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Speed of gravity. Whats the problem.

  1. Nov 6, 2003 #1
    "Speed" of gravity. Whats the problem.

    I keep finding stuff on the net about the speed of gravity and how some people contest it should be c, the speed of light.

    Now I lean more towards an infinite "speed" of gravity, but I really cant see why gravity should be restricted by c. People say that nothing can travel faster than the speed of light. But objects appear to expierience gravitational force direction changes instantly.

    So what's the problem. Nothing has travelled faster than the speed of light. Two object have just experienced accellerations at the same time. Relativity says that masses cannot travel at the speed of light relative to one another, but why should this apply to forces.

    No Thing has travelled faster than light.
  2. jcsd
  3. Nov 6, 2003 #2


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    Re: "Speed" of gravity. Whats the problem.

    SR holds that no information can travel faster than c. An infinite propagation speed for gravity would violate that. GR shows that gravity can have a propagation speed of c without causes the problems that some people seen to think that it would cause.
  4. Nov 6, 2003 #3
    Re: "Speed" of gravity. Whats the problem.

    Because experiments indicate that the laws of relativity are correct.

    No, they don't. Nobody has measured the propagation speed of changes in the gravitational field directly. But we have indirect measurements: the Taylor-Hulse binary pulsar system. The orbits of the neutron stars decay in exactly the way predicted if they were radiating gravitational waves. The rate of decay depends on how quickly the waves travel, so we can infer the speed of gravity from the decay rate. It come out to be c, to within a few percent (if I recall correctly).

    No non-tachyonic field theory consistent with relativity can have field disturbances propagate faster than the speed of light.

    Imaginary masses (tachyons) can propagate faster than the speed of light; likewise, tachyonic fields can propagate disturbances faster than light. But we don't have any experimental evidence of tachyonic fields, just like we don't have any evidence of tachyons. Non-tachyonic particles and fields have a light-speed limit in relativity.
  5. Nov 9, 2003 #4
    But what exactly is wrong with information travelling faster than the speed of light.

    Special relativity shows that masses cannot travel faster than light but why should this hold so for information. the theory says nothing about information.

    What postulates of special relativity does an infinite speed for gravity violate. It doesn't violate any.

    If gravity wasn't infinite then wouldn't that lead to a contridiction. If a large mass was placed in the center of a long rocket, the force it would exert on the front and back of the rocket would be the same according to someone on the rocket, but different according to some one moving relative to the rocket. Wouldn't this lead to a contridiction?

    That aside, why can't information travel faster than light? It doesn't have mass so what is the problem?
  6. Nov 9, 2003 #5
    You can write down relativstic theories in which information travels faster than light. (Maxwellian electromagnetism is not one of those theories.) They are called tachyonic. However, they have, among other things, problems with causality: any influence that propagates from an event A to an event B in one frame will propagate backward in time from B to A in some other frame.

    Special relativity, in the form of the light cone structure of Lorentz geometry, places a constraint on all causal influences, in particle or field theories. It doesn't just constrain particles with mass.

    You can write down tachyonic theories consistent with the postulates of SR, but they aren't consistent with reality.

    No, it wouldn't. (Nor is this true in electromagnetism, replacing masses with charges and gravitational forces with electric forces.)
  7. Dec 1, 2003 #6
    Re: Re: "Speed" of gravity. Whats the problem.

    I agree, the speed of gravity, is creation. It has NO SPEED.
    When we get this right we have the interstellar and Galaxy travel we dream about. Can anyone say "DUNE"

    lets use the MAGNETISM we know about to find the answer. It was done on the ocean once..."other story"
    what is Magnetism.....same as gravity, OR bubbles in a sink, surface tension....same stuff. LARGE scale....

    work it.
    Hans B.
  8. Dec 1, 2003 #7
    awares of the nova explosion?

    I happened to find an news report about 8 to 12 years ago of a super nova testing its speed....
    It was millions of times faster than the speed of light.
    for about some millions of nano seconds.
    but it happened!

    the news and the article vanished. in 1 hour as I want ed to print it...figures. no one would like me anyway.

    cant find it anyplace.

    It haunts me to this day, GUYS, GALS, keep looking for the gravity wave.
  9. Dec 2, 2003 #8

    Ambitwister, hello again.

    I think what I was after in the way of an answer in one of our discussions was this;

    if we measure speed and change by spectral analysis or by optical verification (ie:using our eyes) then we have limited our observations to the use of visible light and those other spectrums of light which we are able to observe.

    Therefore it would seem correct to say that we cannot observe events that are faster than light and will never be able to until we find another method of observation and verification than optical methods. This other method could not include simple hypothetical mathematical formuli because they only prove that we can postulate the conditions for speeds at <c on paper but never actually prove they exist through optical observation.

    This does not bode well for those who wish to prove that information or any other format of events can travel <c.

    Once again, in the spirit of this thread, I'll ask; is it the effect of gravity (caused by the presence of a mass) that travels outward or is it gravity that travels at whatever speed it travels?
    Last edited: Dec 2, 2003
  10. Dec 2, 2003 #9
    Re: <c

    Why? There isn't any law that says that we have to probe an object's speed with something that travels faster than that object.

    Once again, in order for me to answer that question, you need to precisely define what you mean by "the effect of gravity", as opposed to simply "gravity". To be physically meaningful, you have to define it operationally, in terms of an experiment: describe how to carry out two experiments, one of which measures "the speed of gravity" and one which measures "the speed of the effect of gravity". Then I can tell you what our theories of gravity say the outcomes of those experiments will be.
  11. Dec 3, 2003 #10


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    Example: planetary motion. If the "speed" of gravity is infinite, the "force" holding the planet in orbit will be directly on line with the sun. If the speed is finite, there will be a lag between the force and the position relative to the sun. And the effect of this lag would be...
  12. Dec 3, 2003 #11
    Re: Re: <c

    Great, thanks again.

    Experiment #1: measuring the speed of gravity.

    Hypothetically we find a way to identify "gravitons" or waves that have a constitution that is completely separate from the particles, gases and radiation found in the space surrounding a mass and that show an interaction with those particles, gases and radiation that help us determine that these are, in fact, gravity waves or "gravitons". Then we are able to track these waves with or without relying on observations of their interaction and influence on the surrounding space and its contents. In this experiment we have isolated gravity as a separate wave or sub-atomic force and are able to measure its speed.

    Experiment #2: measuring the speed of the effect of gravity.

    Hypothetically we find out that gravity has no radiating waves or "gravitons" and that it is only by observing the actions of particles, gases and radiation surrounding a mass in space that we are able to say the mass is generating a gravitational pull. So that, in this experiment, we pick a region of space with close to zero mass in it but does have some incidental particles, gases and radiation. Then (instantaneously) we introduce a large mass and immediately measure the speed of the influence of the mass by observing the reaction of the particles, gases and radiaion surrounding the newly introduced mass. In this experiment we have only isolated the effect of gravity and the speed at which this effect spreads through space (and matter).
  13. Dec 3, 2003 #12
    Re: "Speed" of gravity. Whats the problem.

    A water wave is rolling towards the beach at few miles per hour. You hit it at hundred mph in your highspeed boat. Is it that the wave travels faster than light, or is it that you crossed that which was already there?

    Crash your plane into the mountain. Ask whether you experienced collisional force direction changes instantly.
  14. Dec 9, 2003 #13
    None of this has answered my initial question.

    In what way would an infinite speed for gravity break the postulates of relativity. Namely.

    All Physical laws are exactly the same in all interial frames.
    All observers will measure the same spped for light regardless of their inertial frame.
  15. Dec 9, 2003 #14


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    You could posssibly contrive to send a message via gravity. That message would travel instantly and would incur the time-travel consequences of FTL in relativity, which contradict causality.
  16. Dec 10, 2003 #15
    But information travelling faster than the speed of light doesn't break any of the postulates.

    Causality isn't violated by faster than light trasmission of data.

    Just because I know that something is coming towards me, before I can see the light from that object, will not violate either of the postulates.
  17. Dec 10, 2003 #16
    Perhaps I'm harping too much on the postulates alone.

    Maybe someone could describe a thought experiment which would show a paradox in having an infinite speed for gravity.
  18. Dec 10, 2003 #17


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    As was stated earlier:

    It doesn't take too much creativity to set up a paradox:

    Person A sends a FTL signal to person B someplace far away. Upon receiving A's signal, B sends a signal to person C who is next to A. Upon receiving B's signal, person C kills person A.

    Then, if the signals are sent in the right way, they both travel backwards in time in a particular reference frame, thus in all reference frames, person C receives B's signal before person A sends a signal, and we have a paradox.
    Last edited: Dec 10, 2003
  19. Dec 10, 2003 #18
    I don't see the paradox.
  20. Dec 10, 2003 #19
    Re: Re: "Speed" of gravity. Whats the problem.

    my understanding from the physics USENET faq is that even with tachyonic field theories, no local field disturbance can propagate faster than light.

    which is nice, since even though there aren t any tachyonic matter fields in the standard model, there are possible field configurations at unstable points in the potential which act like tachyons in some regimes. even these fields do not have signals propagating faster than light.
  21. Dec 10, 2003 #20
    Re: Re: <c

    Hello Ambit, please see the two experiments I have laid out in response to your request. I am interested in finding out if these are one and the same or not. Thank you.
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