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The speed of gravity?

  1. Apr 4, 2006 #1
    I'm just wondering what we know about the speed of gravity.

    a) Is the gravitational force determined by mass or a combination of mass and energy?

    b) As a planet orbits the sun, can we discern its position by detecting its gravitational force instantly and therefore see it slightly ahead of its reflected light? Or does the gravitational force information take time to get to us?

    c) If a star burns up at a great distance away, is the missing gravitational force detectable instantly? Or does it take time for the stellar event to reach us as does the light from that star?

    d) If we were able to create mass and then destroy mass in a repetitive manner, and if gravitational force effect is instantaneous, then can we broadcast information instantly to any other part of the universe? (Thus exceeding the speed of light)?

    The above questions are purely curiosities in my head. But is seems to me if g-force is instantaneous, then that is remarkable. And if g-force changes travel at some velocity then that is equally remarkable!

    Oh well! Any takers?
  2. jcsd
  3. Apr 4, 2006 #2


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    By the four-momentum and the momentum stress (energy is a component of four-momentum) If the particle is massive, its rest mass contributes to the energy, but even massless particles have momentum, and gravitate.

    According to General Relativity gravity travels at the same speed that light does, c.

    Since c is a finite speed, it takes time.

    No. You might produce gravity waves with this technique, but they too would travel at c.

    I quite agree!
  4. Apr 4, 2006 #3
    Thanks! ill check out general relativity again and see what i can find regarding my curiosity!
  5. Apr 5, 2006 #4


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    I'll quote a bit of the article to try and induce more people to read the original in its entirety.

    To put it more simply, when you measure the speed of light, you measure the speed of propagation of a wave. Applying the same definition, the speed of gravity is the speed of gravitational radiation, which is expected to be the same as the speed of light, but has not been measured (yet).

    Looking at the direction of the force from a "point mass" or a "point charge" leads people to misleading ideas about the "speed" of both electromagnetism and gravity - the direction of the force does not actually tell one anything about the speed.

    Asking what happens when a mass or charge "disappears" is also a dead end. When one does the math, one finds that charges don't disappear and neither do masses. This non-disappearance is built directly into the appropriate equations (Maxwell's equations for E&M, Einstein's field equations for gravity). Therefore one cannot solve these equations for what happens when mass/charge disappears, the equations assume that mass and charge cannot disappear.
    Last edited: Apr 5, 2006
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