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Space like time like light like separations?

  1. Apr 22, 2005 #1

    I tried using the search function but it keeps timing out on me. Anyway I'm trying to find a good description of what is meant by space/time/light like separations, could someone help me out? :confused:

    Thanks :wink:
  2. jcsd
  3. Apr 22, 2005 #2


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    Consider two locations in spacetime, say (t1,x1) and (t2,x2) where the ts are time coordinates and the x's are space coordinated (you can think of them either as one dimensional numbers or as three-dimensional vectors). Suppose you send a light signal out from (t1,x1). There are three possibilities:
    1) The light arrives at space location x2 precisely at time t2. Then the two four-dimensional locations are said to be lightlike separated. An example; if you see a star, then the star and your eye are lightlike separated.
    2) The light arrives at x2 before t2. Then (t1,x1) and (t2,x2) are timelike separated, and we say that (t2,x2) is inside the lightcone of (t1,x1). Timelike events can interact, since even a slower-than-light cause can reach x2 from x1 in time (t2-t1).
    3) The light arrives at x2 after t2. Then (t1x1) and (t2,x2) are spacelike separated and cannot causally interact, since a causal influence would have to travel faster than ligh (impossible for real causes) to get from x1 to x2 in time (t2-t1).
  4. Apr 22, 2005 #3
    Thanks that helped!
  5. Jan 16, 2011 #4


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    Just want to thanks selfAdjoint for your simple but great descriptions.
  6. Jan 16, 2011 #5
    Just thought I would add that timelike separated events always happen in the same order according to any inertial observer (to conserve causality), while different observers with different relative velocities can disagree on the temporal order of spacelike events, but this is not a problem because spacelike events are not causally connected.
  7. Jan 16, 2011 #6
    Jumping between the quantum physics folders and the relativity folder is like jumping from one multiple personality disorder to another.

    In quantum electrodynamics, spacelike connectivity is a requirement.
  8. Jan 17, 2011 #7
    Space like separations are events always separated in space for every frame.

    Time like spearations are events always seperated in time for every frame.
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