A lift is falling freely under gravity. An observer in the lift has set up a laser pointer attached to the wall of the lift pointing in her coordinate systems at right angles so that the emerging beam is emitted normal to the wall.

 Bearing in mind the principle of equivalence, what is the pathway of the photons across the lift?

 What would an observer outside the lift standing on the Earth's surface observe about the light beam as the lift falls? It is a glass lift, so both observers can see the beam.

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The principle of equivalence in the context of physics and specifically general relativity states that locally the effects of gravity are the same as ... View the full answer