1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

An interesting scenario

  1. Apr 12, 2008 #1
    Say you had a universe, like our own in every way except that the total matter in existance was one single atom. Now say you introduced another atom of roughly the same dimensions into this universe as far from the original atom as spacially possible.

    How long, if any time at all, would it take for either atom to "observe" the existance of the other? Meaning: Is the force of gravity an instantaneous force, or does the force travel ie... as in a gravity wave?

    If the atoms are say 100 million light years apart, would either be aware of the other's existance untill 100 million years after the 2nd atom was introduced, or would they somehow instantly experiance the distortion of spacetime due to each others mass?

    Would they be able to "feel" each other before they could "see" each other?
  2. jcsd
  3. Apr 12, 2008 #2


    User Avatar
    Science Advisor

    Gravity is not an instantaneous force and propagates at the speed of light. Otherwise, information would travel instantaneously.
  4. Apr 12, 2008 #3
    According to Einstein, gravity propagates at the speed of light.

    On the other hand, nobody know what would happen if you suddenly created matter out of nothing. If you can do one impossible thing, maybe you can do other impossible things. ;)
  5. Apr 12, 2008 #4


    User Avatar
    Homework Helper
    Gold Member

    Gravity is not an instantaneous force. As you said, gravity waves from the second atom will travel to the first at a finite speed, namely, the speed of light.
  6. Apr 12, 2008 #5
    Ha Ha. Very funny. :wink:

    Well good you all have so far confirmed what i was thinking. Thanks guys!
    On the subject of gravity waves...

    I was thinking, we can only (theoretically) maybe detect extreme examples of ridiculous manifestations of matter (black holes, binary neutron stars, type 2 supernova) but, if the theory holds true would i be correct in assuming that every instance of matter occuring in spacetime would distort spacetime and produce "gravity waves" that could theoreticaly be measured even if our current technology does not allow it? So does this mean we are even more so swamped with gravitational waves than we are with EM radiation?

    Say we had "eyes" that could "see" a certain wavelengths of gravity waves instead of eyes that can detect EM radiation. How would our experience of the physical world be different/similar? Would we be provided with more information or less?
  7. Apr 12, 2008 #6


    User Avatar
    Homework Helper
    Gold Member


    It is true that every massive body in the universe will theoretically emit gravity waves as it moves, even if we cannot detect those waves because they are so weak. Whether this means we are more swamped by gravity waves than by E&M waves, I am not sure.
  8. Apr 12, 2008 #7
    Hmmm... i wonder what would happen to a mass' gravitational field if, in relation to the total extent of spacetime surrounding it, it had no motion. ie... an object's only motion is equivalent in direction and magnitude to the expansion of spacetime.

    My point being: is the distortion of spacetime reliant on mass AND acceleration, or simply mass?

    Would it even emit gravity waves?
  9. Apr 13, 2008 #8

    so does that mean that there is a gravitational Doppler effect?
  10. Apr 13, 2008 #9
    There is a difference between a static gravitational field and gravitational waves. Everything has a gravitational field. But only accelerating objects radiate gravitational waves and only if the motion is not spherically symmetric.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook