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 N/A 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. Thanks.