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Homework Help: How to show that light travels through space, but not time

  1. Jun 9, 2012 #1
    1. The problem statement, all variables and given/known data
    Use relativistic equations and γ to demonstrate the following:
    b) Light travels through space but not in time

    2. Relevant equations
    Lorentz factor γ = 1 / √(1 - [v/c]^2)

    Time dilation t = γ t°

    Length contraction L = γ L°

    Mass relativity m = γ m°

    E = m c^2

    3. The attempt at a solution

    To be honest, I'm not exactly sure how to approach this. I don't quite understand the concept behind what the question is really asking for. Furthermore, I don't know how I can use these equations to prove that light does not travel through time.
    Last edited: Jun 9, 2012
  2. jcsd
  3. Jun 10, 2012 #2
    Consider a particle traveling at a uniform velocity relative to the observer. Compare a fixed time interval t0, perhaps 1 second, in that particle's reference frame to the corresponding time interval in the observer's reference frame. Ie., pretend the particle is carrying a clock that ticks once every second.
    How long does the observer have to wait to observe one tick if the particle travels at 0.9c ? At 0.95c? At 0.99c? 0.999c?
    Does this behavior continue if we make the fixed unit of time for a single tick smaller? What does this imply about theoretical particles carrying these same clocks that travel at c?
    With the formulas you have, you can only make a weak implication.
  4. Jun 10, 2012 #3
    Thank you. I can't quite wrap my head around this, but I think I have a good enough idea to somehow explain this. I'll be sure to ask my teacher when I can.
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