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Spacetime curvature

  1. Sep 4, 2015 #1
    Do gravitational forces have to follow spacetime in the same way as light? Or does gravity act in a higher dimension?

  2. jcsd
  3. Sep 4, 2015 #2
    Higher dimensions are not necessary to explain gravity.
    The general theory of relativity provides an explanation of gravity using only the familiar dimensions of space/time and there is much evidence which supports it.
    One of the consequences is that any form of information cannot propagate faster than the universal speed limit, normally referred to as the speed of light.
    I think it is therefore concluded that if the Sun were to somehow mysteriously vanish, the Earth would continue to orbit where he sun had been for about 8 minutes, (the time light takes to reach us from the Sun), before heading off into interstellar space.
  4. Sep 4, 2015 #3
    I can't say I've had the luxury of being able to focus on this area of physics, but I don't know if the speed of light is necessarily the universal speed limit. Some physicists believe gravity waves travel much faster. But time will tell (no pun intended).

    My original question was more in reference to the following idea:

    Imagine you're world as a 2-sphere, and only two masses exist in it, and on opposite sides of the sphere. Would gravity's influence be able to travel the line connecting the two masses (moving in a dimension not described by the 2-sphere - i.e. force vectors pointing towards each other through the center of the sphere), or would the force vectors on the masses point tangent to the 2-sphere?
  5. Sep 4, 2015 #4
    When you start considering extra dimensions you are explicitly redefining your world as something other than a 2 sphere.
    If you do that you could theorize about gravity being propagated trough the additional dimension you introduced.
    However why would this new model be applicable to gravity but not so for light?
  6. Sep 4, 2015 #5
    Ok, thank you. I was just wondering if it was possible that light operated in less dimensions than gravity. The motivation was dark matter.
  7. Sep 4, 2015 #6


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    Staff: Mentor

    It is. There's very good theoretical reasons and an absurd amount of experimental evidence for the speed of light being the universal speed limit.

    Then those physicists don't know what they're talking about. Gravitational waves are required to travel at c by General Relativity. Traveling FTL would bring about some serious paradoxes.

    If you mean the masses are two-dimensional masses on the surface of this sphere, then gravity would act along those two dimensions, not through the third dimension since it doesn't exist in this hypothetical scenario.
  8. Sep 12, 2015 #7


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    twelfthroot2 said: ↑ Some physicists believe gravity waves travel much faster.
    Please cite the peer reviewed sources confirming this statement. Perhaps you meant some physicists have postulated that gravity travels > c ? If a physicist "believes" this, despite the enormous amount of evidence to the contrary, then his/her belief is irrational. Newtonian gravity assumes gravity is instantaneous. It is wrong. If you think about it, it isn't hard to understand that orbital mechanics will place limits on the speed of gravity. What I don't know is how small the range of speed is around c; what are the confidence limits.
    As far as your question about lower dimensions. The definitions of "dimension" become abstract after Freshman physics, but on the non-microscopic scale, General Relativity ( 4 dimensional spacetime) fits all known physics. There is nothing in the physical universe which is just 2 dimensional, everything real has extent in 3 space-like and one time-like dimension. OTOH, see wikipedia "Holographic Principle".
    Last edited: Sep 12, 2015
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