves at a more fundamental level are not only wrong, but nature has no global symmetries at all.

    Update: First author of the paper, Daniel Harlow, has reached out to clarify a point that was not sufficiently appreciated by the author. He relates the following:

    “I wanted to point out that there is one technical problem in your description… our theorem does not apply to any of the symmetries you mention here! And indeed in AdS/CFT they all can be unbroken. The reason is that they are all actually gauge symmetries, not global symmetries. For electric charge I guess you are familiar with that, but in gravitational theory such as general relativity then translations, Lorentz transformations, CPT, etc are also gauge symmetries: they are just diffeomorphisms.

    The difference between a gauge symmetry and a global symmetry is that the presence of gauge charge can be measured from far away, while the presence of a global charge cannot. For example in elecromagnetism if we want to know the total charge in a region, we just have to measure the electric flux through its boundary. Similarly in gravity if we want to know the energy of something, we can measure the fall-off of the metric far away (basically looking for the M in the Schwarzschild metric). This should be compared with for example the Z_2 global symmetry of the Ising model, where there is no way to know that the spins are up in a region without going there and looking at them.

    It isn’t widely appreciated, but in the standard model of particle physics coupled to gravity there is actually only one global symmetry: the one described by the conservation of B-L (baryon number minus lepton number). So this is the only known symmetry we are actually saying must be violated!

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 10:37 am on July 4, 2019 Permalink | Reply
    Tags: "What Is The Smallest Possible Distance In The Universe?", , , , , , Ethan Siegel, , , Planck length,   

    From Ethan Siegel: “What Is The Smallest Possible Distance In The Universe?” 

    From Ethan Siegel
    July 3, 2019

    The Planck length is a lot smaller than anything we’ve ever accessed. But is it a true limit?

    1
    Black holes may be our best option for exploring quantum gravitational effects, as the space very close to the central singularity is where those effects are expected to be most important. However, below a certain distance scale, we are unable to accurately describe the Universe, even in theory. The existence of a smallest distance scale at which the laws of physics presently makes sense is a puzzle yet-to-be-solved for physicists. (NASA/AMES RESEARCH CENTER/C. HENZE)

    If you wanted to understand how our Universe operates, you’d have to examine it at a fundamental level. Macroscopic objects are made up of particles, which can only themselves be detected by going to subatomic scales. To examine the Universe’s properties, you must to look at the smallest constituents on the smallest possible scales. Only by understanding how they behave at this fundamental level can we hope to understand how they join together to create the human-scale Universe we’re familiar with.

    But you can’t extrapolate what we know about even the small-scale Universe to arbitrarily small distance scales. If we decide to go down to below about 10^-35 meters ⁠ — the Planck distance scale ⁠ — our conventional laws of physics only give nonsense for answers. Here’s the story of why, below a certain length scale, we cannot say anything physically meaningful.

    2
    We often visualize space as a 3D grid, even though this is a frame-dependent oversimplification when we consider the concept of spacetime. The question of whether space and time are discrete or continuous, and whether there’s a smallest possible length scale, is still unanswered. However, we do know that below the Planck distance scale, we cannot predict anything with any accuracy at all. (REUNMEDIA / STORYBLOCKS)

    Imagine, if you like, one of the classic problems of quantum physics: the particle-in-a-box. Imagine any particle you like, and imagine that it’s somehow confined to a certain small volume of space. Now, in this quantum game of peek-a-boo, we’re going to ask the most straightforward question you can imagine: “where is this particle?”

    You can make a measurement to determine the particle’s position, and that measurement will give you an answer. But there will be an inherent uncertainty associated with that measurement, where the uncertainty is caused by the quantum effects of nature.

    How large is that uncertainty? It’s related to both ħ and L, where ħ is Planck’s constant and L is the size of the box.

    3
    This diagram illustrates the inherent uncertainty relation between position and momentum. When one is known more accurately, the other is inherently less able to be known accurately. (WIKIMEDIA COMMONS USER MASCHEN)

    For most of the experiments we perform, Planck’s constant is small compared to any actual distance scale we’re capable of probing, and so when we examine the uncertainty we get — related to both ħ and L — we’ll see a small inherent uncertainty.

    But what if L is small? What if L is so small that, relative to ħ, it’s either comparably sized or even smaller?

    This is where you can see the problem start to arise. These quantum corrections that occur in nature don’t simply arise because there’s the main, classical effect, and then there are quantum corrections of order ~ħ that arise. There are corrections of all orders: ~ħ, ~ħ², ~ħ³, and so on. There’s a certain length scale, known as the Planck length, where if you reach it, the higher-order terms (which we usually ignore) become just as important as, or even more important than, the quantum corrections we normally apply.

    4
    The energy levels and electron wavefunctions that correspond to different states within a hydrogen atom, although the configurations are extremely similar for all atoms. The energy levels are quantized in multiples of Planck’s constant, but the sizes of the orbitals and atoms are determined by the ground-state energy and the electron’s mass. Additional effects may be subtle, but shift the energy levels in measurable, quantifiable fashions. Note that the potential created by the nucleus acts like a ‘box’ that confines the electron’s physical extent, similar to the particle-in-a-box thought experiment. (POORLENO OF WIKIMEDIA COMMONS)

    What is that critical length scale, then? The Planck scale was first put forth by physicist Max Planck more than 100 years ago. Planck took the three constants of nature:

    G, the gravitational constant of Newton’s and Einstein’s theories of gravity,
    ħ, Planck’s constant, or the fundamental quantum constant of nature, and
    c, the speed of light in a vacuum,

    and realized that you could combine them in different ways to get a single value for mass, another value for time, and another value for distance. These three quantities are known as the Planck mass (which comes out to about 22 micrograms), the Planck time (around 10^-43 seconds), and the Planck length (about 10^-35 meters). If you put a particle in a box that’s the Planck length or smaller, the uncertainty in its position becomes greater than the size of the box.

    5
    If you confine a particle to a space, and try to measure its properties, there will be quantum effects proportional to Planck’s constant and the size of the box. If the box is very small, below a certain length scale, these properties become impossible to calculate. (ANDY NGUYEN / UT-MEDICAL SCHOOL AT HOUSTON)

    But there’s a lot more to the story than that. Imagine you had a particle of a certain mass. If you compressed that mass down into a small enough volume, you’d get a black hole, just like you would for any mass. If you took the Planck mass — which is defined by the combination of those three constants in the form of √(ħc/G) — and asked that question, what sort of answer would you get?

    You’d find that the volume of space you needed that mass to occupy would be a sphere whose Schwarzschild radius is double the Planck length. If you asked how long it would take to cross from one end of the black hole to the other, the length of time is four times the Planck time. It’s no coincidence that these quantities are related; that’s unsurprising. But what might be surprising is what it implies when you start asking questions about the Universe at those tiny distance and time scales.

    6
    The energy of a photon depends on the wavelength it has; longer wavelength are lower in energy and shorter wavelengths are higher. In principle, there is no limit to how short a wavelength can be, but there are other physics concerns that cannot be ignored. (WIKIMEDIA COMMONS USER MAXHURTZ)

    In order to measure anything at the Planck scale, you’d need a particle with sufficiently high energy to probe it. The energy of a particle corresponds to a wavelength (either a photon wavelength for light or a de Broglie wavelength for matter), and to get down to Planck lengths, you need a particle at the Planck energy: ~10¹⁹ GeV, or approximately a quadrillion times greater than the maximum LHC energy.

    If you had a particle that actually achieved that energy, its momentum would be so large that the energy-momentum uncertainty would render that particle indistinguishable from a black hole. This is truly the scale at which our laws of physics break down.

    7
    The simulated decay of a black hole not only results in the emission of radiation, but the decay of the central orbiting mass that keeps most objects stable. Black holes are not static objects, but rather change over time. For the lowest-mass black holes, evaporation happens the fastest. (EU’S COMMUNICATE SCIENCE)

    When you examine the situation in greater detail, it only gets worse. If you start thinking about quantum fluctuations inherent to space (or spacetime) itself, you’ll recall there’s also an energy-time uncertainty relation. The smaller the distance scale, the smaller the corresponding timescale, which implies a larger energy uncertainty.

    At the Planck distance scale, this implies the appearance of black holes and quantum-scale wormholes, which we cannot investigate. If you performed higher-energy collisions, you’d simply create larger mass (and larger size) black holes, which would then evaporate via Hawking radiation.

    8
    An illustration of the concept of quantum foam, where quantum fluctuations are large, varied, and important on the smallest of scales. The energy inherent to space fluctuates in large amounts on these scales. If you view scales that are small enough, such as approaching the Planck scale, the fluctuations become large enough that they create black holes spontaneously. (NASA/CXC/M.WEISS)

    You might argue that, perhaps, this is why we need quantum gravity. That when you take the quantum rules we know and apply them to the law of gravity we know, this is simply highlighting a fundamental incompatibility between quantum physics and General Relativity. But it’s not so simple.

    Energy is energy, and we know it causes space to curve. If you start attempting to perform quantum field theory calculations at or near the Planck scale, you no longer know what type of spacetime to perform your calculations in. Even in quantum electrodynamics or quantum chromodynamics, we can treat the background spacetime where these particles exist to be flat. Even around a black hole, we can use a known spatial geometry. But at these ultra-intense energy, the curvature of space is unknown. We cannot calculate anything meaningful.

    9
    Quantum gravity tries to combine Einstein’s General theory of Relativity with quantum mechanics. Quantum corrections to classical gravity are visualized as loop diagrams, as the one shown here in white. Whether space (or time) itself is discrete or continuous is not yet decided, as is the question of whether gravity is quantized at all, or particles, as we know them today, are fundamental or not. But if we hope for a fundamental theory of everything, it must include quantized fields. (SLAC NATIONAL ACCELERATOR LAB)

    At energies that are sufficiently high, or (equivalently) at sufficiently small distances or short times, our current laws of physics break down. The background curvature of space that we use to perform quantum calculations is unreliable, and the uncertainty relation ensures that our uncertainty is larger in magnitude than any prediction we can make. The physics that we know can no longer be applied, and that’s what we mean when we say that “the laws of physics break down.”

    But there might be a way out of this conundrum. There’s an idea that’s been floating around for a long time — since Heisenberg, actually — that could provide a solution: perhaps there’s a fundamentally minimal length scale to space itself.

    10
    A representation of flat, empty space with no matter, energy or curvature of any type. If this space is fundamentally discrete, meaning there’s a minimum length scale to the Universe, we should be able to design an experiment that, at least in theory, shows that behavior. (AMBER STUVER, FROM HER BLOG, LIVING LIGO)

    Of course, a finite, minimum length scale would create its own set of problems. In Einstein’s theory of relativity, you can put down an imaginary ruler, anywhere, and it will appear to shorten based on the speed at which you move relative to it. If space were discrete and had a minimum length scale, different observers — i.e., people moving at different velocities — would now measure a different fundamental length scale from one another!

    That strongly suggests there would be a “privileged” frame of reference, where one particular velocity through space would have the maximum possible length, while all others would be shorter. This implies that something that we currently think is fundamental, like Lorentz invariance or locality, must be wrong. Similarly, discretized time poses big problems for General Relativity.

    11
    This illustration, of light passing through a dispersive prism and separating into clearly defined colors, is what happens when many medium-to-high energy photons strike a crystal. If we were to set this up with just a single photon, the amount the crystal moved could be in a discrete number of spatial ‘steps.’ (WIKIMEDIA COMMONS USER SPIGGET)

    Still, there may actually be a way to test whether there is a smallest length scale or not. Three years before he died, physicist Jacob Bekenstein put forth a brilliant idea for an experiment. If you pass a single photon through a crystal, you’ll cause it to move by a slight amount.

    Because photons can be tuned in energy (continuously) and crystals can be very massive compared to a photon’s momentum, we could detect whether the crystal moves in discrete “steps” or continuously. With low-enough energy photons, if space is quantized, the crystal would either move a single quantum step or not at all.

    12
    The fabric of spacetime, illustrated, with ripples and deformations due to mass. However, even though there are many things happening in this space, it does not need to be broken up into individual quanta itself.(EUROPEAN GRAVITATIONAL OBSERVATORY, LIONEL BRET/EUROLIOS)

    At present, there is no way to predict what’s going to happen on distance scales that are smaller than about 10^-35 meters, nor on timescales that are smaller than about 10^-43 seconds. These values are set by the fundamental constants that govern our Universe. In the context of General Relativity and quantum physics, we can go no farther than these limits without getting nonsense out of our equations in return for our troubles.

    It may yet be the case that a quantum theory of gravity will reveal properties of our Universe beyond these limits, or that some fundamental paradigm shifts concerning the nature of space and time could show us a new path forward. If we base our calculations on what we know today, however, there’s no way to go below the Planck scale in terms of distance or time. There may be a revolution coming on this front, but the signposts have yet to show us where it will occur.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 8:20 am on July 2, 2019 Permalink | Reply
    Tags: "Meet The Largest X-Ray Jet In The Universe", , , , , Ethan Siegel, , The active galaxy Pictor A   

    From Ethan Siegel: “Meet The Largest X-Ray Jet In The Universe” 

    From Ethan Siegel
    July 1, 2019

    Discovered by NASA’s Chandra X-ray observatory, it’s powered by a supermassive black hole.

    2019 marks 20 years of NASA’s Chandra, humanity’s most powerful X-ray observatory.

    1
    Artist illustration of the Chandra X-ray Observatory. Chandra is the most sensitive X-ray telescope ever built, and its mission was extended through at least 2024 as the flagship X-ray observatory in the NASA arsenal. (NASA/CXC/NGST TEAM)

    It’s viewed everything from pulsars to colliding gas to galaxy clusters and supermassive black holes.

    2
    A map of the 7 million second exposure of the Chandra Deep Field-South. This region shows hundreds of supermassive black holes, each one in a galaxy far beyond our own. The GOODS-South field, a Hubble project, was chosen to be centered on this original image. Its view of supermassive black holes is only one incredible application of the NASA’s Chandra X-ray observatory. (NASA/CXC/B. LUO ET AL., 2017, APJS, 228, 2)

    In 2015, it set its sights on a galaxy some 485 million light-years away: the radio-loud behemoth known as Pictor A.

    3
    The jet of the active galaxy Pictor A, with X-rays in blue and radio lobes in pink. When galaxies merge together, they’re expected to activate similarly to how this one has. (X-RAY: NASA/CXC/UNIV OF HERTFORDSHIRE/M.HARDCASTLE ET AL., RADIO: CSIRO/ATNF/ATCA)

    When Chandra took a look at it with its X-ray eyes, it saw something unprecendented and spectacular: a jet 300,000 light-years long.

    4
    The X-ray (B&W) and radio (red contours) emissions from the galaxy Pictor A. The greyscale image shows all the X-rays emitted with 500 to 5000 eV of energy, more than enough to ionize any atoms or molecules it encounters. The red contours are radio data shown superimposed atop the X-ray data. (M.J. HARDCASTLE ET AL. (2015), FROM ARXIV.ORG/ABS/1510.08392)

    Like all known active galaxies, Pictor A is powered by a supermassive black hole many millions to billions of times our Sun’s mass.

    5
    The galaxy Centaurus A is the closest example of an active galaxy to Earth, with its high-energy jets caused by electromagnetic acceleration around the central black hole. The extent of its jets are far smaller than the jets that Chandra has observed around Pictor A. (NASA/CXC/CFA/R.KRAFT ET AL.)

    Black holes can accelerate and eject infalling matter, leading to intense emissions.

    6
    A black hole more than six billion times the mass of the Sun powers the X-ray jet at the center of M87, which is many thousands of light-years in extent. If this image looks familiar, it might be: M87 is the first galaxy to have its event horizon imaged directly, owing to the incredible collaborative work of scientists working on the Event Horizon Telescope. (NASA/HUBBLE/WIKISKY)

    The light released spans the spectrum from high-energy X-rays to low-energy radio waves.

    7
    Appearing on a scale far greater than the scale of the galaxy itself, the jet emitted from Pictor A can be seen in the data at various points, thanks to the interactions between these high-energy emissions and the gas in the surrounding environment of the galaxy itself. The ‘hot spot’ at the end of the jet can be seen at the far right of the upper view of this image. (M.J. HARDCASTLE ET AL. (2015), FROM ARXIV.ORG/ABS/1510.08392)

    The radio lobes of gas provide a medium for high-energy X-rays to interact with.

    8
    While distant host galaxies for quasars and active galactic nuclei can often be imaged in visible/infrared light, the jets themselves and the surrounding emission is best viewed in both the X-ray and the radio, as illustrated here for the galaxy Hercules A. The gaseous outflows are highlighted in the radio, and if X-ray emissions follow the same path into the gas, they can be responsible for creating hot spots owing to the acceleration of electrons. (NASA, ESA, S. BAUM AND C. O’DEA (RIT), R. PERLEY AND W. COTTON (NRAO/AUI/NSF), AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

    When these interactions cause electrons to exceed the speed of sound in the gaseous medium, it creates intense shock waves.

    9
    An annotated version of the X-ray/radio composite image of Pictor A, showing the counterjet, the Hot Spot, and many other fascinating features. (X-RAY: NASA/CXC/UNIV OF HERTFORDSHIRE/M.HARDCASTLE ET AL., RADIO: CSIRO/ATNF/ATCA)

    The “hot spot” illustrated on the above NASA image is the definitive evidence of the jet-like nature of these X-rays and accelerated electrons.

    10
    Artist’s impression of an active galactic nucleus. The supermassive black hole at the center of the accretion disk sends a narrow high-energy jet of matter into space, perpendicular to the disc. A blazar about 4 billion light years away is the origin of many of the highest-energy cosmic rays and neutrinos, but even the full suite of active galaxies cannot compete with Pictor A in terms of raw size of the X-ray jet. (DESY, SCIENCE COMMUNICATION LAB)

    Alternative explanations involving boosted CMB photons have been ruled out.

    11
    The most distant X-ray jet in the Universe, from quasar GB 1428, located 12.4 billion light years from Earth. This jet comes from electrons heating CMB photons, but that mechanism is ruled out for Pictor A. (X-RAY: NASA/CXC/NRC/C.CHEUNG ET AL; OPTICAL: NASA/STSCI; RADIO: NSF/NRAO/VLA)

    Pictor A possesses the largest X-ray jet in the known Universe.

    12
    Despite many years of observations, we still don’t know whether the galaxy Pictor A, shown as viewed in optical light (main) and ultraviolet light (inset), is a spiral, elliptical, or irregular galaxy. Superior observations of the galaxy itself have yet to be acquired. (DIGITIZED SKY SURVEY 2 (MAIN); NASA/GALEX (INSET))

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:17 am on June 25, 2019 Permalink | Reply
    Tags: "Is This The Most Massive Star In The Universe?", , , , , Ethan Siegel   

    From Ethan Siegel: “Is This The Most Massive Star In The Universe?” 

    From Ethan Siegel

    June 24, 2019

    1
    The largest group of newborn stars in our Local Group of galaxies, cluster R136, contains the most massive stars we’ve ever discovered: over 250 times the mass of our Sun for the largest. The brightest of the stars found here are more than 8,000,000 times as luminous as our Sun. And yet, there are still likely even more massive ones out there. (NASA, ESA, AND F. PARESCE, INAF-IASF, BOLOGNA, R. O’CONNELL, UNIVERSITY OF VIRGINIA, CHARLOTTESVILLE, AND THE WIDE FIELD CAMERA 3 SCIENCE OVERSIGHT COMMITTEE)

    At the core of the largest star-forming region of the Local Group sits the biggest star we know of.

    Mass is the single most important astronomical property in determining the lives of stars.
    2
    The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. Our Sun is a G-class star, producing light with an effective temperature of around 5800 K and a brightness of 1 solar luminosity. Stars can be as low in mass as 8% the mass of our Sun, where they’ll burn with ~0.01% our Sun’s brightness and live for more than 1000 times as long, but they can also rise to hundreds of times our Sun’s mass, with millions of times our Sun’s luminosity. (WIKIMEDIA COMMONS USER LUCASVB, ADDITIONS BY E. SIEGEL)

    Greater masses generally lead to higher temperatures, greater brightnesses, and shorter lifetimes.

    3
    The active star-forming region, NGC 2363, is located in a nearby galaxy just 10 million light-years away. The brightest star visible here is NGC 2363-V1, visible as the isolated, bright star in the dark void at left. Despite being 6,300,000 times as bright as our Sun, it’s only 20 times as massive, having likely brightened recently as the result of an outburst. (LAURENT DRISSEN, JEAN-RENE ROY AND CARMELLE ROBERT (DEPARTMENT DE PHYSIQUE AND OBSERVATOIRE DU MONT MEGANTIC, UNIVERSITE LAVAL) AND NASA)

    Since massive stars burn through their fuel so quickly, the record holders are found in actively star-forming regions.

    4
    The ‘supernova impostor’ of the 19th century precipitated a gigantic eruption, spewing many Suns’ worth of material into the interstellar medium from Eta Carinae. High mass stars like this within metal-rich galaxies, like our own, eject large fractions of mass in a way that stars within smaller, lower-metallicity galaxies do not. Eta Carinae might be over 100 times the mass of our Sun and is found in the Carina Nebula, but it is not among the most massive stars in the Universe. (NATHAN SMITH (UNIVERSITY OF CALIFORNIA, BERKELEY), AND NASA)

    Luminosity isn’t enough, as short-lived outbursts can cause exceptional, temporary brightening in typically massive stars.

    5
    The star cluster NGC 3603 is located a little over 20,000 light-years away in our own Milky Way galaxy. The most massive star inside it is, NGC 3603-B, which is a Wolf-Rayet star located at the centre of the HD 97950 cluster which is contained within the large, overall star-forming region. (NASA, ESA AND WOLFGANG BRANDNER (MPIA), BOYKE ROCHAU (MPIA) AND ANDREA STOLTE (UNIVERSITY OF COLOGNE))

    Within our own Milky Way, massive star-forming regions, like NGC 3603, house many stars over 100 times our Sun’s mass.

    6
    The star at the center of the Heart Nebula (IC 1805) is known as HD 15558, which is a massive O-class star that is also a member of a binary system. With a directly-measured mass of 152 solar masses, it is the most massive star we know of whose value is determined directly, rather than through evolutionary inferences. (S58Y / FLICKR)

    As a member of a binary system, HD 15558 A is the most massive star with a definitive value: 152 solar masses.

    7
    The Large Magellanic Cloud, the fourth largest galaxy in our local group, with the giant star-forming region of the Tarantula Nebula (30 Doradus) just to the right and below the main galaxy. It is the largest star-forming region contained within our Local Group. (NASA, FROM WIKIMEDIA COMMONS USER ALFA PYXISDIS)

    However, all stellar mass records originate from the star forming region 30 Doradus in the Large Magellanic Cloud.

    8
    A large section of the Tarantula Nebula, the largest star-forming region in the Local Group, imaged by the Ciel Austral team. At top, you can see the presence of hydrogen, sulfur, and oxygen, which reveals the rich gas and plasma structure of the LMC, while the lower view shows an RGB color composite, revealing reflection and emission nebulae. (CIEL AUSTRAL: JEAN CLAUDE CANONNE, PHILIPPE BERNHARD, DIDIER CHAPLAIN, NICOLAS OUTTERS AND LAURENT BOURGON)

    Known as the Tarantula Nebula, it has a mass of ~450,000 Suns and contains over 10,000 stars.

    9
    The star forming region 30 Doradus, in the Tarantula Nebula in one of the Milky Way’s satellite galaxies, contains the largest, highest-mass stars known to humanity. The largest collection of bright, blue stars shown here is the ultra-dense star cluster R136, which contains nearly 100 stars that are approximately 100 solar masses or greater. Many of them have brightnesses that exceed a million solar luminosities. (NASA, ESA, AND E. SABBI (ESA/STSCI); ACKNOWLEDGMENT: R. O’CONNELL (UNIVERSITY OF VIRGINIA) AND THE WIDE FIELD CAMERA 3 SCIENCE OVERSIGHT COMMITTEE)

    The central star cluster, R136, contains 72 of the brightest, most massive classes of star.

    10
    The cluster RMC 136 (R136) in the Tarantula Nebula in the Large Magellanic Cloud, is home to the most massive stars known. R136a1, the greatest of them all, is over 250 times the mass of the Sun. While professional telescopes are ideal for teasing out high-resolution details such as these stars in the Tarantula Nebula, wide-field views are better with the types of long-exposure times only available to amateurs. (EUROPEAN SOUTHERN OBSERVATORY/P. CROWTHER/C.J. EVANS)

    The record-holder is R136a1, some 260 times our Sun’s mass and 8,700,000 times as bright.

    11
    An ultraviolet image and a spectrographic pseudo-image of the hottest, bluest stars at the core of R136. In this small component of the Tarantula Nebula alone, nine stars over 100 solar masses and dozens over 50 are identified through these measurements. The most massive star of all in here, R136a1, exceeds 250 solar masses, and is a candidate, later in its life, for photodisintegration. (ESA/HUBBLE, NASA, K.A. BOSTROEM (STSCI/UC DAVIS))

    Stars such as this cannot be individually resolved beyond our Local Group.

    12
    An illustration of the first stars turning on in the Universe. Without metals to cool down the stars, only the largest clumps within a large-mass cloud can become stars. Until enough time has passes for gravity to affect larger scales, only the small-scales can form structure early on. Without heavy elements to facilitate cooling, stars are expected to routinely exceed the mass thresholds of the most massive ones known today. (NASA)

    With NASA’s upcoming James Webb Space Telescope, we may discover Population III stars, which could reach thousands of solar masses.

    NASA/ESA/CSA Webb Telescope annotated

    13

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:26 am on June 17, 2019 Permalink | Reply
    Tags: "How Did This Black Hole Get So Big So Fast?", , , , , Ethan Siegel   

    From Ethan Siegel: “How Did This Black Hole Get So Big So Fast?” 

    From Ethan Siegel
    June 17, 2019

    1
    This image of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun, was created from images taken from surveys made by both the Sloan Digital Sky Survey and the UKIRT Infrared Deep Sky Survey. The quasar appears as a faint red dot close to the centre. This quasar was the most distant one known from 2011 until 2017, and is seen as it was just 770 million years after the Big Bang. Its black hole is so massive it poses a challenge to modern cosmological theories of black hole growth and formation.(ESO/UKIDSS/SDSS)

    It’s not impossible according to physics, but we truly don’t know how this object came to exist.

    Out in the extremities of the distant Universe, the earliest quasars can be found.

    2
    HE0435–1223, located in the centre of this wide-field image, is among the five best lensed quasars discovered to date, where the lensing phenomenon magnifies the light from distant objecst. This effect enables us to see quasars whose light was emitted when the Universe was less than 10% of its current age. The foreground galaxy creates four almost evenly distributed images of the distant quasar around it. (ESA/HUBBLE, NASA, SUYU ET AL.)

    Supermassive black holes at the centers of young galaxies accelerate matter to tremendous speeds, causing them to emit jets of radiation.

    3
    While distant host galaxies for quasars and active galactic nuclei can often be imaged in visible/infrared light, the jets themselves and the surrounding emission is best viewed in both the X-ray and the radio, as illustrated here for the galaxy Hercules A. (NASA, ESA, S. BAUM AND C. O’DEA (RIT), R. PERLEY AND W. COTTON (NRAO/AUI/NSF), AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

    What we observe enables us to reconstruct the mass of the central black hole, and explore the ultra-distant Universe.

    4
    The farther away we look, the closer in time we’re seeing towards the Big Bang. The current record-holder for quasars comes from a time when the Universe was just 690 million years old. (ROBIN DIENEL/CARNEGIE INSTITUTION FOR SCIENCE)

    Recently, a new black hole, J1342+0928, was discovered to originate from 13.1 billion years ago: when the Universe was 690 million years old, just 5% of its current age.

    5
    As viewed with our most powerful telescopes, such as Hubble, advances in camera technology and imaging techniques have enabled us to better probe and understand the physics and properties of distant quasars, including their central black hole’s properties. (NASA AND J. BAHCALL (IAS) (L); NASA, A. MARTEL (JHU), H. FORD (JHU), M. CLAMPIN (STSCI), G. HARTIG (STSCI), G. ILLINGWORTH (UCO/LICK OBSERVATORY), THE ACS SCIENCE TEAM AND ESA (R))

    It has a mass of 800 million Suns, an exceedingly high figure for such early times.

    6
    This artist’s rendering shows a galaxy being cleared of interstellar gas, the building blocks of new stars. Winds driven by a central black hole are responsible for this, and may be at the heart of what’s driving this active ultra-distant galaxy behind this newly discovered quasar. (ESA/ATG MEDIALAB)

    Even if black holes formed from the very first stars, they’d have to accrete matter and grow at the maximum rate possible — the Eddington limit — to reach this size so rapidly.

    7
    The active galaxy IRAS F11119+3257 shows, when viewed up close, outflows that may be consistent with a major merger. Supermassive black holes may only be visible when they’re ‘turned on’ by an active feeding mechanism, explaining why we can see these ultra-distant black holes at all. (NASA’S GODDARD SPACE FLIGHT CENTER/SDSS/S. VEILLEUX)

    Fortunately, other methods may also grow a supermassive black hole.

    When new bursts of star formation occur, enormous quantities of massive stars are created.

    8
    The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, demonstrating that not all stars need to go supernova or experience a stellar cataclysm to form a black hole.(NASA/ESA/C. KOCHANEK (OSU))

    These can either directly collapse or go supernova, creating large numbers of massive black holes which then merge and grow.

    9
    Simulations of various gas-rich processes, such as galaxy mergers, indicate that the formation of direct collapse black holes should be possible. A combination of direct collapse, supernovae, and merging stars and stellar remnants could produce a young black hole this massive. Complementarily, present LIGO results indicate that black holes merge every 5 minutes somewhere in the Universe. (L. MAYER ET AL. (2014), VIA ARXIV.ORG/ABS/1411.5683)

    Only ~20 black holes this large should exist so early in the Universe.

    10
    An ultra-distant quasar showing plenty of evidence for a supermassive black hole at its center. How these black holes got so massive so quickly is a topic of contentious scientific debate, but may have an answer that fits within our standard theories. We are uncertain whether that’s true or not at this juncture. (X-RAY: NASA/CXC/UNIV OF MICHIGAN/R.C.REIS ET AL; OPTICAL: NASA/STSCI)

    Is this problematic for cosmology? More data will eventually decide.

    See the full article here .

    five-ways-keep-your-child-safe-school-shootings

    Please help promote STEM in your local schools.

    Stem Education Coalition

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
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