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Help understanding a SR equation!

  1. Oct 22, 2011 #1
    Hi all, I'm currently a stage 1 Physics student at uni; and in the process of learning Special Relativity.

    I was reading my textbook and came across this in the Time and Space section:

    Suppose an observer in frame S measures two flashbulbs going off at the same time t but at different x-coordinates x1 and x2. Then an observer in frame S` would measure the time interval t`1 - t`2 between the flashbulbs going to be:

    t`1 - t`2 = [itex]\gamma[/itex]((x2-x1)*(u/c)^2)

    I was wondering why it's t`1 - t`2 instead of the usual t`2 - t`1. I assume it's to prevent the final answer being negative? I feel like I'm being stupid asking this :|
  2. jcsd
  3. Oct 22, 2011 #2

    Doc Al

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    Staff: Mentor

    It's just to get rid of a minus sign. The full transformation is:
    [tex]\Delta t' = \gamma(\Delta t - v\Delta x/c^2)[/tex]

    Since Δt = 0, you get:
    [tex]\Delta t' = -\gamma(v\Delta x/c^2)[/tex]

    Where Δt' is the usual t'2 - t'1. But if you switch the order you can get rid of the minus sign on the right hand side.
  4. Oct 22, 2011 #3
    Yeah that makes sense now, cheers! :D
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