Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of gravity

  1. Dec 19, 2006 #1
    Since nothing can go faster then the speed of light, how about gravity? While the earth is rotating around the sun, we are being influenced by its gravity and light. What if the Sun suddenly disappears? It would take somewhere around 8 minutes for people on earth to finally realize that the Sun is gone... How about its gravity? The instant the Sun disappears, would we be slingshoted into outerspace or would we have to wait 8 minutes for that to happen too?

    I'm guessing if that's the case... gravity is as fast as the speed of light.

    And if that's true... is that true for ALL the forces? Do they all go at the speed of light?
  2. jcsd
  3. Dec 19, 2006 #2

    If the Sun dissappeared, it would take approx. 8 minutes for the light to stop and the sun's gravity to take no more effect.
  4. Dec 19, 2006 #3
    So you are saying that gravity is as fast as light. Is this true for all the forces ex: magnetic, electromegnatic, attraction between charged particles, etc...?
  5. Dec 19, 2006 #4


    User Avatar

    - Yes, if the sun suddenly dissapears, we would only feel that after 8 minutes.
    - Yes, gravitational waves propagate at the speed of light.
    ( More reading here: http://metaresearch.org/cosmology/speed_of_gravity.asp [Broken] )

    What other "forces" might you be interested in? If you reffer to the remaining interactions:
    - strong force propagates at light speed (as gluons are massless)
    - weak force propagates at speeds below c (as the carriers have mass)

    Classical "forces" (like elasticity, friction, etc) obviously don't make objects move at the speed of light, although the field propagates at the speed of light.

    Those things you mentioned ( magnetic, electromegnatic, attraction between charged particles ) are actually just the same thing as light.
    Last edited by a moderator: May 2, 2017
  6. Dec 21, 2006 #5

    Chris Hillman

    User Avatar
    Science Advisor

    Some pedantic caveats

    1. According to gtr (and most related theories of gravitation), gravitational radiation and electromagnetic radiation move at the same speed (at least, in "empty space", i.e. region not filled with matter). In particular, in gtr one can study both gravitational and electromagnetic plane wave solutions, or plane waves combining both types of radiation, and in the latter case, the wavefronts of the gravitational and the EM radiation are the same.

    2. Even in flat spacetime, there are many distinct but operationally significant notions of distance, and thus of velocity, over noninfinitesimal regions. For this reason, you should speak of the "infinitesimal speed" or some such phrase signalling this caveat.

    3. In gtr, the principle of conservation of mass-energy prevents one from studying a scenario in which the Sun simply "vanishes". One can however study a scenario which which an isolated object suddenly and asymmetrically redistributes its matter (e.g. an exploding bomb in deep space). In such a case, we would expect the explosion to produce both gravitational and electromagnetic radiation, and the wavefronts would expand at the same speed.

    Which should be understood as shorthand for: if our Sun exploded, it would take 8 minutes for the bad news to reach us. (As a practical concern, the EM radiation would be far more dangerous than any gravitational radiation. But while the Sun is expected to eventually misbehave badly, this is not expected to happen for some billions of years, and the expected scenario would initially pose problems for life on Earth rather different from what one might guess from this "eight minute scenario".)
  7. Dec 22, 2006 #6
    That's what the difference between einstein's gravity and newtonion's gravity,the latter one was somewhat unable to explain this.u can imagine ripples of space time warp being disappearing first from center and then ripples travel in the form of gravitational waves which reach us by 8 min.

    I had my first general theory of relativity class started by this same question(if the sun disappears suddenly) and luckily i knew the answer.
    Last edited: Dec 22, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook