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Speed of Space-Time

  1. Aug 16, 2009 #1
    Ok, here me out on this one. I got thinking a while back after I read some very amazing books quantum physics and theory of relativity and and a thought just came to me. I might just be crazy though.

    My friend and I were discussing light speed and how gravity seems to "bend" light. We all know of these gravitational "lenses" that are created by galaxies millions of light years away. But then I got thinking of the speed of light and gravity.

    I made up a scenario of the disappearing sun. One day the sun disappears. Now, we all know that light takes 8 minutes or so to travel from the sun to the Earth. If the sun disappeared we would not know until 8 minutes later. Then I thought of this question: If light takes 8 minutes to reach Earth from the sun, how long will it take before we notice the change of gravity from the sun not being there?

    In one of the books I read (I think it was the Fabric of the Cosmos), it explains that light and gravity have the same "speed" so to speak. If light and gravity have the speed, that means that the space-time is changing at the same rate as the speed of light. (When the sun disappeared, the space-time that was distorted will change back to flat space at the same rate as the speed of light).

    What I'm saying is that the warping of space-time (gravity) will change at the same rate as the speed of light.

    It's late and I had no coffee, so I could be out of my mind. What do you guys think?
  2. jcsd
  3. Aug 16, 2009 #2


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    Welcome to PF!

    Hi dream431ca! Welcome to PF! :smile:
    Yes, that's basically correct …

    gravity is believed to move at the speed of light, so the gravitational effect of any change in the Sun (for example, the Sun pulsating in and out … it's difficult to see how it could actually move away :wink:) should reach us at the same time as we see it. :smile:
  4. Aug 16, 2009 #3
    Have a look at this
    http://www.metaresearch.org/cosmology/speed_of_gravity.asp" [Broken]
    Last edited by a moderator: May 4, 2017
  5. Aug 16, 2009 #4


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    Re: Welcome to PF!

    Gravity doesn't move, it is already there. Statements like the above lead to wrong conclusions, about the direction of the gravitational pull of a moving gravity source. Gravity is not like light, but more like the electrical field. It is only the changes in gravity that propagate at light speed.
    You mean the right thing, but laymen oft misunderstand this explanation.
  6. Aug 16, 2009 #5


    Staff: Mentor

    Van Flandern is a pretty notorious anti-relativity crank. Have a look only if you have a bunch of spare time and absolutely nothing better to do.
  7. Aug 16, 2009 #6


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    "There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes. In particular, one does not think of particles as moving through space-time, or as following along their world-lines. Rather, particles are just in space-time, once and for all, and the world-line represents, all at once, the complete life history of the particle." R. Geroch, General Relativity from A to B (University of Chicago Press, Chicago, 1978), pp. 20-21.
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