Tagged: Speed of Gravity Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 11:07 am on November 25, 2016 Permalink | Reply
    Tags: , , , Speed of Gravity,   

    From New Scientist: “Gravity may have chased light in the early universe” 


    New Scientist

    23 November 2016
    Michael Brooks

    Getting up to speed. Manuela Schewe-Behnisch/EyeEm/Getty

    It’s supposed to be the most fundamental constant in physics, but the speed of light may not always have been the same. This twist on a controversial idea could overturn our standard cosmological wisdom.

    In 1998, Joao Magueijo at Imperial College London, proposed that the speed of light might vary, to solve what cosmologists call the horizon problem. This says that the universe reached a uniform temperature long before heat-carrying photons, which travel at the speed of light, had time to reach all corners of the universe.

    The standard way to explain this conundrum is an idea called inflation, which suggests that the universe went through a short period of rapid expansion early on – so the temperature evented out when the cosmos was smaller, then it suddenly grew. But we don’t know why inflation started, or stopped. So Magueijo has been looking for alternatives.

    Now, in a paper to be published 28 November in Physical Review, he and Niayesh Afshordi at the Perimeter Institute in Canada have laid out a new version of the idea – and this one is testable. They suggest that in the early universe, light and gravity propagated at different speeds.

    If photons moved faster than gravity just after the big bang, that would have let them get far enough for the universe to reach an equilibrium temperature much more quickly, the team say.

    A testable theory

    What really excites Magueijo about the idea is that it makes a specific prediction about the cosmic microwave background (CMB). This radiation, which fills the universe, was created shortly after the big bang and contains a “fossilised” imprint of the conditions of the universe.

    CMB per ESA/Planck
    CMB per ESA/Planck

    In Magueijo and Afshordi’s model, certain details about the CMB reflect the way the speed of light and the speed of gravity vary as the temperature of the universe changes. They found that there was an abrupt change at a certain point, when the ratio of the speeds of light and gravity rapidly went to infinity.

    This fixes a value called the spectral index, which describes the initial density ripples in the universe, at 0.96478 – a value that can be checked against future measurements. The latest figure, reported by the CMB-mapping Planck satellite in 2015, place the spectral index at about 0.968, which is tantalisingly close.


    If more data reveals a mismatch, the theory can be discarded. “That would be great – I won’t have to think about these theories again,” Magueijo says. “This whole class of theories in which the speed of light varies with respect to the speed of gravity will be ruled out.”

    But no measurement will rule out inflation entirely, because it doesn’t make specific predictions. “There is a huge space of possible inflationary theories, which makes testing the basic idea very difficult,” says Peter Coles at Cardiff University, UK. “It’s like nailing jelly to the wall.”

    That makes it all the more important to explore alternatives like varying light speeds, he adds.

    John Webb of the University of New South Wales in Sydney, Australia, has worked for many years on the idea that constants may vary, and is “very impressed” by Magueijo and Afshordi’s prediction. “A testable theory is a good theory,” he says.

    The implications could be profound. Physicists have long known there is a mismatch in the way the universe operates on its smallest scales and at its highest energies, and have sought a theory of quantum gravity to unite them. If there is a good fit between Magueijo’s theory and observations, it could bridge this gap, adding to our understanding of the universe’s first moments.

    “We have a model of the universe that embraces the idea there must be new physics at some point,” Magueijo says. “It’s complicated, obviously, but I think ultimately there will be a way of informing quantum gravity from this kind of cosmology.”

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

  • richardmitnick 3:40 pm on April 29, 2016 Permalink | Reply
    Tags: , , Speed of Gravity,   

    From Ethan Siegel: “Why Does Gravity Move At The Speed Of Light?” 

    Starts with a Bang

    Ethan Siegel

    Image credit: European Gravitational Observatory, Lionel BRET/EUROLIOS.

    If you looked out at the Sun across the 93 million miles of space that separate our world from our nearest star, the light you’re seeing isn’t from the Sun as it is right now, but rather as it was some 8 minutes and 20 seconds ago. This is because as fast as light is — moving at the speed of light — it isn’t instantaneous: at 299,792.458 kilometers per second (186,282 miles per second), it requires that length of time to travel from the Sun’s photosphere to our planet. 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.

    Image credit: NASA/JPL-Caltech, for the Cassini mission.

    But is that right? If the Sun were to simply wink out of existence, would the Earth immediately fly off in a straight line, or would it continue orbiting the Sun’s location for another 8 minutes and 20 seconds? 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 that transformation isn’t instantaneous. Because spacetime is a fabric, that transition would have to occur in some sort of “snapping” motion, which would send very large ripples — i.e., gravitational waves — through the Universe, propagating outward like ripples in a pond.

    Image credit: Sergiu Bacioiu from Romania, under c.c.-2.0 generic.

    The speed of those ripples is determined the same way the speed of anything is determined in relativity: by their energy and their mass. Since gravitational waves are massless yet have a finite energy, they must move at the speed of light! Which means, if you think about it, that the Earth isn’t directly attracted to the Sun’s location in space, but rather to where the Sun was located a little over 8 minutes ago.

    Image credit: David Champion, Max Planck Institute for Radio Astronomy.

    If that were the only difference between Einstein’s theory of gravity and Newton’s, we would have been able to instantly conclude that Einstein’s theory was wrong. The orbits of the planets were so well studied and so precisely recorded for so long (since the late 1500s!) that 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. 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!

    But in General Relativity, there’s another piece to the puzzle that matters a great deal: the orbiting planet’s velocity as it moves around the Sun. The Earth, for example, since it’s also moving, kind of “rides” over the ripples traveling through space, coming down in a different spot from where it was lifted up. It looks like we have two effects going on: each object’s velocity affects how it experiences gravity, and so do the changes that occur in gravitational fields.

    Image credit: LIGO/T. Pyle, of a model of distorted space in the Solar System.

    What’s amazing is that the changes in the gravitational field felt by a finite speed of gravity and the effects of velocity-dependent interactions cancel almost exactly! The inexactness of the cancellation is what allows us to determine, observationally, if Newton’s “infinite speed of gravity” model or Einstein’s “speed of gravity = speed of light” model matches with our Universe. In theory, we know that the speed of gravity should be the same as the speed of light. But the Sun’s force of gravity out here, by us, is far too weak to measure this effect. In fact, it gets really hard to measure, because if something moves at a constant velocity in a constant gravitational field, there’s no observable affect at all. What we’d want, ideally, is a system that has a massive object moving with a changing velocity through a changing gravitational field. In other words, we want a system that consists of a close pair of orbiting, observable stellar remnants, at least one of which is a neutron star.

    Access mp4 video here .

    As one or both of these neutron stars orbit, they pulse, and the pulses are visible to us here on Earth each time the pole of a neutron star passes through our line-of-sight. 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%!

    Image credit: NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer, via http://www.mpg.de/7644757/W002_Physics-Astronomy_048-055.pdf.

    That’s an indirect measurement, of course. We were able to do another 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 measure the speed of gravity, ruling out an infinite speed and determining that the speed of gravity was between 2.55 × 10^8 and 3.81 × 10^8 meters-per-second, completely consistent with Einstein’s predictions.

    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. Image credit: Fomalont et al. (2000), ApJS 131, 95-183, via http://www.jive.nl/svlbi/vlbapls/J0842+1835.htm.

    Ideally, we’d be able to measure the speed of these ripples directly, from the direct detection of a gravitational wave. LIGO just saw the first one, after all! Unfortunately, due to our inability to correctly triangulate the location from which these waves originated, we don’t know from which direction the waves were coming. By calculating the distance between the two independent detectors (in Washington and Louisiana) and measuring the difference in the signal arrival time, we can determine that the speed of gravity is consistent with the speed of light, but can only place an absolute constraint that it’s equal to the speed of light within 70%.

    The gravitational wave arrival at the two detectors in WA and LA, with an uncertain origin to their direction. Image credit: Diego Blas, Mikhail M. Ivanov, Ignacy Sawicki, Sergey Sibiryakov, via https://arxiv.org/abs/1602.04188.

    Still, it’s the indirect measurements from very rare pulsar systems that give us the tightest constraints. The best results, at the present time, tell us that the speed of gravity is between 2.993 × 10^8 and 3.003 × 10^8 meters per second, which is an amazing confirmation of General Relativity and a terrible difficulty for alternative theories of gravity that don’t reduce to General Relativity! (Sorry, Newton!) And now you know not only what the speed of gravity is, but where to look to figure it out!

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

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

Compose new post
Next post/Next comment
Previous post/Previous comment
Show/Hide comments
Go to top
Go to login
Show/Hide help
shift + esc
%d bloggers like this: