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Mobile communication with electromagnetic waves and theory of relativity

  1. Dec 26, 2009 #1
    Since the combined speed of electormagnetic waves can be 'c' and that they travel in space at 'c' leaves no component for time, hence the time on a watch on a beam of electromagnetic wave does not change at all.

    With this understanding, assume two stationary mobile phone users U1 and U2.

    U1's mobile phone emits an electromagnetic wave at time T1 which according to U2's watch reaches him at T2. However, as per the wave, it is still time T1.

    Considering spacetime to be a book with events in a region of space R being arranged as pages of the book according to the time at which they occured (T1 and T2 are 2 different pages of the book), U1 and U2 have moved to the page corresponding to time T2 while the wave always remains in the page correspoding to time T1.

    Now how are the two users U1 and U2 ever able to communicate with each other? The wave and the recepient are never on the same page of the spacetime book.
  2. jcsd
  3. Dec 26, 2009 #2


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    A watch could never travel at the speed of light...

    For that analogy to be applicable, there would have to be absolute time and absolute space - a "preferred" reference frame. Relativity showed that that is not the case.
  4. Dec 26, 2009 #3
    Ok, lets just leave out the watch. That was put in to indicate that no time has passed as per the wave and it still T1 according to it.

    Accepted that there is no absolute space and no absolute time, but there is absolute spacetime and this spacetime does have different events on difference slices of spacetime.
    Hence the two users would have moved to a different slice of spacetime
  5. Dec 26, 2009 #4
    While it is "still T1 per the wave" it's also true that it was "already T2 per the wave" when the wave was emitted. T1 and T2 are the same "page" per the wave, using your analogy.
  6. Dec 26, 2009 #5


    Staff: Mentor

    With a little bit of lag.
  7. Dec 26, 2009 #6


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    The point is that no physical system capable of measuring the passage of time (that's what we mean by "clock" or "watch" in relativity) could ever travel at the speed of light. A photon has no concept of time.
    Yeah, but then the question of whether or not two events are on the same slice depends on how you choose your slices. There is no one special "correct" way to slice spacetime. The book analogy is a bad - well, misleading - one because it makes you think that the particular way events are grouped on to pages of the book is The One True Way Of Slicing, but that's just not the case. (Everybody takes a while to get used to the concept :wink:)

    Al68 had a decent way of putting it, I think. I suppose you could say that from the photon's "perspective" (photons don't really have a perspective, but whatever), the entire book is all on one big piece of parchment. And as you can imagine, the concept of "pages" makes no sense when there's just one of them.
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