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Einstein was right about the speed of gravity

  1. Apr 17, 2003 #1
    [SOLVED] Einstein was right about the speed of gravity

    Here's the link.
    I just read an article on MSNBC. I always thought gravity travelled at the speed of light anyways. Makes sense in soooo many different respects.
    Anyway, thought I'd post that bit of info.
  2. jcsd
  3. Apr 18, 2003 #2
    There is still intense debate as to whether the expiriment really measured the speed of gravity.
  4. Apr 18, 2003 #3
    How would light speed gravity restrict possible universe topologies?
  5. Apr 18, 2003 #4
    Well, it would stop changes in the topology of the universe from changing instantly, and I would assume stop gravity from being the same strength everywhere originating from a body (after all, if it were to travel instantly, why should it get weaker with distance?). Not only that, it keeps information in accordance with SR.
  6. Apr 18, 2003 #5
    This is, in fact, under great scrutiny, while the author maintains the errors do not lie in his mathematics, but in ones who challenge.

    First of all, Clifford Will published a paper in the Astrophysical Journal in which he explained that a "deflection caused by the finite speed of gravity should indeed occur, but that it is some 10,000 times smaller than the shift the radio team predicted and observed."

    It is true that the effect exists, but the VLBA's 10-microarcsecond resolution just isn't enough resolution to see it.

    A much better test is coming up though, when the LIGO is done. All the experiment with the VLBA did was indirectly measure the speed of light.

    The actual effect that matter has on gravitational waves is very minimal, it could travel great distances with almost no dispersion or scattering. For example, if a GW propagated through the universe once, its dispersion would only be one [lamb]. Although, GW's can show redshifting and can be lensed, much like light. The only restriction, that I know of, on the topology of the universe and GW's is that the GW's [lamb] must be less than the radius of the curvature of space-time.
    Last edited: Apr 18, 2003
  7. Apr 18, 2003 #6
    Could this tie in with Wheyl Tensor?
  8. Apr 18, 2003 #7
    I'm not sure what the Weyl Tensor is, but I do know that the curvature is described as a rank 4-Riemann tensor, R sub(abcd). I don't know if this actually helps or not though.

    NOTE: ABCD being alpha, beta, gamma, and delta.
  9. Apr 19, 2003 #8


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    How could GR look if speed of light was unequal to speed of gravity anyways?

  10. Apr 19, 2003 #9
    Heres a link to describe Weyl Tensor

    http://mathworld.pdox.net/math/w/w088.htm [Broken]
    Last edited by a moderator: May 1, 2017
  11. Apr 19, 2003 #10
    Well Hurkyl, I don't think it would be consistent with SR then (in other words, I don't think it would be internally consistent, with the ONLY exception being when Vgravity < c). Otherwise information could travel FTL. Where it slower than c, I would imagine then it might add a sort of dampening effect and allow one to better sheild gravity, perhaps even managing to be able to manipulate spacetime so that light would not bend around massive bodies.
  12. Apr 19, 2003 #11
    Nice article, nice testing, Black Holes prove that gravity can exceed light, as no light escapes the gravitational pull of a Black Hole.

    Hummmm, makes me wonder why?
  13. Apr 19, 2003 #12
    Don't confuse the magnitude of the force of gravity and its effect on the escape velocity with actual velocity of gravity. Gravity does not "come out" of a black hole, as it is the local curvature of spacetime around the hole.

    edit: If I recall correctly, the black hole is a 90 degree 'rotation' of spacetime.
  14. Apr 20, 2003 #13
    Ahem, I haven't forgotten, that it goes in, as proven by matter falling across it's event horizon as evidenced by Hubble's film of that event, Broadcast imagery, on TV.

    (Go Figure)
  15. Apr 26, 2003 #14
    do you believe that quantum gravity could have an answer?
  16. Apr 26, 2003 #15
    across the event horizon? not into the black hole/EH?
    Do you mean to say that gravity goes in to a black hole? it doesn't; its the cause of the going in of matter.
  17. Apr 26, 2003 #16
    how do you imagine gravity travels?
  18. Apr 27, 2003 #17
    Please tell me the difference between something that crosses the Event Horizon of a Black Hole, and something the goes into a Black Holes 'environment', that is JUST behind the event horizon, that the object, going into it, crossed??

    Gravity absents the space inside a Black Hole, (other side of the event Horizon!) of EMR! ....and it will 'suck in' matter, as well, quickly too!
  19. Apr 27, 2003 #18
    Frame shifting.
  20. Apr 27, 2003 #19
    never mind what I said. I had an idea about it, but I guess its a little self-contradictory.
  21. Apr 28, 2003 #20
    NOTHING wrong with having ideas, nothing wrong with having ideas that end up, once reflected against others, being not quite the right answer, cause, sometimes knowing/realizing (finding out it's...) the wrong way, leads you to the right one!

    My Apologies if I came across, just a little bit to strong, sometimes I don't know my own strength, well. sorta do, but , ya know, sometimes, well, HUH, I, Errrr, Ummmm, weillllll, ya see, it's like this, (Insert "Yadda-Yadda-Yadda" here)

  22. Apr 28, 2003 #21
    i don't know how it answers my previous q:?

    i meant in a sense of:

    what is it "travel"?
    what does it take for one thing to travel?
    what kind of things do travel?
    why "Gravity" would be such a thing(a traveling thing)?
  23. Apr 28, 2003 #22
    Are you familiar with wave mechanics, or should I just start from the beginning??

    BTW, gravity is composed of theoretical/hypothetical particles called gravitons.
  24. Apr 29, 2003 #23
    In response to several replies that quoted my previous post:

    Gravity does not go in. It is an attractive force, best seen as a warping of spacetime. Imagine a 90 degree drop in spacetime occuring at the event horizon. With this we see it more of a distortion effect, as it should be.

    And in all liklihood quantum gravity would indeed provide an answer, but we are a few years away from having a very nice quantum theory of gravity.
  25. Apr 29, 2003 #24
    I don't know a thing about quantum gravity. Can you tell me at least a few things??
  26. Apr 30, 2003 #25


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    Program: Develop a quantum approach to spacetime that is background free. Ordinary quantum physics has the physics happening on a fixed background space and time, or spacetime. This is just like Newton. But Einstein's General Relativity built the physics into spacetime, and the spacetime into physics. "The scenery got into the act".

    Quantum gravity researchers are very strong on background indpendence.

    There are several schools of quantum gravity research, but the one most famous in North America is Loop Quantum Gravity (LQG) which currently uses a concept of quantum foam, more or less the successor to the original loops. Names to google: Ashtekar, Baez, Perimeter Institute. Successes: proof that area and volume (but not length) are quantized in their theory. Also they can prove the Hawking-Beckenstein blackhole entropy formula from (their) first principles, up to an undetermined parameter, the Immirzi parameter. Lots of recent discussion about whether they can get rid of the Immirzi parameter problem by doing some fairly major redefinitions of their quantum foam.

    Quantum foam consists of quantized four dimensional triangular pyramids - simplexes - carrying spinor values and filling spacetime. where they cut into some surface (say a black hole event horizon) they give it a quantum of area. Smolin has some material on LQG in his book "Three Roads to Quantum Gravity". I don't know of any other popular presentation of it.
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