Equivalence Principle: Photon Fall Near Earth's Surface

k3r0
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Homework Statement


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).

Homework Equations


N/A


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.

Thanks.
 
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k3r0 said:

Homework Statement


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).

Homework Equations


N/A


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.

Thanks.

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.
 
Thanks, I've got it now!
 

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