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Speed of Gravity

  1. Jan 4, 2004 #1
    Does gravity propagate at the speed of light?
  2. jcsd
  3. Jan 4, 2004 #2
    You know, with all the fancy gadgets we are building to attempt to find gravity waves from outerspace, you wonder why we haven't devised an experiment to measure the speed of gravitational changes on the smaller scale.
    I know it is difficult to evacuate a large enough volume to have a significant massive object contained inside, but it could be done. I would guess with confidence, that gravitational effects travel at light speed.
    Wouldn't it be interesting though, if we found out it traveled at 137.036 times light speed or something? We would have a good reason for the fine structure coupling constant. But no research efforts are on the front to determine this "speed" as far as I've read on the web. I hope someone will look into that soon though, it could be worthwile.

  4. Jan 4, 2004 #3
    I have an idea of a cheap way it could be done.
    -In a vacuum tube, have two 1kg weights (for balance) on a small motor, and spin them really fast at a constant rotational speed.
    -Then at given distances from the spinning weights, have very small masses suspended also in the tube.
    -monitor the system over a long period of time (so the oscillations have time to stabilize). And use light of a fixed wavelength to measure the positions of the smaller masses at specific times.

    The posistions of the small masses relative to the light should tell us something more than just the spring constant of the device used to fix them .
  5. Jan 4, 2004 #4
    Clever idea but ...Alas.
    The experiment would have to be conducted far away from massive bodies , like the Earth. The difference of G attraction on the space shuttle on the ground and in orbit is almost negligible. The effect of the Earth on the experiment would swamp any difference ( altho it does exist)

    Secondly what you would measure is more demonstrative of the inverse square law then vG.

    To do this we would have to devise a real world experiment that parallels Einstein's thought experiment of the disappearance of the Sun and it's effect on Earth. I.E have a body that exerts a gravitational attraction on another body and then have that effect disappear instantainiously. We can do that with an electromagnet and a piece of iron , where we stop the current flow to the EM, but not Grav.
  6. Jan 4, 2004 #5


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    But the very reason for that is because it will take the most sensitive equipment we have to detect the most powerfull gravity waves (like those theoretically produced by neutron stars in close orbit around one another).
  7. Jan 5, 2004 #6
    I doubt that any experimental result would differ from the theoretical predictions, but you never know. After all, there are gravitational effects such as those of dark matter which we still don't understand, so perhaps the theory is incomplete.
  8. Jan 5, 2004 #7
    Quantum theory does have non-locality, but it remains true that no signal can travel faster than light. So it is improbable that gravity waves travel faster than light. If you think about it, it is also absurd that they travel at a speed less than light: Imagine, the sun suddenly disappearing, and the Earth still travelling in its orbit even as we see the sun winking out!
  9. Jan 6, 2004 #8

    jimmy p

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    There was a report in a New Scientist magazine a while back (i cant remember which one) where apparently some scientist had used some data and equations, as you do, and worked out the speed of gravity. It is slightly slower than the speed of light, sorry to be unglamourous, but i dont know the exact figure, maybe someone with more patience and/or knowledge would care to fill in my blanks.

    In fact, what Tron said above, is true though, if the sun burned out, then after 8 minutes it would be dark and cold, and a little time after that, the earth would spin off into space...worrying thought that!
    Last edited: Jan 6, 2004
  10. Jan 7, 2004 #9
  11. Jan 7, 2004 #10
    - The gravitational effect of an oscillating mass at a given frequency, gravity should transfer this same frequency (or another harmonic one) to the smaller masses. This slight movement at a specific frequency is what would have to be anaylized, and then the specific phase difference would reveal the speed that gravity transfers the energy. This way, things like people walking by, or sprinkler's making the ground outside wet, etc. shouldn't effect the results.

    - This still wouldn't be easy to measure, though, as most of you point out. Because say we hold the smaller masses in the vacuum via magnetic field for less friction, then something as slight as radio waves - or noise from any electric motors, would distort the effect. The advantage, though, would be that a small amount of force would be cummulative over large spans of time at a discrete frequency.

    - Since these gravity wave detectors have to know the precise position of the moon, I wonder if somehow this can be tracked to a precise time, and then compared with it's observed position (via light speed). Does anyone know someone involved in those studies to see if that would even be feesible?
  12. Jan 9, 2004 #11


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    This is one reference to the experiment that I think you are referring to (there are many others):

    Note that the interpretation of the results received at least one significant challenge:


    In your suggested experiment, does the periodic signal become easier to detect because of its known frequency (so the faint signal may be found even if it's several OOM below the noise level), or that the magnitude of the effect is cumulative? If the latter, why would this amplification happen?

    As mentioned here and in the reference jimmy p provided, the best chance of making an observational test would be through timing differences in neutrino, EM, and gravitational wave signals from a supernova or colliding neutron star/BH event.
  13. Jan 9, 2004 #12
    Cool links - Kind of funny the way the news goes in waves!
    I tend to agree that celestial data may provide more convincing evidence, at least if gravity if very close to light speed (obviously if it were substantially different, the nova events and gravity waves would be tricky to link.
    What I had in mind was that both effects are important. The discernable frequency of movement would allow us to neglect most noise. And if the apparatus it set-up right, the pull of the moving source mass would accumulate on the smaller masses. However, looking at some of the possible sources of overwhealming distortion in the period range, I don't think it would be easy. Confining the "sensor" masses into a plane parallel to the plane of the source mass would also be extremely important if comparison of light wavelengths is going to be used to detect the phase difference. The more you think about it, the more you realize why it hasn't been done yet.
  14. Jan 13, 2004 #13
    light takes 8.3 minutes to get from the sun to us. the planet does not get pulled to where the sun looks like it is. it gets pulled "ahead" of where it looks like. it gets pulled to where the sun looks like it will be, in 8.3 minutes time, where the sun ACUALLY is. there for, gravity is instant. but, its not. gravity doesnt acually move, it doesnt get from the sun to us where we are instantly, it just didnt HAVE to move. think about a solid pole. if you push one end, and the other end (1 AU away) would move at the same time.
  15. Jan 13, 2004 #14
    Gara, if a solid metal pole 1 AU long were pushed at one end, and the other end moved at the same time, the transfer would be faster than the speed of light. Not possible. In fact, mechanical force propagation(pushing a rod) is actually far slower than the speed of light anyway, so it would take a LONG time before the end of the rod moved(if one could even input the incredible amount of force necessary to do it in the first place)
  16. Jan 13, 2004 #15
    okay maybe the pole way of explaining it isnt right, but the fact the earth is attracted to where the sun is, and not where the sun looks like it is, remains.
  17. Jan 14, 2004 #16
    I would submit that you can have it both ways. That gravitational waves propagate at C, and the location of an object is known istantly. So what we would have is a gravitational wave that propagates away from the sun and reaches us over 8 minutes later, but the wave follows the position of the sun.
    I am confident what I said will be misunderstood, but there it is. Take it or leave it.[b(]
  18. Jan 23, 2010 #17
    if the sun where to be dropped into our solar system, then ripples in time space, gravity, would reach us in 8 minutes, it seem the only plausible explanation, to say that the effects would be instantaneous then this would say that gravity has an infinitesimally small momentum, this is impossible as we know infinity and any other variation of infinity cannot be true. Therefore gravity traveling at the speed of light seems like the only possible solution.
  19. Feb 5, 2010 #18
    Wait, you are worried about effects being instantaneous after you instantaneously dropped the whole sun? I don't like logical reasoning from impossible scenarios. :)

    Can anyone confirm Gara, point to some references about gravity feeling instant. Do planets and galaxies spin as they should with gravity delay? Anyone can prove this wrong so I can go on with my life?

    EDIT: Just read Tronks link. Bollocks.
    Last edited: Feb 5, 2010
  20. Feb 10, 2010 #19
    Question concerning the speed of gravity: does gravity propagate at the speed of light?

    Empty space has certain physical properties, including springiness and inertia, and it momentarily compresses or stretches when an oscillating gravitational wave passes through it. (These properties of space are related to Newton’s universal gravitational constant, and to a gravitational constant that is the gravitational analogue of the magnetic force constant in electromagnetism, and they can be written in terms of these constants). The speed of gravitational waves through empty space is: s = square root of the ratio of the springiness of space (in its spatial dimensions) to its inertia (See, for example, chapter 3, pg 32 of the teacher’s edition of the fascinating little book entitled: Similarities in Wave Behavior, by Dr. John N Shive, director of education and training Bell Telephone Laboratories, Waverly Press, Inc., third printing 1964. So the speed of gravitational waves has to be finite. Although we have not yet detected or measured the speed of gravitational waves (we have one experiment, but it is in dispute), some believe its speed to be the speed of light on theoretical grounds, as noted elsewhere in these replies. (If it is the speed of light, that has some important physical ramifications for the relationship between fields). But the important point here is that its speed has to be finite.

    But, as has also been noted in these replies, it has been documented in the literature that the force of gravity, say between the Earth and the Sun, is along the direction of a line connecting the current position of the Sun with the current position of the Earth, and that if the earth were attracted to the retarded position of the Sun (to the point occupied by the Sun 8 minutes earlier or whatever) instead of the current position, the solar orbits of the Earth and planets would be unstable. But we know from experience that they are not unstable, all of which seems to imply that the gravitational influence of the Sun reaches the Earth instantaneously. So we have a paradox (or at least an apparent paradox).

    No reason to despair, though, for the laws of physics are incomplete, and, as such, there exist physical anomalies that cannot be explained with the current laws of physics. Unfortunately, such things are often swept under the rug because they cannot be explained, and are even suppressed, but paradoxes and anomalies lead to important advances in the laws of physics--they are critically important clues.

    UltraPi1 is correct, it is possible to have gravitational influences traveling at finite speed while at the same time having planets respond to the instantaneous position of the Sun. It works like this: the Earth is attracted to both the retarded position of the Sun and to its advanced position (that is, to where it was eight minutes ago, and to where it will be eight minutes from now)--the gravitational influence of the Sun travels forwards in time to the Earth from the Sun’s retarded position at the speed of light, it also travels backwards in time to the Earth from the Sun’s advanced position at the speed of light. And the vector sum of the two pulls is towards the instantaneous position of the Sun, with a magnitude equal to one half of the sum of the two pulls (at least when the sun is moving at constant speed). This gives the uncanny appearance of action at a distance when in fact it is not. It was Wheeler and Feynman who first proposed this theory, except I think they had electromagnetic interactions in mind rather than gravitational ones, and it was referred to as “The Wheeler-Feynman absorber theory”--a theory of interaction between particles that radiate a traveling wave and particles that absorb that wave no matter how far apart they may be. A brief general discussion of it can be found at http://en.wikipedia.org/wiki/Wheeler-Feynman_absorber_theory (This paradox, by the way, is similar to the Einstein-Podolsky-Rosen paradox in quantum mechanics, which also gives the appearance of action at a distance, but which can be resolved in the same way--quantum mechanics involves backwards in time causality, which accounts for some of its strangeness).

    The laws of physics are symmetric with respect to things traveling forward in time and those traveling backwards in time. The Maxwell’s equations and the wave equation for electromagnetic waves and Schrödinger’s equation for matter waves, for example, all have two possible solutions: a retarded solution and an advanced one--the total solution being the sum or superposition of the two. But as Feynman emphatically points out in his Lectures on Physics, electromagnetic forces between electric charges in our world involve the retarded position only, and never the advanced position. Therefore the advanced solution is normally discarded since it is assumed that it does not physically exist in our world. And the Wheeler-Feynman absorber theory, when applied to electromagnetic forces, has to be adjusted to make the advanced solution cancel out. This time asymmetry for electromagnetic phenomena begs for an explanation--but that is still a work in progress. In any case, gravitational forces between gravitational charges (masses) appear to involve both advanced and retarded solutions in the real world. Which is interesting.

    In conclusion, gravitational influences, including gravitational waves, propagate at a finite speed (probably at the speed of light), but gravitational influences, unlike electromagnetic influences, are accounted for by both retarded and advanced waves, giving the uncanny appearance of action at a distance and instantaneous propagation for gravitational disturbances.
  21. Feb 11, 2010 #20
    Thanks for the reply, It actually did enlighten me somewhat. But kind of hard to swallow.

    If sun does not move linearly and constantly what then?

    Do we receive information (instantly via gravity wave) from sun about it's whereabouts in 8 minutes?

    I believe that any theory can be made to work consistently with proven ones with enough math and imagination. We just have to pick simple and beautiful ones and try to stick with them and keep them consistent with each other. I am not physicist in any way, but please tell me that piling theories on top of each other and filling holes with them is not the only way research works? I mean, if we gave Feynman and friends dogma that maximum speed in universe is 100km/h I'm sure they would find a way to make it work. And maybe it is even true, but we have simpler and prettier theories that point that it is probably not. I don't have hard time believing that advanced wave theory works, it is that I just don't find it pretty description of manifestation of world around us, that's all. And I am sure that we will have better one. But i guess that is another topic.
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