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Speed of Gravity Controversy

  1. Dec 11, 2004 #1

    Les Sleeth

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    At another science forum site someone asked about the speed of gravity. I posted a link to a news article about Kopeikin's experiment and published results: http://www.nrao.edu/pr/2003/gravity/

    Some excerpts are:

    I’d thought this was all but accepted by scientists, until another member there posted this link to a paper rebutting Kopeikin's results: http://www.metaresearch.org/media and links/press/SOG-Kopeikin.asp

    Here are some relevant quotes from that paper:

    I have a few questions. Is there general agreement now that Kopeikin’s team failed to measure the speed of gravity? Is it generally agreed that the speed of gravitational force is virtually instantaneous? In the article above they cited the following thought experiment which seems to make sense:

    “A common thought experiment asks: ‘What would happen to the Earth's orbit if the Sun suddenly ceased to exist?’ The answer is now clear. The usual relationship ‘force is the gradient of the potential’ would instantly end. The Sun's potential field would then begin to dissipate, taking 8.3 minutes to dissipate out to the distance of the Earth's orbit; so effects such as light-bending and clock-slowing would persist for that long. But the Newtonian component of gravitational force, the force that keeps Earth in its orbit, would cease almost instantly, and Earth would fly off along a straight line like a weight on a spinning merry-go-round that broke free from its moorings.”

    Mainly I wanted to ask this. If the speed of gravity is virtually instantaneous, what are people’s thoughts about what gravity is doing to space that would cause that? For example, would it be improper to see mass as having a “constricting effect” on space?

    In that case, the constricting effect would simultaneously affect every place the force extends, and then disappear concurrently everywhere if, as in the example above, the Sun suddenly ceased to exists.
     
    Last edited: Dec 11, 2004
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  3. Dec 11, 2004 #2

    Integral

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    I do not think that there is a lot of controversy about the speed of gravity. It is pretty well accepted that it is c, we just need to find a way to measure it. There was some controversy around the paper you mention, due to the claims of finally having found a way to measure the speed of gravity. Critical analysis later found errors in the methods, which invalidated the results. We remain without experimental verification of the theory.
     
  4. Dec 11, 2004 #3

    jcsd

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    Assuming GR is correct or at the very least it gives a near-perfect model for a very wide variety of situations then gravity propagates at c, few scientists would disgaree with these assumptions much less relativsts.

    Howver this is specifically the propagtion speed of gravity thta is being talked about is the propagation speed and the article plays semantic games with this and it also seems to make claims that are at the very least contraversial, it doesn't seem to be a greta source of information. However it is right in one respect as from what I see most people agree that Kopeikin's results don't match his conclusion and thta he was not in actuality measuring the speed of gravity.
     
  5. Dec 11, 2004 #4
    A plague on both their houses. Integal is right.
     
  6. Dec 11, 2004 #5

    pervect

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    The answer to this old chestnut is in the sci.physics.faq. Let me know if you need the link, I suspect you or anyone else who is interested can find it if they look around (but if you can't find it, I'll post the link). The very easy answer is that the sun cannot cease to exist, this violates some important conservation laws.

    What one _can_ do, in principle at least, is to blow up the sun. To avoid getting the Earth caught in the explosion, it's convenient to imagine splitting the sun into two parts, one goes "up", the other goes "down". If one does this it will take 8 minues for the disturbance in gravity to propagate to the Earth.

    This is quite comparable to electromagnetism. One might ask the question "What happens to the electric field at some distant point if one makes a charge disappear". The answer is the same, the conservation of charge means that one cannot make a charge disappear, or create charges - the best one can do is to create a dipole by separating a postive and negative charge. Gravity is very similar, though since the masses have the same sign, one actually creates a quadropole moment by separating them.
     
  7. Dec 11, 2004 #6

    Les Sleeth

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    I am really curious about this, so I hope you'll indulge me a bit more. I don't know if you read the entire paper rebutting Kopeikin's results, but what did you think of these so-called experiements:

    Are these "experiments considered relevant?


    I am confused a bit (a lot probably). Can you explain why you see critique of Kopeikin's work as semantics? Does the speed of gravity refer to what happens in a gravitational field, or does it refer to the force of gravity? In that thought experiment where the Sun disappears suddenly, will it take 8.3 minutes for Earth to feel the loss of the Sun's gravitational attraction, or will that happen almost instantly?

    As an aside, one of the posters at that other site asked "I would like to know how you account for the fact that the instantaneous position of target bodies are required for any successful orbital insertion?" Do you see how that relevant to this argument?
     
  8. Dec 11, 2004 #7

    jcsd

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    What I mean by semantics is thatbin some infact most parts they are clearly not talking about the propagation speed of gravity, infact they seem to be talking about (quasi-)static gravitational fields (as there's no gravity waves propagting in these fields not very relevant) and certain irrelvant limitations placed on theories of gravity by observations.
     
  9. Dec 11, 2004 #8

    jcsd

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    Sorry I missed this bit out.

    No it's not really relavnt as we're talking about th espeed of propagtion of gravity, i.e. the speed at which changes in the garvutional field propagate.
     
  10. Dec 11, 2004 #9

    Les Sleeth

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    Three more quick questions for you.

    So, it is true then that Kopeikin's interpretation about the meaning of the speed of gravity is correct regardless of whether his experiment demonstrated it's nearness to c, is that correct?

    If so, can you answer my question of how long it would take for Earth to feel the gravitational effects of the Sun suddenly disappearing. Is it the time it takes for light to travel from the Sun to Earth?

    Finally, is the problem here the source of the critique of Kopeikin's study? It is Meta Research and Tom Van Flandern. Is this just another TD guy with another theory?
     
  11. Dec 11, 2004 #10

    ohwilleke

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    I have to agree that I have doubts that this is a meaningful question.

    1. In GR gravity is a function of mass-energy density.
    2. Mass-energy is conserved.
    3. Therefore, the only way the a gravitation field from a particular localized clump of mass-energy, say, the sun, can change over time is for the displacement of the mass-energy to change.
    4. The mass-energy itself is subject to limiting its changes in displacement to something less than c as a result of SR.

    Thus, this places some hard limits on the extent to which gravitational energy can change.

    I suppose that the ideal kind of thought experiment you'd like to have is similar to the experience of an observer seeing lightning before hearing thunder. If gravity propogates faster than light, than you would as an observe, feel a tug of gravity for the distant event before you had visual evidence of it. For example, suppose that a star ejected a highly massive clump of matter straight towards you (it may emit another in the opposite direction). You would suddenly feel a strong raviational pull towards the emitting star before seeing any evidence of its emission.

    Alternately, you could have two space ships with precise measurments of their own locations. They go a long way from each other and agree in advance that one will move in a particular direction at a particular local time. Suppose that the ships are also emitting radio waves at each other. You should see a discrepency.

    I'd imagine that the uncertainies involved in such precise detection of subtle events would make it very hard to estimate.
     
  12. Dec 11, 2004 #11

    pervect

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    The speed of gravity is usually interpred exactly in the same way that the speed of light is - the electric field of a charge spreads out to infinity (well, that's an idealization, actually), but when you wiggle it, changes in the field propagate at a certain speed - that speed is 'c', the "speed of light". If you "wiggle" a mass, the gravitational field may propagate out to infinity, but changes in the field as a result of the wiggling are expected to propagate at 'c'. Because gravity is such a weak force, nobody has been able to experimentally measure gravity waves (the anology to electromagnetic radiation) yet, much less measure their speed.

    The fact that the field of a uniformly moving charge always points towards the current position of the charge, not the retarded position, doesn't have anything meaningful to say about the speed at which light travels - nor does it have anything meaningful to say about the speed at which gravity travels.

    We don't go around saying that the speed of light is faster than the speed of light because the electric field points towards the current position of the charge - neither should we say that the speed of gravity is faste than the speed of light because the gravitational field points (approximately, at low velocities) towards the current position of the mass.

    You really should read the sci.physics.faq, it explains this nicely.

    It also mentions that because you can't destroy charge (nor mass), you can't answer the question "what would happen if mass/charge were suddenly to disappear". The best answer is that neither one can disappear, they are conserved quantities.
     
  13. Dec 11, 2004 #12

    Les Sleeth

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    Thanks, I most definitely will read it.

    I don't think I see too clearly why the hypothetical question can't be considered however. I suspect I listened too much to that second article I referenced, but I could see there might be a difference between effects within the field, and how quickly gravity would disappear if its source went absent. It seems like one concept says, "this is how fast gravity moves EM through it," and the other concept says "this is how fast gravity would disappear if its source were no longer there."

    Is this a meaningless distinction?
     
  14. Dec 11, 2004 #13

    Chronos

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    Tom Van Flandern is among the most famous pottery magicians on the net. He proposes instaneous propogation of gravity and claims the gps system uses no relativistic corrections [which is totally wrong]. Kopeikin's conclusions have, however, been challenged by a number of other credible sources. Here is a more recent paper by a fairly well respected authority
    http://arxiv.org/abs/gr-qc/0403060
     
  15. Dec 11, 2004 #14

    Les Sleeth

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    Thanks Chronos, and everyone else. I think I see it. Geez, it's just more evidence that something making sense doesn't make it true. No wonder everyone is so conservative about considering new theories. :smile:
     
  16. Dec 12, 2004 #15
    Birkhoff's Theory also says that gravity waves propagate at the speed of light. If the Sun suddenly disappeared, Earth would continue to orbit for another 8 minutes. That's a neat idea ... one by one, starting with Mercury, the planets would head off at a tangent ... and we couldn't know it was happening by observation of Mercury and Venus ... by the time the light from them reached Earth, so would the lack of gravity and sunlight. But the Moon would continue to orbit Earth ... that is truly weird.
     
  17. Dec 12, 2004 #16

    Andrew Mason

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    I may be showing a lack of appreciation for some of the subtlties of General Relativity, but if gravity is the property of all matter and energy, how can gravity travel any faster than matter and energy travel?

    Why does it have to really move at all? Matter changing into energy and vice versa should have absolutely no effect on the total gravitational field.

    So why do we have to invent a theory of the graviton and the speed at which it is supposed to move? No one has ever detected a graviton despite serious attempts for over 30 years. Perhaps there is a reason for that.

    AM
     
  18. Dec 12, 2004 #17

    Nereid

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    There was PF another thread on a similar topic recently Les, here (it also contains a link to sci.physics.faq does gravity travel at the speed of light).

    To take an 'experimental' look at this. First, LIGO, and other gravitational wave detectors, are starting to come online (there's a link on the LIGO page to Einstein@Home, where you can contribute spare CPU cycles on your PC to help make the detection!) and hopes are high that an 'inspiral event' (e.g. two neutron stars orbiting every closer until they merge/collide), or an asymmetric supernova will be 'seen' in the next few years. If either event also generates 'observable' EM (e.g. light, radio, gammas), or even neutrinos, we will also have some solid results on the 'speed' of gravity. AFAIK, Kopeikin's is the only half-way credible observation of the speed of gravity so far, and as Chronos pointed out, he got his sums wrong.

    Second, observations of some neutron star binaries (with at least one member a pulsar) show decaying orbits. The rates of decay very nicely match the hypothesis that energy is being lost from the systems in the form of gravitational radiation, as predicted by GR (some hard working scientists got Nobels for this). Since gravity travels at c in GR, these results provide indirect support for GR and the speed of gravity being c (this is explained in more detail in the Baez page).

    Third, and most generally, GR has passed all its obsevational and experimental tests to date - including one which would have shown a deviation if GR were out by as little as 1 part in 20,000. While these tests have only probed GR in a limited range of domains (astonishing as that might seem - the tests go right up to the whole universe, size-wise), one can say that there is no experimental or observational evidence, direct or indirect, that even hints at the speed of gravity being anything other than c.
     
  19. Dec 12, 2004 #18

    Nereid

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    Andrew, that same thread I mentioned in my previous post also discussed the graviton, cf gravitational radiation. As mentioned in that thread, the graviton is not a key part of GR (indeed, you could argue it isn't in GR at all); what GR does predict is gravitational radiation (aka 'waves'), and there's very good observational results - albeit indirect - for that!
     
  20. Dec 12, 2004 #19

    Andrew Mason

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    What I don't get, and I don't pretend to grasp the mathematics of GR, is how mass can be created or destroyed. It can be converted into energy but the energy has the same relativistic mass. Is total gravity not the same? I can see that the gravitational field at a distance to change with time. But that change should not occur any more rapidly than the change of position of the mass which produced the gravitational field.

    AM
     
  21. Dec 12, 2004 #20

    jcsd

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    The speed at which the arrangement of matter changes will affect the properties of the wave e.g. it's wavelength, but they don't affect the speed at which the wave will propagate. Think of an osciallting charge, the speed at which it oscillates affects the frequency of the em wave produced, but it doesn't affect the speed at which that wave propagates.
     
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