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Interchangeability of time and space

  1. Jul 30, 2011 #1
    Recently, when I was talking with my friends, one of them noted that as an object approaches c, there takes places an exchange in the characteristics of time and space. In simple words, what does this mean, if true?
  2. jcsd
  3. Jul 30, 2011 #2


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    Nothing like that happens when an object approaches c. For example, even if the relative velocity between observers A and B is 0.999999999999999c, A and B still agree on whether the relationship between events P and Q is, say, timelike.

    Something like that does happen when you cross the event horizon of a black hole.

    Does your friend really mean "approaches," or does s/he mean "surpasses?" If the latter, then the question doesn't really make sense, because a material observer can't surpass c by any continuous process of acceleration. We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=511170
  4. Jul 30, 2011 #3
    Thanks. He surely meant approaches. Now I dunno if he was bluffing or so. I can't recollect his exact words but what I read after I Googled was like, "in the early 1900s, Minkowski and Einstein found that space and time are interchangeable and replaced them with spacetime for rigorous scientific purposes."

    How far is that valid, do you think?
  5. Jul 30, 2011 #4
    Also, here is what was trying to convey, which I suppose is highly improbable
    "at velocities that we deal with in normal life, we have the freedom in the space co-ordinates, but time always flows by in one specific direction. So, as we approach higher velocities, tending to c, is it that we have the freedom of our time axis, but not for the space co-ordinate axis?"

    Can you make any sense out of it?
  6. Jul 30, 2011 #5
    Oh wait. You are Benjamin Crowell, aren't you? OMG :O Your lightandmatter texts are the gospel truth for me, here in India! I'd better go and read your book on relativity again!
  7. Jul 30, 2011 #6


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    Well, there's a certain symmetry between the way space and time are treated in relativity, but they're not totally interchangeable.

    This way of saying it doesn't really make sense to me, because it seems to be saying that at low speeds X happens, but at high speeds Y happens. An observer going at .99999999999c in some frame can also be considered as being at rest in her own frame.
  8. Jul 30, 2011 #7


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    The inter - changing of space - like coordinates and time - like coordinates has a specific meaning in GR. As Ben already stated, past the EH of a black hole the space - like and time - like coordinates inter - change in the metric. I don't quite understand the statement you got from your google search regarding the inter - changing; maybe it is talking about proper time consisting of both time - like and space - like components.
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