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Q & A: falling light near Earth

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Most recent answer: 12/30/2017
Does a light beam fall 4.9 m in 1 second? Assume that it is being reflected back and forth between two parallel mirrors.
- Jeffrey Wetherhold (age 55)
Allentown, PA, USA

Yes, that's what happens. If the light didn't fall exactly like the mirrors, then you could notice the difference from within the mirror-box, and determine that you were in a gravitational field. That would violate the Equivalence Principal, the claim that no local measurement can determine a uniform gravitational field. That's a well-confirmed general physical law, the basis of General Relativity.

Mike W.

(published on 12/30/2017)

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