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The Speed of Force?

  1. Aug 7, 2003 #1
    Hey all,
    I have a question that I've been thinkin about, that I find very interesting:

    What is the speed of force? For example, say there was some sort of force applied to you from 4 light years away... you wouldn't SEE anything happening for 4 years, but how long would it take to feel the effects of it?

    For now, keep it simple-- just use gravitational force (don't reply with "well there are different kinds of force etc etc")

    The reason I ask, is because we can use this as communication between other stars rather than light which takes 4 years to reach the closest star.
  2. jcsd
  3. Aug 8, 2003 #2
    Changes in the gravitational force must obey the speed of light limit like everything else. For example, if a star suddenly collapses into a black hole, the gravitational force at some distance from it will increase. This doesn't happen instantaneously -- the change in the field will propagate outwards in the form of gravity waves.
  4. Aug 8, 2003 #3
    OK but..
    how fast do gravity waves travel? I don't see why it would be the same as light speed (by the way, if it acted the same way as light did, the force would also not be able to escape a black hole).
  5. Aug 8, 2003 #4
    As far as I know, GR theory predicts they travel at nearly the speed of light. As for the exact reasons, I don't know.

    I'm not sure I understand what you mean by the force escaping from a BH. Do you mean the BH's own gravitational force? In that case, GR predicts that that it doesn't need to "escape" -- it is just there, due to the curvature of space-time generated by the BH's immense mass-density.

    There is another theory of gravity, based on particles called gravitons. They are supposed to be like photons -- massless and traveling at the speed of light. They would "carry" the gravitational force, much like photons carry the EM force. In this case, the gravitons would have to escape the BH in order to carry information about its gravity. How would they do this? I don't think anyone truly knows -- gravitons have not been detected and are purely theoretical. If they did exist, it's possible they could tunnel quantum mechanically out of the BH, or that they actually travel faster than light (thus exceeding the necessary escape velocity from the event horizon). At this point it's all speculation.

    Sorry, that's all I can say, GR is not my specialty. Maybe someone else can explain things a little better.
  6. Aug 8, 2003 #5
  7. Aug 8, 2003 #6
    I've heard that gravity waves have been estimated to travel at c*10^10m/s.
  8. Aug 8, 2003 #7


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    Re: Re: The Speed of Force?

    Your point is correct, but your example is flawed. From the outside, there is no difference between the gravitational field of a star of a given mass and a black hole of the same mass.
  9. Aug 8, 2003 #8


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    Gravity waves are predicted to travel at the speed of light. There is no conclusive observational evidence yet of their speed, or even their existance. However, LIGO is busily gathering information to confirm or refute the waves predicted by Einstein's equations.
  10. Aug 8, 2003 #9
    These is a good question hidden in there! Let me say it this way:

    Suppose there are gravitons, and they are the carrier of the gravitational force. How is it that they can escape a black hole? They would have to be able to because we can feel the gravitational attraction of a black hole outside its horizon.
  11. Aug 8, 2003 #10

    Chi Meson

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    There is a serious group of astrophysicists that maintain that the speed of the gravity force carrier, the "graviton," travels at least 20 billion times the speed of light. They point out that the angular momentum of galaxies would not be conserved otherwise. (do a seach for "speed of gravity") It's a convincing argument and these guys are not crackpots.

    But just about a year ago, all eyes were on one of Jupiter's moons as it passed in front of a distant ... what, a quasar?... a distant thingy. Blah Blah Blah it was calculated that the speed of gravity was identical to the speed of light.

    This supports the notion of general relativity that gravity is not really a force, but an inertial effect. Yadda Yadda. Yadda.

    PS, the electromagnetic force is limited to the speed predicted by James Clerk Maxwell, bound by the permittivity and the permeability of a vacuum to the electric and magnetic fields. That speed is the speed of light.

    And the graviton people have not given in; it's a ripe topic! If the graviton does travel 20 billion c, then all we have to do is toggle mass in and out of existance to send hyper c e-mail!
  12. Aug 10, 2003 #11
    Re: Re: Re: The Speed of Force?

    Sure there is. The gravitational field is much "steeper" closer to the black hole than it is closer to a star of the same mass.
  13. Aug 15, 2003 #12
    there is no direct observational evidence. but there is some very compelling indirect evidence for gravitational waves. binary pulsar system PSR1913+16 was observed to be collapsing. its periods of revolution have been measured for about 30 years, and the fit to the predictions of GR due to gravity waves is excellent. you should check out the data fit, you can t distinguish the measured date points from the theoretical fit. taylor and hulse got the nobel for this. (even einstein never got a GR nobel)
  14. Aug 15, 2003 #13

    Les Sleeth

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    Within the last two weeks I read a fairly large and highly priortized article in the newspaper about scientists confirming Einstein's prediction that gravity travels at light speed.

    I found this link:


    Since I threw the newspaper article away, I am not sure that link is what it refers to. It seemed to me the article was about something more recent.
  15. Aug 15, 2003 #14
    I have also heard that gravitation travels at the speed of light. If our Sun was to miraculously vanish, we'd feel it in 8 mins - if I'm not mistaken.
  16. Aug 15, 2003 #15


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    You mean T von Flandern? Actually, they are. They have made serious errors in their mathematics. (As an example, they have applied directly a speed of gravity on a newtonian model, without realising that GR predicts it as a matter of spacetime - as a result, time will also be affected, compensating for that effect.)

    I am not sure, but I think these are one and the same. GR implies the prescence of light-speed force carriers, as photons mediate the electromagnetic force. Gravitons are simply the particle equivalent of gravity waves.

    Hmm.... I heard the answer to this once... lemme try and reproduce it. The trouble is time dilation again. The thing is, the gravity we feel from the blackhole is "old gravity" from before the blackhole formed, which is trapped by time dilation.
  17. Aug 16, 2003 #16

    The speed of force must be the same as that of light.

    This helps explain why you cannot push something faster than light. If the speed at which force moves is similar to light, then as things speed up their masses don't increase, but the effect of force diminishes. A sort of quasi-mass increase similar to Einstein's mass increase but different in origin.
    Last edited: Aug 22, 2003
  18. Aug 16, 2003 #17

    Les Sleeth

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    Excellent point. I've surmised that what you point out may suggest radiation and gravitational force are polar manifestations of the same thing.
    Last edited: Aug 16, 2003
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