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What is a 'discrete reversal of the time direction'?

  1. May 18, 2014 #1
    In my book it says that the Lorentz group O(3,1) includes 'discrete reversals of the time direction'.

    What on earth does this mean?

    So I get that time reversal is making time go backward instead of forward, but what is the word 'discrete' doing there?

    Thanks in advance!
  2. jcsd
  3. May 18, 2014 #2


    Staff: Mentor

    Because time is either reversed or it's not; there's no continuous group of transformations that reverse time a little bit, then a little bit more, etc. There's just one single transformation that reverses time.
  4. May 18, 2014 #3


    User Avatar
    Science Advisor

    ##t\rightarrow -t##. It's discrete simply because it is not in the connected component of the identity. In other words you cannot reach the time reversal by a continuous series of infinitesimal Lorentz transformations (rotations + boosts) starting from the identity. If you're familiar with QM then time reversal symmetry corresponds to an anti-unitary representation of the Lorentz group.
  5. May 18, 2014 #4
    Ohh ok yes that makes perfect sense, Thanks!:D
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