I am telling the reader this story in the hope of impelling him or her to find their own story and start a wordpress blog. We all have a story. Find yours.
The oldest post I can find for this blog is “From FermiLab Today: Tevatron is Done” at the End of 2011 (but I am not sure if that is the first post, just the oldest I could find.
But the origin goes back to 1985, Timothy Ferris Creation of the Universe PBS, November 20, 1985, available in different videos on YouTube; The Atom Smashers, PBS Frontline November 25, 2008, centered at Fermilab, not available on Youtube; and The Big Bang Machine, with Sir Brian Cox of U Manchester and the ATLAS project at the LHC at CERN.
In 1993, our idiot Congress pulled the plug on The Superconducting Super Collider, a particle accelerator complex under construction in the vicinity of Waxahachie, Texas. Its planned ring circumference was 87.1 kilometers (54.1 mi) with an energy of 20 Tev per proton and was set to be the world’s largest and most energetic. It would have greatly surpassed the current record held by the Large Hadron Collider, which has ring circumference 27 km (17 mi) and energy of 13 TeV per proton.
If this project had been built, most probably the Higgs Boson would have been found there, not in Europe, to which the USA had ceded High Energy Physics.
The project’s director was Roy Schwitters, a physicist at the University of Texas at Austin. Dr. Louis Ianniello served as its first Project Director for 15 months. The project was cancelled in 1993 due to budget problems, cited as having no immediate economic value.
Some where I learned that fully 30% of the scientists working at CERN were U.S. citizens. The ATLAS project had 600 people at Brookhaven Lab. The CMS project had 1,000 people at Fermilab. There were many scientists which had “gigs” at both sites.
I started digging around in CERN web sites and found Quantum Diaries, a “blog” from before there were blogs, where different scientists could post articles. I commented on a few and my dismay about the lack of U.S recognition in the press.
Those guys at Quantum Diaries, gave me access to the Greybook, the list of every institution in the world in several tiers processing data for CERN. I collected all of their social media and was off to the races for CERN and other great basic and applied science.
Since then I have expanded the list of sites that I cover from all over the world. I build html templates for each institution I cover and plop their articles, complete with all attributions and graphics into the template and post it to the blog. I am not a scientist and I am not qualified to write anything or answer scientific questions. The only thing I might add is graphics where the origin graphics are weak. I have a monster graphics library. Any science questions are referred back to the writer who is told to seek his answer from the real scientists in the project.
The blog has to date 900 followers on the blog, its Facebook Fan page and Twitter.I get my material from email lists and RSS feeds. I do not use Facebook or Twitter, which are both loaded with garbage in the physical sciences.
Echoes in gravitational wave signals suggest that the event horizon of a black hole may be more complicated than scientists currently think.
Gravitational waves Werner Benger-ZIB-AEI-CCT-LSU
Research from the University of Waterloo reports the first tentative detection of these echoes, caused by a microscopic quantum “fuzz” that surrounds newly formed black holes.
Gravitational waves are ripples in the fabric of space-time, caused by the collision of massive, compact objects in space, such as black holes or neutron stars.
“According to Einstein’s Theory of General Relativity, nothing can escape from the gravity of a black hole once it has passed a point of no return, known as the event horizon,” explained Niayesh Afshordi, a physics and astronomy professor at Waterloo. “This was scientists’ understanding for a long time, until Stephen Hawking used quantum mechanics to predict that quantum particles will slowly leak out of black holes, which we now call Hawking radiation.
Afshordi and his coauthor Jahed Abedi from Max-Planck-Institut für Gravitationsphysik in Germany, have reported the first tentative findings of these repeating echoes, providing experimental evidence that black holes may be radically different from what Einstein’s theory of relativity predicts, and lack event horizons.
They used gravitational wave data from the first observation of a neutron star collision, recorded by the LIGO/Virgo gravitational wave detectors.
MIT /Caltech Advanced aLigo
Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018
The echoes observed by Afshordi and Abedi match the simulated echoes predicted by models of black holes that account for the effects of quantum mechanics and Hawking radiation.
“Our results are still tentative because there is a very small chance that what we see is due to random noise in the detectors, but this chance becomes less likely as we find more examples,” said Afshordi. “Now that scientists know what we’re looking for, we can look for more examples, and have a much more robust confirmation of these signals. Such a confirmation would be the first direct probe of the quantum structure of space-time.”
In just half a century, the University of Waterloo, located at the heart of Canada’s technology hub, has become a leading comprehensive university with nearly 36,000 full- and part-time students in undergraduate and graduate programs.
Consistently ranked Canada’s most innovative university, Waterloo is home to advanced research and teaching in science and engineering, mathematics and computer science, health, environment, arts and social sciences. From quantum computing and nanotechnology to clinical psychology and health sciences research, Waterloo brings ideas and brilliant minds together, inspiring innovations with real impact today and in the future.
As home to the world’s largest post-secondary co-operative education program, Waterloo embraces its connections to the world and encourages enterprising partnerships in learning, research, and commercialization. With campuses and education centres on four continents, and academic partnerships spanning the globe, Waterloo is shaping the future of the planet.
14 Jan 2020
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University of Birmingham UK
tel: +44 (0)121 414 2772. r.lockwood@bham.ac.uk
A new study by a group of researchers at the University of Birmingham has found that collisions of supermassive black holes may be simultaneously observable in both gravitational waves and X-rays at the beginning of the next decade.
An image of the use of Athena and LISA to observe the same source. Credits: R.Buscicchio (University of Birmingham), based on content from NASA, ESA, IFCA, the Athena Community Office, G. Alexandrov, A. Burrows
ESA/Athena spacecraft depiction
Gravity is talking. Lisa will listen. Dialogos of Eide
ESA/NASA eLISA space based, the future of gravitational wave research
The European Space Agency (ESA) has recently announced that its two major space observatories of the 2030s will have their launches timed for simultaneous use. These missions, Athena, the next generation X-ray space telescope and LISA, the first space-based gravitational wave observatory, will be coordinated to begin observing within a year of each other and are likely to have at least four years of overlapping science operations.
According to the new study, published this week in Nature Astronomy, ESA’s decision will give astronomers an unprecedented opportunity to produce multi-messenger maps of some of the most violent cosmic events in the Universe, which have not been observed so far and which lie at the heart of long-standing mysteries surrounding the evolution of the Universe.
They include the collision of supermassive black holes in the core of galaxies in the distant universe and the “swallowing up” of stellar compact objects such as neutron stars and black holes by massive black holes harboured in the centres of most galaxies.
The gravitational waves measured by LISA will pinpoint the ripples of space time that the mergers cause while the X-rays observed with Athena reveal the hot and highly energetic physical processes in that environment. Combining these two messengers to observe the same phenomenon in these systems would bring a huge leap in our understanding of how massive black holes and galaxies co-evolve, how massive black holes grow their mass and accrete, and the role of gas around these black holes.
These are some of the big unanswered questions in astrophysics that have puzzled scientists for decades.
Dr Sean McGee, Lecturer in Astrophysics at the University of Birmingham and a member of both the Athena and LISA consortiums, led the study. He said, “The prospect of simultaneous observations of these events is uncharted territory, and could lead to huge advances. This promises to be a revolution in our understanding of supermassive black holes and how they growth within galaxies.”
Professor Alberto Vecchio, Director of the Institute for Gravitational Wave Astronomy, University of Birmingham, and a co-author on the study, said: “I have worked on LISA for twenty years and the prospect of combining forces with the most powerful X-ray eyes ever designed to look right at the centre of galaxies promises to make this long haul even more rewarding. It is difficult to predict exactly what we’re going to discover: we should just buckle up, because it is going to be quite a ride”.
During the life of the missions, there may be as many as 10 mergers of black holes with masses of 100,000 to 10,000,000 times the mass of the sun that have signals strong enough to be observed by both observatories. Although due to our current lack of understanding of the physics occurring during these mergers and how frequently they occur, the observatories could observe many more or many fewer of these events. Indeed, these are questions which will be answered by the observations.
In addition, LISA will detect the early stages of stellar mass black holes mergers which will conclude with the detection in ground based gravitational wave observatories. This early detection will allow Athena to be observing the binary location at the precise moment the merger will occur.
Birmingham has been challenging and developing great minds for more than a century. Characterised by a tradition of innovation, research at the University has broken new ground, pushed forward the boundaries of knowledge and made an impact on people’s lives. We continue this tradition today and have ambitions for a future that will embed our work and recognition of the Birmingham name on the international stage.
Researchers install a new quantum squeezing device into one of LIGO’s gravitational wave detectors. Image: Lisa Barsotti
A close-up of the quantum squeezer which has expanded LIGO’s expected detection range by 50 percent. Image: Maggie Tse
Just a year ago, the National Science Foundation-funded Laser Interferometer Gravitational-wave Observatory, or LIGO, was picking up whispers of gravitational waves every month or so. Now, a new addition to the system is enabling the instruments to detect these ripples in space-time nearly every week.
Since the start of LIGO’s third operating run in April, a new instrument known as a quantum vacuum squeezer has helped scientists pick out dozens of gravitational wave signals, including one that appears to have been generated by a binary neutron star — the explosive merging of two neutron stars.
The squeezer, as scientists call it, was designed, built, and integrated with LIGO’s detectors by MIT researchers, along with collaborators from Caltech and the Australian National University, who detail its workings in a paper published today in the journal Physical Review Letters.
What the instrument “squeezes” is quantum noise — infinitesimally small fluctuations in the vacuum of space that make it into the detectors. The signals that LIGO detects are so tiny that these quantum, otherwise minor fluctuations can have a contaminating effect, potentially muddying or completely masking incoming signals of gravitational waves.
“Where quantum mechanics comes in relates to the fact that LIGO’s laser is made of photons,” explains lead author Maggie Tse, a graduate student at MIT. “Instead of a continuous stream of laser light, if you look close enough it’s actually a noisy parade of individual photons, each under the influence of vacuum fluctuations. Whereas a continuous stream of light would create a constant hum in the detector, the individual photons each arrive at the detector with a little ‘pop.’”
“This quantum noise is like a popcorn crackle in the background that creeps into our interferometer, and is very difficult to measure,” adds Nergis Mavalvala, the Marble Professor of Astrophysics and associate head of the Department of Physics at MIT.
With the new squeezer technology, LIGO has shaved down this confounding quantum crackle, extending the detectors’ range by 15 percent. Combined with an increase in LIGO’s laser power, this means the detectors can pick out a gravitational wave generated by a source in the universe out to about 140 megaparsecs, or more than 400 million light years away. This extended range has enabled LIGO to detect gravitational waves on an almost weekly basis.
“When the rate of detection goes up, not only do we understand more about the sources we know, because we have more to study, but our potential for discovering unknown things comes in,” says Mavalvala, a longtime member of the LIGO scientific team. “We’re casting a broader net.”
The new paper’s lead authors are graduate students Maggie Tse and Haocun Yu, and Lisa Barsotti, a principal research scientist at MIT’s Kavli Institute for Astrophysics and Space Research, along with others in the LIGO Scientific Collaboration.
Quantum limit
LIGO comprises two identical detectors, one located at Hanford, Washington, and the other at Livingston, Louisiana. Each detector consists of two 4-kilometer-long tunnels, or arms, each extending out from the other in the shape of an “L.”
MIT /Caltech Advanced aLigo
Caltech/MIT Advanced aLigo Hanford, WA, USA installation
Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
To detect a gravitational wave, scientists send a laser beam from the corner of the L-shaped detector, down each arm, at the end of which is suspended a mirror. Each laser bounces off its respective mirror and travels back down each arm to where it started. If a gravitational wave passes through the detector, it should shift one or both of the mirrors’ position, which would in turn affect the timing of each laser’s arrival back at its origin. This timing is something scientists can measure to identify a gravitational wave signal.
The main source of uncertainty in LIGO’s measurements comes from quantum noise in a laser’s surrounding vacuum. While a vacuum is typically thought of as a nothingness, or emptiness in space, physicists understand it as a state in which subatomic particles (in this case, photons) are being constantly created and destroyed, appearing then disappearing so quickly they are extremely difficult to detect. Both the time of arrival (phase) and number (amplitude) of these photons are equally unknown, and equally uncertain, making it difficult for scientists to pick out gravitational-wave signals from the resulting background of quantum noise.
And yet, this quantum crackle is constant, and as LIGO seeks to detect farther, fainter signals, this quantum noise has become more of a limiting factor.
“The measurement we’re making is so sensitive that the quantum vacuum matters,” Barsotti notes.
Putting the squeeze on “spooky” noise
The research team at MIT began over 15 years ago to design a device to squeeze down the uncertainty in quantum noise, to reveal fainter and more distant gravitational wave signals that would otherwise be buried the quantum noise.
Quantum squeezing was a theory that was first proposed in the 1980s, the general idea being that quantum vacuum noise can be represented as a sphere of uncertainty along two main axes: phase and amplitude. If this sphere were squeezed, like a stress ball, in a way that constricted the sphere along the amplitude axis, this would in effect shrink the uncertainty in the amplitude state of a vacuum (the squeezed part of the stress ball), while increasing the uncertainty in the phase state (stress ball’s displaced, distended portion). Since it is predominantly the phase uncertainty that contributes noise to LIGO, shrinking it could make the detector more sensitive to astrophysical signals.
When the theory was first proposed nearly 40 years ago, a handful of research groups tried to build quantum squeezing instruments in the lab.
“After these first demonstrations, it went quiet,” Mavalvala says.
“The challenge with building squeezers is that the squeezed vacuum state is very fragile and delicate,” Tse adds. “Getting the squeezed ball, in one piece, from where it is generated to where it is measured is surprisingly hard. Any misstep, and the ball can bounce right back to its unsqueezed state.”
Then, around 2002, just as LIGO’s detectors first started searching for gravitational waves, researchers at MIT began thinking about quantum squeezing as a way to reduce the noise that could possibly mask an incredibly faint gravitational wave signal. They developed a preliminary design for a vacuum squeezer, which they tested in 2010 at LIGO’s Hanford site. The result was encouraging: The instrument managed to boost LIGO’s signal-to-noise ratio — the strength of a promising signal versus the background noise.
Since then, the team, led by Tse and Barsotti, has refined its design, and built and integrated squeezers into both LIGO detectors. The heart of the squeezer is an optical parametric oscillator, or OPO — a bowtie-shaped device that holds a small crystal within a configuration of mirrors. When the researchers direct a laser beam to the crystal, the crystal’s atoms facilitate interactions between the laser and the quantum vacuum in a way that rearranges their properties of phase versus amplitude, creating a new, “squeezed” vacuum that then continues down each of the detector’s arm as it normally would. This squeezed vacuum has smaller phase fluctuations than an ordinary vacuum, allowing scientists to better detect gravitational waves.
In addition to increasing LIGO’s ability to detect gravitational waves, the new quantum squeezer may also help scientists better extract information about the sources that produce these waves.
“We have this spooky quantum vacuum that we can manipulate without actually violating the laws of nature, and we can then make an improved measurement,” Mavalvala says. “It tells us that we can do an end-run around nature sometimes. Not always, but sometimes.”
This research was supported, in part, by the National Science Foundation. LIGO was constructed by Caltech and MIT.
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KAGRA gravitational wave detector, Kamioka mine in Kamioka-cho, Hida-city, Gifu-prefecture, Japan
Japan’s Kamioka Gravitational-wave Detector (KAGRA) will soon team up with the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) and Europe’s Virgo in the search for subtle shakings of space and time known as gravitational waves. Representatives for the three observatories signed a memorandum of agreement (MOA) about their collaborative efforts today, October 4. The agreement includes plans for joint observations and data sharing.
“This is a great example of international scientific cooperation,” says Caltech’s David Reitze, executive director of the LIGO Laboratory. “Having KAGRA join our network of gravitational-wave observatories will significantly enhance the science in the coming decade.”
“At present, KAGRA is in the commissioning phase, after the completion of its detector construction this spring. We are looking forward to joining the network of gravitational-wave observations later this year,” says Takaaki Kajita, principal investigator of the KAGRA project and co-winner of the 2015 Nobel Prize in Physics.
In 2015, the twin detectors of LIGO, one in Washington and the other in Louisiana, made history by making the first direct detection of gravitational waves, a discovery that earned three of the project’s founders—Caltech’s Barry Barish, Ronald and Maxine Linde Professor of Physics, Emeritus, and Kip Thorne, Richard P. Feynman Professor of Theoretical Physics, Emeritus; and MIT’s Rainer Weiss, professor of physics, emeritus—the 2017 Nobel Prize in Physics. Since then, LIGO and its partner Virgo have identified more than 30 likely detections of gravitational waves, mostly from colliding black holes.
“The more detectors we have in the global gravitational-wave network, the more accurately we can localize the gravitational-wave signals on the sky, and the better we can determine the underlying nature of cataclysmic events that produced the signals.” says Reitze.
For instance, in 2017, Virgo and the two LIGO detectors were able together to localize a merger of two neutron stars to a patch of sky about 30 square degrees in size, or less than 0.1 percent of the sky. This was a small enough patch to enable ground-based and space telescopes to pinpoint the galaxy that hosted the collision and observe its explosive aftermath in light.
“These findings amounted to the first time a cosmic event had been observed in both gravitational waves and light and gave astronomers a first-of-its kind look at the spectacular smashup of neutron stars,” says Virgo Collaboration spokesperson Jo van den Brand of Nikhef (the Dutch National Institute for Subatomic Physics) and Maastricht University in the Netherlands.
With KAGRA joining the network, these gravitational-wave events will eventually be narrowed down to patches of sky that are only about 10 square degrees, greatly enhancing the ability of light-based telescopes to carry out follow-up observations. For its initial run, KAGRA will operate at sensitivities that are likely too low to detect gravitational waves, but with time, as the performance of the instrumentation is improved, it will reach sensitivities high enough to join the hunt.
Having a fourth detector will also increase the overall detection rate, helping scientists to probe and understand some of the most energetic events in the universe.
KAGRA is expected to come online for the first time in December of this year, joining the third observing run of LIGO and Virgo, which began on April 1, 2019.
MIT /Caltech Advanced aLigo
VIRGO Gravitational Wave interferometer, near Pisa, Italy
The Japanese detector will pioneer two new approaches to gravitational-wave searches. It will be the first kilometer-scale gravitational-wave observatory to operate underground, which will dampen unwanted noise from winds and seismic activity; and it will be the first to use cryogenically chilled mirrors, a technique that cuts down on thermal noise.
“These features could supply a very important direction for the futureof gravitational-wave detectors with much higher sensitivities. Therefore, we should make every effort, for the global gravitational-wave community, to prove that the underground site and the cryogenic mirrors are useful,” says Kajita.
The new MOA also includes the German-British GEO600 detector. Although GEO600 is not sensitive enough to detect gravitational-wave signals from distant black hole and neutron star collisions, it has been important for testing new technologies that will be key for improving future detectors. In addition, LIGO India is expected to join the network of observatories in 2025, signifying the beginning of a truly global effort to catch ripples in the fabric of space and time.
Additional information about the gravitational-wave observatories:
LIGO is funded by NSF and operated by Caltech and MIT, which conceived of LIGO and lead the project. Financial support for the Advanced LIGO project was led by the NSF with Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council) and Australia (Australian Research Council-OzGrav) making significant commitments and contributions to the project. Approximately 1,300 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration. A list of additional partners is available at https://my.ligo.org/census.php.
The Virgo Collaboration is currently composed of approximately 480 scientists, engineers, and technicians from about 96 institutes from Belgium, France, Germany, Hungary, Italy, the Netherlands, Poland, and Spain. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa in Italy, and is funded by Centre National de la Recherche Scientifique (CNRS) in France, the Istituto Nazionale di Fisica Nucleare (INFN) in Italy, and Nikhef in the Netherlands. A list of the Virgo Collaboration members can be found at http://public.virgo-gw.eu/the-virgo-collaboration/. More information is available on the Virgo website at http://www.virgo-gw.eu.
The KAGRA project is supported by MEXT (Ministry of Education, Culture, Sports, Science, and Technology-Japan). KAGRA is hosted by the Institute for Cosmic Ray Research (ICRR), the University of Tokyo, and co-hosted by High Energy Accelerator Research Organization (KEK) and the National Astronomical Observatory of Japan (NAOJ). The KAGRA collaboration is composed of more than 360 individuals from more than 100 institutions from 15 countries/regions. The list of collaborators’ affiliations is available at http://gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA/KSC#KAGRAcollaborators. More information is available on the KAGRA website at https://gwcenter.icrr.u-tokyo.ac.jp/en/.
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A black widow pulsar and its small stellar companion, viewed within their orbital plane. Powerful radiation and the pulsar’s “wind” – an outflow of high-energy particles — strongly heat the facing side of the star to temperatures twice as hot as the sun’s surface. The pulsar is gradually evaporating its partner, which fills the system with ionized gas and prevents astronomers from detecting the pulsar’s radio beam most of the time. NASA’s Goddard Space Flight Center/Cruz deWilde
Second fastest spinning radio pulsar known is a gamma-ray pulsar, too. Multi-messenger observations look closely at the system and raise new questions.
An international research team led by the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover has discovered that the radio pulsar J0952-0607 also emits pulsed gamma radiation. J0952-0607 spins 707 times in one second and is 2nd in the list of rapidly rotating neutron stars. By analyzing about 8.5 years worth of data from NASA’s Fermi Gamma-ray Space Telescope, LOFAR radio observations from the past two years, observations from two large optical telescopes, and gravitational-wave data from the LIGO detectors, the team used a multi-messenger approach to study the binary system of the pulsar and its lightweight companion in detail.
Gran Telescopio Canarias at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, Spain, sited on a volcanic peak 2,267 metres (7,438 ft) above sea level
TFC HiPERCAM mounted on the Gran Telescopio Canarias,
ESO/NTT at Cerro La Silla, Chile, at an altitude of 2400 metres
ESO La Silla NTT ULTRACAM is an ultra fast camera capable of capturing some of the most rapid astronomical events. It can take up to 500 pictures a second in three different colours simultaneously. It was designed and built by scientists from the Universities of Sheffield and Warwick (United Kingdom), in collaboration with the UK Astronomy Technology Centre in Edinburgh. ULTRACAM employs the latest in charged coupled device (CCD) detector technology in order to take, store and analyse data at the required sensitivities and speeds. CCD detectors can be found in digital cameras and camcorders, but the devices used in ULTRACAM are special because they are larger, faster and most importantly, much more sensitive to light than the detectors used in today’s consumer electronics products. Since it was built, it has operated at the William Herschel Telescope, the New Technology Telescope, and the Very Large Telescope. It is now permanently mounted on the Thai National Telescope.
NASA/Fermi LAT
NASA/Fermi Gamma Ray Space Telescope
ASTRON LOFAR European Map
ASTRON LOFAR Radio Antenna Bank, Netherlands
Their study published in The Astrophysical Journal shows that extreme pulsar systems are hiding in the Fermi catalogues and published in the Astrophysical Journal today shows that extreme pulsar systems are hiding in the Fermi catalogues and motivates further searches. Despite being very extensive, the analysis also raises new unanswered questions about this system.
MIT /Caltech Advanced aLigo
Pulsars are the compact remnants of stellar explosions which have strong magnetic fields and are rapidly rotating.
Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, Cambridge University, taken for the Daily Herald newspaper in 1968. Denied the Nobel.
Dame Susan Jocelyn Bell Burnell at work on first plusar chart 1967 pictured working at the Four Acre Array in 1967. Image courtesy of Mullard Radio Astronomy Observatory.
Dame Susan Jocelyn Bell Burnell 2009
Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com
They emit radiation like a cosmic lighthouse and can be observable as radio pulsars and/or gamma-ray pulsars depending on their orientation towards Earth.
The fastest pulsar outside globular clusters
PSR J0952-0607 (the name denotes the position in the sky) was first discovered in 2017 by radio observations of a source identified by the Fermi Gamma-ray Space Telescope as possibly being a pulsar. No pulsations of the gamma rays in data from the Large Area Telescope (LAT) onboard Fermi had been detected. Observations with the radio telescope array LOFAR identified a pulsating radio source and – together with optical telescope observations – allowed to measure some properties of the pulsar. It is orbiting the common center of mass in 6.2 hours with a companion star that only weighs a fiftieth of our Sun. The pulsar rotates 707 times in a single second and is therefore the fastest spinning in our Galaxy outside the dense stellar environments of globular clusters.
Searching for extremely faint signals
Using this prior information on the binary pulsar system, Lars Nieder, a PhD student at the AEI Hannover, set out to see if the pulsar also emitted pulsed gamma rays. “This search is extremely challenging because the Fermi gamma-ray telescope only registered the equivalent of about 200 gamma rays from the faint pulsar over the 8.5 years of observations. During this time the pulsar itself rotated 220 billion times. In other words, only once in every billion rotations was a gamma ray observed!” explains Nieder. “For each of these gamma rays, the search must identify exactly when during each of the 1.4 millisecond rotations it was emitted.”
This requires combing through the data with very fine resolution in order not to miss any possible signals. The computing power required is enormous. The very sensitive search for faint gamma-ray pulsations would have taken 24 years to complete on a single computer core. By using the Atlas computer cluster at the AEI Hannover it finished in just 2 days.
MPG Institute for Gravitational Physics Atlas Computing Cluster
A strange first detection
“Our search found a signal, but something was wrong! The signal was very faint and not quite where it was supposed to be. The reason: our detection of gamma rays from J0952-0607 had revealed a position error in the initial optical-telescope observations which we used to target our analysis. Our discovery of the gamma-ray pulsations revealed this error,” explains Nieder. “This mistake was corrected in the publication reporting the radio pulsar discovery. A new and extended gamma-ray search made a rather faint – but statistically significant – gamma-ray pulsar discovery at the corrected position.”
Having discovered and confirmed the existence of pulsed gamma radiation from the pulsar, the team went back to the Fermi data and used the full 8.5 years from August 2008 until January 2017 to determine physical parameters of the pulsar and its binary system. Since the gamma radiation from J0952-0607 was so faint, they had to enhance their analysis method developed previously to correctly include all unknowns.
The pulse profile (distribution of gamma-ray photons during one rotation of the pulsar) of J0952-0607 is shown at the top. Below is the corresponding distribution of the individual photons over the ten years of observations. The greyscale shows the probability (photon weights) for individual photons to originate from the pulsar. From mid 2011 on, the photons line up along tracks corresponding to the pulse profile. This shows the detection of gamma-ray pulsations, which is not possible before mid 2011. L. Nieder/Max Planck Institute for Gravitational Physics.
Another surprise: no gamma-ray pulsations before July 2011
The derived solution contained another surprise, because it was impossible to detect gamma-ray pulsations from the pulsar in the data from before July 2011. The reason for why the pulsar only seems to show pulsations after that date is unknown. Variations in how much gamma rays it emitted might be one reason, but the pulsar is so faint that it was not possible to test this hypothesis with sufficient accuracy. Changes in the pulsar orbit seen in similar systems might also offer an explanation, but there was not even a hint in the data that this was happening.
Optical observations raise further questions
The team also used observations with the ESO’s New Technology Telescope at La Silla and the Gran Telescopio Canarias on La Palma to examine the pulsar’s companion star. It is most likely tidally locked to the pulsar like the Moon to the Earth so that one side always faces the pulsar and gets heated up by its radiation. While the companion orbits the binary system’s center of mass its hot “day” side and cooler “night” side are visible from the Earth and the observed brightness and color vary.
These observations create another riddle. While the radio observations point to a distance of roughly 4,400 light-years to the pulsar, the optical observations imply a distance about three times larger. If the system was relatively close to Earth, it would feature a never-seen-before extremely compact high density companion, while larger distances are compatible with the densities of known similar pulsar companions. An explanation for this discrepancy might be the existence of shock waves in the wind of particles from the pulsar, which could lead to a different heating of the companion. More gamma-ray observations with Fermi LAT observations should help answer this question.
Searching for continuous gravitational waves
Another group of researchers at the AEI Hannover searched for continuous gravitational wave emission from the pulsar using LIGO data from the first (O1) and second (O2) observation run. Pulsars can emit gravitational waves when they have tiny hills or bumps. The search did not detect any gravitational waves, meaning that the pulsar’s shape must be very close to a perfect sphere with the highest bumps less than a fraction of a millimeter.
Rapidly rotating neutron stars
Understanding rapidly spinning pulsars is important because they are probes of extreme physics. How fast neutron stars can spin before they break apart from centrifugal forces is unknown and depends on unknown nuclear physics. Millisecond pulsars like J0952-0607 are rotating so rapidly because they have been spun up by accreting matter from their companion. This process is thought to bury the pulsar’s magnetic field. With the long-term gamma-ray observations, the research team showed that J0952-0607 has one of the ten lowest magnetic fields ever measured for a pulsar, consistent with expectations from theory.
Einstein@Home searches for test cases of extreme physics
“We will keep studying this system with gamma-ray, radio, and optical observatories since there are still unanswered questions about it. This discovery also shows once more that extreme pulsar systems are hiding in the Fermi LAT catalogue,” says Prof. Bruce Allen, Nieder’s PhD supervisor and Director at the AEI Hannover. “We are also employing our citizen science distributed computing project Einstein@Home to look for binary gamma-ray pulsar systems in other Fermi LAT sources and are confident to make more exciting discoveries in the future.”
General relativity suggests the spacetime oddities can be fully described by their mass and spin.
After two black holes collide and meld into one, the new black hole “rings” (illustrated), emitting gravitational waves before settling down into a quiet state. M. Isi/MIT, NASA
For black holes, it’s tough to stand out from the crowd: Donning a mohawk is a no-no.
Ripples in spacetime produced as two black holes merged into one suggest that the behemoths have no “hair,” scientists report in the Sept. 13 Physical Review Letters. That’s another way of saying that, as predicted by Einstein’s general theory of relativity, black holes have no distinguishing characteristics aside from mass and the rate at which they spin (SN: 9/24/10).
“Black holes are very simple objects, in some sense,” says physicist Maximiliano Isi of MIT.
Detected by the Advanced Laser Interferometer Gravitational-Wave Observatory, LIGO, in 2015, the spacetime ripples resulted from a fateful encounter between two black holes, which spiraled around each other before crashing together to form one big black hole (SN: 2/11/16).
MIT /Caltech Advanced aLigo
In the aftermath of that coalescence, the newly formed big black hole went through a period of “ringdown.” It oscillated over several milliseconds as it emitted gravitational waves, similar to the way a struck bell vibrates and makes sound waves before eventually quieting down.
Reverberating black holes emit gravitational waves not at a single frequency, but with additional, short-lived frequencies known as overtones — much like a bell rings with multiple tones in addition to its main pitch.
Measuring the ringing black hole’s main frequency as well as one overtone allowed the researchers to compare those waves with the prediction for a hairless black hole. The results agreed within 20 percent.
That result still leaves some wiggle room for the no-hair theorem to be proved wrong. But, “It’s a clear demonstration that the method works,” says physicist Leo Stein of the University of Mississippi in Oxford, who was not involved with the research. “And hopefully the precision will increase as LIGO improves.”
The researchers also calculated the mass and spin of the black hole, using only waves from the ringdown period. The figures agreed with the values estimated from the entire event — including the spiraling and merging of the original two black holes — and so reinforced the idea that the resulting black hole’s behavior was determined entirely by its mass and spin.
But just as a mostly bald man may sport a few strands, black holes could reveal some hair on closer inspection. If they do, that might lead to a solution to the information paradox, a puzzle about what happens to information that falls into a black hole (SN: 5/16/14). For example, in a 2016 attempt to resolve the paradox, physicist Stephen Hawking and colleagues suggested that black holes might have “soft hair” (SN: 4/3/18).
“It could still be that these objects have more mysteries to them that will only be revealed by future, more sensitive measurements,” Isi says.
Last Wednesday, a gravitational wave detection gave astronomers quite the surprise. As researchers were going about their work at the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of gravitational waves rolled in just minutes apart.
Gravitational waves. Credit: MPI for Gravitational Physics/Werner Benger
Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018
Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
VIRGO Gravitational Wave interferometer, near Pisa, Italy
The first, labelled S190828j, was picked up by all three of LIGO’s gravitational wave detectors at 06:34 am, coordinated universal time.
MIT /Caltech Advanced aLigo
VIRGO Gravitational Wave interferometer, near Pisa, Italy
The second, S190828l, was measured at 06:55 – a mere 21 minutes later.
Both seemed to be the run-of-the-mill dying screams of black holes as they squish together. But here’s why it’s so surprising: astronomers wouldn’t expect to see a pair of signals in such quick succession.
In fact, this is only the second time two detections have rolled in on the same day. What’s more, at first glance they also seemed to echo from more or less the same patch of sky.
“This is a genuine “Uh, wait, what?; We’ve never seen that before…” moment in gravitational wave astronomy,” astrophysicist Robert Routledge from McGill University later tweeted, after openly speculating that it mightn’t be a mere coincidence.
Non-scientists — this is a genuine “Uh, wait, what? We’ve never seen that before…….” moment in gravitational wave astronomy. If you’d like to see how double-checks and confirmations and conclusions occur – pay attention, in real time. Happening now.
— Robert Rutledge (@rerutled) August 28, 2019
Nobody can blame Routledge for getting excited. Unexpected events like this are what discoveries are made of, after all. As he said, this is science in real time.
One possibility briefly kicked around was that S190828j and S190828l were actually the same wave, divided by some sort of distortion in space before being roughly thrown together again. This would have been huge.
Gravitational lensing – the warping effect an intervening mass has on space, as described by general relativity – can divide and duplicate the rays of light from far-off objects. It has become a useful tool for astronomers in the measurement of distances.
Gravitational Lensing NASA/ESA
If this had indeed been a two-for-one deal, it would be the first time a gravitational wave had been observed through a gravitational lens.
Alas, it’s now looking pretty unlikely. As the hours passed, new details emerged indicating the two signals don’t overlap enough to be originating from the same source.
If this were a lensing event, you’d expect the two localizations to sit more or less right on top of each other. They have similar shapes and appear in the same part of the sky, but they don’t really overlap: pic.twitter.com/lqvigNhyBl
— Robert McNees (@mcnees) August 28, 2019
So close, and yet so far. Right now, this twin event is looking more like a coincidence.
To look on the bright side, we now live in an age where the detection of the crash-boom of galactic giants isn’t a rare event, but rather an endless peel of thunder we can record and measure with an insane level of accuracy. It’s hard to believe the first collision was detected only a few years ago.
Scientists face a problem in the wake of freaky events like this one. On the one hand, wild speculations have a habit of taking on a life of their own when discussed so frankly in a public space, transforming into an established fact while barely half baked.
But time can be of the essence when we’re scanning a near-infinite amount of sky for clues, too. By throwing ideas out broadly, different groups of researchers can turn their attention to a phenomenon and collect data while it’s still hot.
This is what scientists do best – stumble across odd events, throw out ideas, and debate which ones deserve to be inspected and which should be abandoned.
If there’s more to S190828j and S190828l than meets the eye, we’ll let you know. For now, we can be disappointed that there was no Earth-shaking discovery, while still being amazed that we have the technology to discover it at all.
We really ought to celebrate the ‘disappointments’ a little more often.
Researchers decipher the history of supermassive black holes in the early universe.
At Western University
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07.18.19
From Wired
Meredith Fore
NASA
A pair of researchers at Western University in Ontario, Canada, developed their model by looking at quasars, which are supermassive black holes.
Astronomers have a pretty good idea of how most black holes form: A massive star dies, and after it goes supernova, the remaining mass (if there’s enough of it) collapses under the force of its own gravity, leaving behind a black hole that’s between five and 50 times the mass of our Sun. What this tidy origin story fails to explain is where supermassive black holes, which range from 100,000 to tens of billions of times the mass of the Sun, come from. These monsters exist at the center of almost all galaxies in the universe, and some emerged only 690 million years after the Big Bang. In cosmic terms, that’s practically the blink of an eye—not nearly long enough for a star to be born, collapse into a black hole, and eat enough mass to become supermassive.
One long-standing explanation for this mystery, known as the direct-collapse theory, hypothesizes that ancient black holes somehow got big without the benefit of a supernova stage. Now a pair of researchers at Western University in Ontario, Canada—Shantanu Basu and Arpan Das—have found some of the first solid observational evidence for the theory. As they described late last month in The Astrophysical Journal Letters, they did it by looking at quasars.
Quasars are supermassive black holes that continuously suck in, or accrete, large amounts of matter; they get a special name because the stuff falling into them emits bright radiation, making them easier to observe than many other kinds of black holes. The distribution of their masses—how many are bigger, how many are smaller, and how many are in between—is the main indicator of how they formed.
Astrophysicists at Western University have found evidence for the direct formation of black holes that do not need to emerge from a star remnant. The production of black holes in the early universe, formed in this manner, may provide scientists with an explanation for the presence of extremely massive black holes at a very early stage in the history of our universe.
After analyzing that information, Basu and Das proposed that the supermassive black holes might have arisen from a chain reaction. They can’t say exactly where the seeds of the black holes came from in the first place, but they think they know what happened next. Each time one of the nascent black holes accreted matter, it would radiate energy, which would heat up neighboring gas clouds. A hot gas cloud collapses more easily than a cold one; with each big meal, the black hole would emit more energy, heating up other gas clouds, and so on. This fits the conclusions of several other astronomers, who believe that the population of supermassive black holes increased at an exponential rate in the universe’s infancy.
“This is indirect observational evidence that black holes originate from direct-collapses and not from stellar remnants,” says Basu, an astronomy professor at Western who is internationally recognized as an expert in the early stages of star formation and protoplanetary disk evolution.
Basu and Das developed the new mathematical model by calculating the mass function of supermassive black holes that form over a limited time period and undergo a rapid exponential growth of mass. The mass growth can be regulated by the Eddington limit that is set by a balance of radiation and gravitation forces or can even exceed it by a modest factor.
“Supermassive black holes only had a short time period where they were able to grow fast and then at some point, because of all the radiation in the universe created by other black holes and stars, their production came to a halt,” explains Basu. “That’s the direct-collapse scenario.”
But at some point, the chain reaction stopped. As more and more black holes—and stars and galaxies—were born and started radiating energy and light, the gas clouds evaporated. “The overall radiation field in the universe becomes too strong to allow such large amounts of gas to collapse directly,” Basu says. “And so the whole process comes to an end.” He and Das estimate that the chain reaction lasted about 150 million years.
The generally accepted speed limit for black hole growth is called the Eddington rate, a balance between the outward force of radiation and the inward force of gravity. This speed limit can theoretically be exceeded if the matter is collapsing fast enough; the Basu and Das model suggests black holes were accreting matter at three times the Eddington rate for as long as the chain reaction was happening. For astronomers regularly dealing with numbers in the millions, billions, and trillions, three is quite modest.
“If the numbers had turned out crazy, like you need 100 times the Eddington accretion rate, or the production period is 2 billion years, or 10 years,” Basu says, “then we’d probably have to conclude that the model is wrong.”
There are many other theories for how direct-collapse black holes could be created: Perhaps halos of dark matter formed ultramassive quasi-stars that then collapsed, or dense clusters of regular mass stars merged and then collapsed.
For Basu and Das, one strength of their model is that it doesn’t depend on how the giant seeds were created. “It’s not dependent on some person’s very specific scenario, specific chain of events happening in a certain way,” Basu says. “All this requires is that some very massive black holes did form in the early universe, and they formed in a chain reaction process, and it only lasted a brief time.”
The ability to see a supermassive black hole forming is still out of reach; existing telescopes can’t look that far back yet. But that may change in the next decade as powerful new tools come online, including the James Webb Space Telescope, the Wide Field Infrared Survey Telescope, and the Laser Interferometer Space Antenna—all of which will hover in low Earth orbit—as well as the Large Synoptic Survey Telescope, based in Chile.
NASA/ESA/CSA Webb Telescope annotated
NASA/WFIRST
Gravity is talking. Lisa will listen. Dialogos of Eide
ESA/LISA Pathfinder
ESA/NASA eLISA space based, the future of gravitational wave research
LSST Camera, built at SLAC
LSST telescope, currently under construction at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.
In the next five or 10 years, Basu adds, as the “mountain of data” comes in, models like his and his colleague’s will help astronomers interpret what they see.
Avi Loeb, one of the pioneers of direct-collapse black hole theory and the director of the Black Hole Initiative at Harvard, is especially excited for the Laser Interferometer Space Antenna. Set to launch in the 2030s, it will allow scientists to measure gravitational waves—fine ripples in the fabric of space-time—more accurately than ever before.
“We have already started the era of gravitational wave astronomy with stellar-mass black holes,” he says, referring to the black hole mergers detected by the ground-based Laser Interferometer Gravitational-Wave Observatory.
Its space-based counterpart, Loeb anticipates, could provide a better “census” of the supermassive black hole population.
For Basu, the question of how supermassive black holes are created is “one of the big chinks in the armor” of our current understanding of the universe. The new model “is a way of making everything work according to current observations,” he says. But Das remains open to any surprises delivered by the spate of new detectors—since surprises, after all, are often how science progresses.
MIT /Caltech Advanced aLigo
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Caltech/MIT Advanced aLigo Hanford, WA, USA installation
Caltech/MIT Advanced aLigo detector installation Livingston, LA, USA
LSC LIGO Scientific Collaboration
Cornell SXS, the Simulating eXtreme Spacetimes (SXS) project
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger
Gravity is talking. Lisa will listen. Dialogos of Eide
ESA/eLISA the future of gravitational wave research
Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018
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)
See the full WIRED article here .
See the full Western University article here .
The University of Western Ontario (UWO), corporately branded as Western University as of 2012 and commonly shortened to Western, is a public research university in London, Ontario, Canada. The main campus is on 455 hectares (1,120 acres) of land, surrounded by residential neighbourhoods and the Thames River bisecting the campus’s eastern portion. The university operates twelve academic faculties and schools. It is a member of the U15, a group of research-intensive universities in Canada.
The university was founded on 7 March 1878 by Bishop Isaac Hellmuth of the Anglican Diocese of Huron as the Western University of London, Ontario. It incorporated Huron University College, which had been founded in 1863. The first four faculties were Arts, Divinity, Law and Medicine. The Western University of London became non-denominational in 1908. Beginning in 1919, the university has affiliated with several denominational colleges. The university grew substantially in the post-World War II era, as a number of faculties and schools were added to university.
Western is a co-educational university, with more than 24,000 students, and with over 306,000 living alumni worldwide. Notable alumni include government officials, academics, business leaders, Nobel Laureates, Rhodes Scholars, and distinguished fellows. Western’s varsity teams, known as the Western Mustangs, compete in the Ontario University Athletics conference of U Sports.
richardmitnick
12:10 pm on July 18, 2019 Permalink
| Reply Tags: "Northwestern leads effort to detect new types of cosmic events", A new kind of gravitational-wave detector that is small enough to fit on a tabletop, Gravitation Wave Astonomy ( 2 ), Multimessenger astrophysics, Northwestern University ( 12 ), Physicists and astronomers begin development of miniature gravitational-wave detector.
Physicists and astronomers begin development of miniature gravitational-wave detector.
A team of physicists and astronomers from Northwestern University is poised to lead gravitational-wave astronomy into its next evolution. The W. M. Keck Foundation has awarded $1 million, which will be used to develop a prototype for a new kind of gravitational-wave detector that is small enough to fit on a tabletop and powerful enough to detect cosmic events that existing astronomical equipment cannot.
“This is the start of the next phase of gravitational-wave and multi-messenger astronomy,” said Andrew Geraci, principal investigator on the project. “This tabletop sensor will be able to observe events we’ve never seen before, expanding our understanding of space and the universe.”
Geraci is an associate professor of physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a faculty member of the Center for Fundamental Physics (CFP) and the Center for Interdisciplinary Exploration and Research for Astrophysics (CIERA) at Northwestern.
The Levitated Sensor Detector will extend the spectrum of detectable gravitational waves to higher frequencies, possibly opening a window to the types of events that are related to the mysterious dark matter in the universe. It will complement research being conducted at the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo observatories, which occupy several acres of land in the United States and Italy, by observing smaller cosmic events that produce waves at a frequency LIGO and Virgo cannot detect.
“If you think of gravitational waves like sound waves, the frequency we are trying to capture with levitated sensors is sort of like a dog whistle,” said Vicky Kalogera, project co-investigator and CIERA director.
“Dogs are capable of hearing sound frequencies that the human ear cannot perceive. Similarly, levitated sensors will pick up frequencies that LIGO and Virgo cannot detect,” said Kalogera, who also is the Daniel I. Linzer Distinguished University Professor of Physics and Astronomy and one of the foremost astrophysicists in the LIGO Scientific Collaboration (LSC).
Northwestern scientists will spend two years building and testing the levitated sensor prototype, which will consist of two arms in an L-formation, each measuring about one meter in length. The levitated sensor will then operate for a one-year observing run, the results of which will be shared with the international gravitational-wave and astronomy communities.
Levitated sensors will be capable of detecting cosmic events that produce gravitational waves with a frequency of greater than 10 kHz. These events are thought to be the result of small cosmic events involving black holes similar in mass to Earth’s sun, and primordial black holes produced in the earliest days of the universe. Astrophysicists theorize but have so far been unable to prove that these events are responsible for the creation of dark matter. Other dark matter candidates such as axions, hypothetical particles that are predicted to be a component of cold dark matter, also can be searched for using the device.
Development of this tabletop detector coincides with the development of a third type of detector on the opposite end of the frequency spectrum. The European Space Agency plans to launch LISA, a space-based gravitational-wave detector that spans tens of millions of miles, in the early 2030s.
ESA/LISA Pathfinder
ESA/NASA eLISA
ESA/NASA eLISA space based, the future of gravitational wave research
“Just like electromagnetic astronomy has telescopes and gamma-ray detectors and more, the gravitational-wave community is now developing the tools needed to detect events on all parts of the spectrum,” project co-investigator Shane Larson said. “LISA will detect the big events; LIGO and Virgo pick up the medium events; and LSD will detect the smallest cosmic events.”
Larson is associate director of CIERA and a research associate professor of physics and astronomy at Northwestern. He also is a member of the LISA Consortium and the LSC.
The project, titled “A novel tabletop gravitational-wave detector for frequencies > 10 kHz,” is supported by the W. M. Keck Foundation.
The grant counts toward We Will. The Campaign for Northwestern. The funds raised through the “We Will” Campaign are helping realize the transformational vision set forth in Northwestern’s strategic plan and solidifying the University’s position among the world’s leading research universities. More information on the “We Will” Campaign is available at http://wewill.northwestern.edu.
Based in Los Angeles, the W. M. Keck Foundation was established in 1954 by the late W. M. Keck, founder of the Superior Oil Company. The Foundation’s grant making is focused primarily on pioneering efforts in the areas of medical research and science and engineering. The Foundation also maintains a Southern California Grant Program that provides support for the Los Angeles community, with a special emphasis on children and youth. For more information, please visit http://www.wmkeck.org.
On May 31, 1850, nine men gathered to begin planning a university that would serve the Northwest Territory.
Given that they had little money, no land and limited higher education experience, their vision was ambitious. But through a combination of creative financing, shrewd politicking, religious inspiration and an abundance of hard work, the founders of Northwestern University were able to make that dream a reality.
In 1853, the founders purchased a 379-acre tract of land on the shore of Lake Michigan 12 miles north of Chicago. They established a campus and developed the land near it, naming the surrounding town Evanston in honor of one of the University’s founders, John Evans. After completing its first building in 1855, Northwestern began classes that fall with two faculty members and 10 students.
Twenty-one presidents have presided over Northwestern in the years since. The University has grown to include 12 schools and colleges, with additional campuses in Chicago and Doha, Qatar.
Northwestern is recognized nationally and internationally for its educational programs.
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)
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.
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.
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.
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.
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.
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.
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.
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.
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.
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%!
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)
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.
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.
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
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.”
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)
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
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.
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!
“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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