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

Relative velocity.

  1. Aug 21, 2008 #1

    Movement is relative I understand. Whether atom A passes atom B in empty space or B passes A is only a question of viewpoint. One can only be moving or still compared to something else. Somewhere at the edge of the visible universe everything is moving away from me with almost the speed of light. Is it then theoreticaly just as well to say that the edge of the universe i immovable and I am moving towards my own middle with almost the speed of light? (Of course I realize some practical problems here. I'm just wondering whether this would be a correct interpretation of the laws of physics :-D )

    best regards, Henrik
  2. jcsd
  3. Aug 21, 2008 #2
    The universe is not expanding in the sense that you are using. There are no edges of the universe, but it is finite. There is no middle of the universe.

    Although, here's an example that I think you're might be looking for. Let's say you and another object are in an isolated frame. To you, the object appears to be traveling at .5c, but in reality it doesn't matter whether you say the object is traveling at .5c, or to say that you are traveling at -.5c.

    We usually never assume this property because in our world/life, you are always aware of when you gain velocity, and so when something else appears to be traveling at v, and you are not aware of any force that may have changed your velocity, we assume that indeed that object is traveling at v. But again, it does not matter. As long as you treat the other relativistic transforms accordingly, you can exchange the velocity of an object, v, for the velocity of the 'stationary' observer, -v.
  4. Aug 21, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This part is OK.

    Actually a lot of stuff in the visible region is moving away with a speed greater than c. (The distance from us increases by more than 299792458 meters each second). This doesn't violate the "speed limit" of relativity, which says that nothing can have a faster speed than c relative to something in its vicinity. (There are other threads about this. I suggest you do a search).

    No, but you can say that a galaxy far away isn't moving and that you are moving relative to it. You can also say that neither of you is moving, but the the distance between you is still increasing fast because of the expansion of space.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook