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Why can't we see the light before we turn on the bulb?

  1. May 25, 2010 #1
    Pardon me if there is an obvious answer, I have not spent too much time thinking about it.

    Having stumbled upon the "light cone." (Just the spacetime diagram centered around the event of turning on a light source.) I have the following question:

    I am not going to be too detailed, but why can't see the light from a light bulb we switched on before we switched it on, if the light emits in the past direction as well as the future direction?
  2. jcsd
  3. May 25, 2010 #2


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    Gold Member

    It does not emit in the past direction.

    Only events within the past light cone can affect the light. Anything outside the light cone is causally unconnected to the light bulb.
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