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Gravity hysteresis.

  1. Mar 30, 2004 #1

    wolram

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    can anyone tell me if gravity suffers hysteresis effects? as gravity
    travels at C i imagine this effect would only be relevant to ,a high
    speed massive body with an highly eccentric orbit, i also wondered
    if this effect had implications at the quantum level.
     
  2. jcsd
  3. Mar 31, 2004 #2

    Njorl

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    Do you know of good sites with arguments about the speed of gravity. I have heard some fairly simple yet cogent arguments that it is not observably finite.

    I bring this up because I have heard that binary pulsar systems exhibit no trace of effects of retarded gravitaional potentials. If anything observable were to exhibit the behavior you ask about, it would be binary pulsars.

    Njorl
     
  4. Mar 31, 2004 #3

    wolram

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    NJORI.

    Do you know of good sites with arguments about the speed of gravity. I have heard some fairly simple yet cogent arguments that it is not observably finite.
    --------------------------------------------------------------------------
    the speed of G has been accepted by main stream science as = to C
    but i have read papers that disagree, i will find anti C articles and
    post soon.
     
    Last edited: Mar 31, 2004
  5. Mar 31, 2004 #4

    Stingray

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    What exactly do you mean by hysteresis? There is a path dependence that exists - essentially gravitational waves being emitted and then scattering back to influence the particle in the future. I'm not sure that I'd call that hysteresis though.

    You're right that it has more relevance for very extreme motions though.

    What do you mean by quantum implications?

    Njorl, binary pulsars give the most evidence that the speed of gravity is c. I don't know what you're referring to.
     
  6. Apr 1, 2004 #5

    wolram

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    STINGRAY.
    when i use the term "hysteresis", i use it in its simplest form as such,
    "literally, to be late. It describes systems that do not directly follow the forces applied to them", and from this i inferred that a massive body
    traveling at high speed will have a delayed reaction to gravity, if gravity travels at C.
    at the other end of the mass scale particles are traveling at relativistic
    speeds and i thought that reaction to gravity would have some time
    dependence.
     
    Last edited: Apr 1, 2004
  7. Apr 1, 2004 #6

    Stingray

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    I'm still not sure exactly how to answer your question. Test particles move according to what the fields (and their derivatives) are at whatever point they are occupying. In this sense everything is completely local, and there is no hysteresis.

    The gravitational force is always zero in terms of strict definitions (for test particles), but if you adopt a more intuitive viewpoint, then the forces are velocity dependent. This is closely analogous to the presence of a vxB term in the electromagnetic force. Do you consider vacuum electromagnetism to show hysteresis?
     
  8. Apr 1, 2004 #7

    wolram

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    STINGRAY.
    The gravitational force is always zero in terms of strict definitions (for test particles), but if you adopt a more intuitive viewpoint, then the forces are velocity dependent. This is closely analogous to the presence of a vxB term in the electromagnetic force. Do you consider vacuum electromagnetism to show hysteresis?
    --------------------------------------------------------------------------
    the dynamics of vacuum magnetism are beyond my learning, but
    intuitively i would say yes, as perpetual motion is an impossibility
    all systems must have losses of some kind, so a solid body in an
    magnetic Field would need time for internal changes to occur
    before reaction could happen.
     
  9. Apr 1, 2004 #8

    wolram

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  10. Apr 1, 2004 #9

    Njorl

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    From what I've read, the decaying orbits of binary pulsars strongly support the predictions of GR insofar as the radiation of gravitaion is concerned but that the arguments that said radiation travels at c are not necessarily supported by these decaying orbits.

    I could easily be wrong. Let me look for it for a while, unless you have a reference at your fingertips.

    Njorl
     
  11. Apr 1, 2004 #10

    TeV

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    Associate and myself constructed causal model which shows the following:
    1)If the velocity of propagation of gravity (Tg wave spacetime disturbance) does not match TEM wave velocity (light) than must be higher than c (not smaller than c under no circumstance)
    2)If Tg Wave propagate superluminal it is "faster"* higher its frequency.

    Result 2 induces unexceptable consequences (like violating Gauge invariance principle and such) so we concluded "speed of gravity" should equal c.

    The paper is currently in peer reviewed process.If it passes ,will be published by the end of this year.
    ____
    * With respect to distant Minkowski space point.
     
  12. Apr 1, 2004 #11

    wolram

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    http://www.mathpages.com/rr/s6-08/6-08.htm

    An even more impressive example of the phase-lag cancellation effects of numerator dynamics involves the "force of gravity" on a massive test particle orbiting a much more massive source of gravity, such as the Earth orbiting the Sun. In the case of Einstein's gravitational field equations the "numerator dynamics" cancel out not only the first-order phase effects (like the uniform velocity effect in electromagnetism) but also the second-order phase effects, so that the "force of gravity" on an orbiting points directly at the gravitating source at the present instant, even though the source (e.g., the Sun) is actually undergoing non-uniform motion. In the two-body problem, both objects actually orbit around the common center of mass, so the Sun (for example) actually proceeds in a circle, but the "force of gravity" exerted on the Earth effectively anticipates this motion.
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    if i am reading this correctly the answer is yes but not realy.
     
    Last edited: Apr 1, 2004
  13. Apr 1, 2004 #12

    Nereid

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    The text wolfram is quoting from explores the mathematics of GR, in this section 'sources in motion'. It does not question - or seek to question - that 'the speed of gravity' is c; it examines what sorts of orbits and orbital dynamics may be expected under GR, particularly where one massive body is in orbit around an even more massive body.

    Whats' really cool is the last two sentences: "The important point to realize is that the fact that the Earth's gravitational acceleration always points directly at the Sun's present position does not imply that the "force of gravity" is transmitted instantaneously. It merely implies that there are velocity and acceleration terms in the transfer function (i.e., numerator dynamics) that effectively cancel out the phase lag in a simple periodic pattern of motion."
     
  14. Apr 1, 2004 #13

    wolram

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    NEREID,
    you are good for the brain, may i prescribe 4 doses a day?
     
  15. Apr 1, 2004 #14

    Njorl

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    That was one of the things I had come across that raised questions about speed of g=c, that Earth's gravitational acceleration vector was not parallel to the light from the sun.

    Now, IIRC from my E&M course so long ago, you don't need to use retarded potentials for moving charges, unless they are accelerating. If gravity works analagously to electrodynamics, you would need to use retarded gravitational potentials because the Earth is accelerating. Right?

    1. Am I just remembering wrongly?
    2. Is the acceleration insignificant?
    3. Is an object in a closed orbit considered to be not accelerating due to the warping of space?

    Thanks,
    Njorl

    BTW, I'm enjoying this. I've been doing the same old stuff for years now (solid state), and I like feeling like a student again for a while. Plus, there are no exams!
     
  16. Apr 1, 2004 #15

    Stingray

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    I should of clarified, but I meant that the experiments are in good agreement with GR, which in turn implies speed of gravity = c (To be completely correct, it travels at speeds <=c -- there are "components" which will not travel at light speed due to a scattering effect off the curved spacetime. This is sort of analogous to the addition of a mass to a scalar field theory if you're familiar with that.).

    Anyway, theories with speeds of gravity that aren't exactly c are not ruled out AFAIK, but an infinite propagation speed is I think conclusively inconsistent with gravitational radiation effects.

    Njorl, that would be quite a miraculous cancellation if EM did not require retarded integrals for uniformly moving charges. It might be true, but I don't remember it.

    All of these things become very difficult to state even in linearized GR for fast-moving objects. Forget about it in the full theory. A particle with infinitesimal mass in an orbit is considered to be unaccelerated, but this is not the real two body problem. Its a hard (mostly unsolved) problem.
     
  17. Apr 2, 2004 #16

    TeV

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    That was actually the point what we examine:Does the effect of changing spacetime curvatory always decelerate speed of light in comparation with speed of light in inital gravity field of the same source?
    We modelled the symmetrical field configuration situation of spherical stationary object lineary INCREASING its mass*.What would happen with velocity of sequence of gravity potential pulse fronts compared with velocity c in start up configuration ?If it doesn't equal c,we show it can't be slower than c in this modell,just higher.It is contrary to what can be expected.Note that it is important to specify condition of c for the purpose of comparation.
    Consider this result just preliminary,becouse it's not completely known in present theory (althought some modells exists ) how to realise condition (*) without local source of energy that disturb in reaction gravity potential parameters.
     
  18. Apr 2, 2004 #17

    wolram

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    by TEV.
    If it doesn't equal c,we show it can't be slower than c in this modell,just higher.
    --------------------------------------------------------------------------
    am i reading this incorrectly or are you saying that speed of G>C ?
     
  19. Apr 2, 2004 #18

    Stingray

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    TeV, it can be shown that the gravitational Green function has support inside (and on) the light cone when linearizing off of a curved background. To me, this is the proper definition of what it should mean to say light travels at <=c.

    For example, if you consider a small particle in orbit around the other one, the radiation reaction effects come from the integrated effect of the "slower than light" portion of the Green function.
     
  20. Apr 2, 2004 #19

    TeV

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    No.The model is quite artificial.Despite causality preserved it requires local source of energy which interacts with spherical mass and rises its gravitational potential while at the same time doesn't have gravitational energy by itself.For instance,we suspect that this source might be vacuum energy.How that can happen is not disscused or wether is possible for that matter.The consequences would be odd .Specificaly in this geometry,without classical gravity waves .The system gets the energy.Both velocities of light and gravity may be higher than c in vacuum (on other hand this isn't tachyon physics).
    .With rotation of the object we get another strange consequence like possibility of antigravity effects.This isn't rigorously forbidden in the equations of GR,but is strange.
    Not mentioning other problems,we concluded that real nature of gravity propagation matches c.
     
  21. Apr 2, 2004 #20

    wolram

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    TEV.
    this sounds intriguing, obviously i am far below your level, and maybe
    wont understand," i would like to try," I'm sure members on this forum
    would be interested in your research.
     
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