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

Is theory of relativity really relative ?

  1. Sep 17, 2014 #1
    Einstein made the theory based on Maxwell saying that electromagnetic waves travel with same speed from the view of all inertial & non - inertial frame. Then with what speed relative to a person sitting on light does light travel ?
     
  2. jcsd
  3. Sep 17, 2014 #2

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    How does one sit on light?

    In fact no person can travel at the speed of light, so the question itself is a contradiction. :)
     
  4. Sep 17, 2014 #3

    jtbell

    User Avatar

    Staff: Mentor

    Such a person is impossible.

    Even leaving out material people or objects, there is no inertial reference frame in which a light beam/ray/photon is at rest, so it's meaningless (in the context of relativity theory) to ask what things "look like" from that point of view.
     
  5. Sep 17, 2014 #4
    I'm nt saying to sit on it.. Jst have light as your frame of reference...
     
  6. Sep 17, 2014 #5
    and when u say nothing can travel at speed of light, my dear friend, we've neutrinos having speed greater than that of light. einstein's theory is on the verge of being proved wrong
     
  7. Sep 17, 2014 #6
    You shouldn't be in theoretical physics if you make statements like that.
     
  8. Sep 18, 2014 #7

    ShayanJ

    User Avatar
    Gold Member

    You don't follow scientific news. That was a technical problem leading to more errors in the measurements and neutrinos weren't moving faster than light.

    You mean theoretical physics is where you can say anything contradictory?!
    Imagine two physicists at rest relative to each other standing far from each other. One of them shines a laser at the other one which the other one surely receives. Now imagine another physicist passing by at the speed of light in the opposite direction to the direction of motion of the laser pulse. According to him, the pulse never reaches the other guy. But the fact that the pulse reaches the other guy or not shouldn't depend on the frame of reference. So no one can go at the speed of light(or faster!).

    About a photon's frame, according to one of the SR's postulates, light moves with speed c in vacuum relative to any inertial frame. Now if we assume that we can associate to light, an inertial frame, a frame which light is at rest with respect to it, the postulate says that light should go at speed c relative to it. CONTRADICTION!
     
    Last edited: Sep 18, 2014
  9. Sep 18, 2014 #8
    This sounds like it might be a useful argument. Unfortunately I can't make any sense of it, can you confirm that you have said exactly what you mean?
     
  10. Sep 18, 2014 #9

    ShayanJ

    User Avatar
    Gold Member

    Yes.
    Where is the first place that you feel its hazy? I mean, as you read, there should be somewhere that you start to feel you don't understand it. Where is that place?
     
  11. Sep 18, 2014 #10
    Sort of the third sentence (passing by whom? both, I suppose), but definitely the fourth (there are three guys, who is the "other" guy?). By the fifth sentence I am just reading words without understanding them.

    Could you possibly draw a diagram?
     
  12. Sep 18, 2014 #11

    Nugatory

    User Avatar

    Staff: Mentor

  13. Sep 18, 2014 #12

    ShayanJ

    User Avatar
    Gold Member

    Oh...right...I should explain more clearly.
    There are two stationary physicists S1 and S2 located far from each other. And there is a third physicist moving at the speed of light in a straight line connecting S2 and S1, from S2 to S1, who we call M.
    S1 shines a laser at S2 which, by experience, we know S2 receives.
    But in the frame of M, the laser pulse doesn't reach S2.
    But the fact that the laser pulse reaches S2 or not shouldn't depend on the frame of reference.
     
    Last edited: Sep 18, 2014
  14. Sep 18, 2014 #13
    Yeah, I believe so. I was trying to understand what he said above but I couldn't. There were just words put together in a meaningless way. @Shyan please try to write in proper English.
     
  15. Sep 18, 2014 #14
    OK, thanks for clarifying, I understand the layout now. I do have another question though; how does M detect whether S2 receives the pulse?
     
  16. Sep 18, 2014 #15

    ChrisVer

    User Avatar
    Gold Member

    I think the easiest way to see that rest frame is impossible, is by looking at the photon that satisfies [itex]E=pc[/itex] or [itex]m=0[/itex],
    Now a rest frame for the photon ([itex]p=0[/itex]) would also imply [itex]E=0[/itex]. So you have a massless, no-energy, no-momentum particle? That's nothingness.
     
    Last edited: Sep 18, 2014
  17. Sep 18, 2014 #16

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There's no inertial frame of reference that's comoving with light, in special relativity or general relativity (see the FAQ that Nugatory linked to), so what theory would you like us to use to answer your question?

    If you keep making claims like that, you will find that we have very little tolerance for them in this forum.
     
  18. Sep 18, 2014 #17

    ChrisVer

    User Avatar
    Gold Member

    As for M, that's not even possible... M is causally disconnected from S2... no signal sent from S2's worldlines can reach M...no?
     
    Last edited: Sep 18, 2014
  19. Sep 18, 2014 #18

    ShayanJ

    User Avatar
    Gold Member

    Yeah, Now that I think, I myself doubt that argument. But I read it in a book which I don't remember what book was it right now and I can't find it.
     
  20. Sep 18, 2014 #19

    Dale

    Staff: Mentor

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is theory of relativity really relative ?
  1. Theory of Relativity (Replies: 2)

  2. Theory of Relativity (Replies: 10)

  3. Theory of Relativity (Replies: 27)

  4. Theory of Relativity? (Replies: 22)

Loading...