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Equivalence Principle

  1. Jan 30, 2014 #1
    1. The problem statement, all variables and given/known data
    A photon near the surface of the Earth travels a horizontal distance of 3 km. How far (in meters) does the photon 'fall' in this time? (Hint: think equivalence principle).

    2. Relevant equations

    3. The attempt at a solution
    My understanding of the equivalence principle is that experiments carried out in a 'free fall' frame will have the same results as an experiment done floating in space (far from a large body). I'm confused because the photon near the surface of earth is in neither of these situations -- so would the answer be that it doesn't 'fall' at all? I would expect it to travel horizontally only.

  2. jcsd
  3. Jan 30, 2014 #2


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    The usual picture is to imagine an experiment carried out in an elevator (3km wide), where you shoot a beam of photons straight across the elevator. If you carry this out in space it will travel straight across. If you are in the same elevator at the earth's surface in free fall, accelerating towards the center of the earth, from the view point of someone in the elevator it will also travel straight across. You are supposed to think of how it looks from the view point of someone who is not in the elevator and standing on the earth's surface.
  4. Jan 30, 2014 #3
    Thanks, I've got it now!
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