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**Unformatted text preview: **g = 0): obs obs hf E = . Energy conservation tells us that the initial energy (at the emitter) must equal the final energy (at the detector) . Thus, ) / 1 ( 2 c gd hf E hf E source obs obs + = = = and hence ( ) 2 / 1 f c gd f obs + and ( ) 2 / 1 c gd obs . These are the same answers we arrived at using the equivalence principle. From this point of view, the photon frequency increased because its kinetic energy increased as it dropped from high to low gravitational potential. Earth M e R e Inertial frame in a gravity field emitter detector Photon Direction E source E observer d This approach gives the right answer for gd/c 2 << 1!...

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