Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of gravity

  1. Aug 9, 2009 #1
    General relativity describes gravity as simply the curvature of space-time. I believe that I read that the effects of this curvature are instantaneous. Is this curvature considered information? Would this mean that the information travels faster than the speed of light?
     
  2. jcsd
  3. Aug 9, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    The local effects of the curvature are instant - just because the curvature is already there. Changes in gravity propogate at the speed of light.

    Yes a change in gravity is definetly information, if the sun dissapeared we wouldn't feel a change in gravity until the loss of light was notices
     
  4. Aug 10, 2009 #3
    Aren't the 'speed of gravity' and 'changes in gravity' different things?

    The speed of gravity must vary according to how far away you are from the mass that is causing it.
     
  5. Aug 12, 2009 #4
    seems like both terms are referring to propagation of gravitational waves, which are considered instantaneous physical responses.
     
  6. Aug 12, 2009 #5

    JesseM

    User Avatar
    Science Advisor

    Gravitational waves move at the speed of light, what do you mean by "instantaneous physical response"?
     
  7. Aug 12, 2009 #6
    The curvature of s/t is essentially an instant response to the presentence of matter.
    looking at the geometry visualized by s/time in SR, we see that the grav field that exists is a local instantaneous response to the presence of matter. since matter travels along geodesic lines, locality is important. the varied reasons associated with the propagation of g waves, show well that they move at c. I have seen reasonable evidence that the gravity/matter response takes place faster than gravitational propogation.
     
  8. Aug 12, 2009 #7

    JesseM

    User Avatar
    Science Advisor

    Well, locally the electromagnetic field also responds instantaneously to the behavior of charged matter--how could it be otherwise, when "locally" means the distances you're looking at are infinitesimal?
    Pretty sure the curvature at any given point can be determined solely from knowledge of the curvature and matter distribution in the past light cone of that point, and likewise that it would be impossible to send a message using gravity that would arrive faster than a message using electromagnetism. If you don't disagree with either of these statements, you need to give a more precise definition of what you mean when you talk about the speed of the "gravity/matter response".
     
  9. Aug 12, 2009 #8

    atyy

    User Avatar
    Science Advisor

    Not sure about this - Minkowski and Schwarzschild solutions are both vacuum solutions (same local matter content), but their spacetime curvature is different.
     
  10. Aug 12, 2009 #9

    JesseM

    User Avatar
    Science Advisor

    But that's why I said "can be determined solely from knowledge of the curvature and matter distribution in the past light cone of that point" rather than "can be determined solely from knowledge of the matter distribution in the past light cone of that point". I don't know for a fact that this is true, but if it wasn't it seems that by measuring the local curvature at a point you could get information about events outside the past light cone of that point, which would lead to causality violations.
     
  11. Aug 12, 2009 #10

    DaveC426913

    User Avatar
    Gold Member

    A field, by definition, means there is an existing value for it at every single point in the space. This is true for a gravitational field just like a magnetic field. It doesn't take time to be affected by an existing magnetic field - or gravitational field.

    However, changes to the value of that field propogate at c. That includes any action that causes the field to climb from zero or fall to zero (such as switching on or off a magnetic device or creating/destroying mass).
     
  12. Aug 12, 2009 #11

    atyy

    User Avatar
    Science Advisor

    Maybe something like this? Assume you and your measuring apparatus (laser, clock, ...) are not so massive as to significantly affect spacetime curvature. You can always measure your local curvature. Suppose you measure spacetime around you to be curved, and you are in vacuum - that certainly means you are not in Minkowski spacetime - can you use that information to deduce the distribution of matter far away? I think not, because the local curvature could be consistent with many distributions of matter (or black holes) far away in spacetime.

    Roughly, to infer the matter distribution in the entire "past" and "future", you need to measure the matter and spatial curvature of one spacelike slice of spacetime. So you still have to measure properties of faraway points of this spacelike slice, and you so will be restricted by the speed of light.

    Edit: this is not satisfactory, since if there is curvature where you are, then you do know immediately that you are not in globally Minkowski spacetime, which means you have eliminated the possibility of one entire past and future at arbitrary distances ... :confused:
     
    Last edited: Aug 12, 2009
  13. Aug 12, 2009 #12
    yep, all i was sayin'.
     
  14. Aug 13, 2009 #13

    This has been discussed (debated) much on the internet and even here I'm sure.

    If you mean how fast does the force of gravity act between massive bodies (due to curvature or propogation) then a case could be made that we would have to conclude from the stability of earth's orbit around the sun that it is extremely fast (if not almost instantaneous), otherwise a propogation delay would cause a gravitational "aberration" as viewed from earth which would place the solar grav. force in a non-central position which would change the earth's orbital angular momentum.... which is not observed.

    Not talking about gravitational radiation, (i.e., waves) here.....but simply the force of normal gravity interaction between massive bodies.

    :biggrin:
     
    Last edited: Aug 13, 2009
  15. Aug 13, 2009 #14

    JesseM

    User Avatar
    Science Advisor

    But there is no need to conclude any actual signal is moving that fast, this lack of aberration can be explained in terms of the local dynamics of the gravitational field...it's analogous to the way the electromagnetic force we feel from a moving charged body is always directed at its current position rather than its retarded position as long as it's moving at constant speed, no matter how far away it is (and if the body is accelerated, then the force will continue to be directed at the position it would have been if it had continued to move at constant velocity, until an electromagnetic wave created by the acceleration has had time to reach us and give us an 'update'). This is discussed in this entry from the Usenet Physics FAQ:
     
  16. Aug 16, 2009 #15
    What is this talk about 'force'? I thought the GR standpoint was that gravity was not a force.
     
  17. Aug 16, 2009 #16

    JesseM

    User Avatar
    Science Advisor

    In that section I was talking about electromagnetism, not gravity. But the passage I quoted from this site noted that in the weak-field approximation, gravity can be treated as a sort of force similar to electromagnetism:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Speed of gravity
  1. Speed of Gravity. (Replies: 2)

  2. The Speed of Gravity (Replies: 15)

  3. Speed of gravity (Replies: 1)

  4. Speed of gravity? (Replies: 28)

Loading...