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I Photon Path in Free Falling lift?

  1. Mar 13, 2017 #1
    I just started reading general theory of relativity. I have some elementary questions. Not an english speaker so bear with me.

    I am reading the thought experiment which describes path of a photon in a free falling lift. For an observer inside the lift, the photon path is a straight line. But for an observer standing on earth, the photon will travel a curved path, which is the shortest path taken by it to maintain a constant distance from the floor of the lift. My question is how does this relate to the bending of spacetime near earth? Will the photon still travel a curved path if there is no lift and only an observer on earth? How does the frame of reference of the lift effect the path of photon as seen by an observer on earth? Please help me if I am missing a point here.
  2. jcsd
  3. Mar 13, 2017 #2


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    The point is that the path must be straight in the free falling frame (see Equivalence Principle), so it must curve in the ground frame.
  4. Mar 13, 2017 #3


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    First of all, you (or the textbook author) really should not be talking about photons in the GR setting unless you are ready to do quantum field theory in a curved space-time. Whenever they write photon, substitute "light pulse".

    A frame of reference can never have any effect on a physical process. They might describe physical processes differently but the process itself is the same. In this case, it is the path of a light pulse in a curved space-time.
  5. Mar 13, 2017 #4
    There is nothing special about it being a"lift." You can imagine the same experiment with the lift replaced by a thin wire cage. You can then make the wires fewer and fewer in number and thinner and thinner. There is no physical effect of the lift other than that it is in free fall.
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