## Gravitational Red ShiftAccording to the principle of equivalence from general relativity,any frequency shift which can be shown to arise from acceleration of a radiating source could also be produced by the appropriate gravitational field. Thus the expected shift in radiation frequency in a gravitational field can be related to the relativistic doppler shift experienced from an accelerating light source. ## Gravity and the photon
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## Harvard Tower Experiment
By just using the expression for gravitational potential energy near the Earth, and using the m in the relativistic energy expression, the gain in energy for a photon which falls distance h is Comparing the energy shifts on the upward and downward paths gives a predicted difference The measured difference was The success of this experiment owed much to the care of Pound and Rebka in preparing the source. They electroplated cobalt-57 onto the surface of a thin sheet of iron and then heated the combination at 1220 K for an hour. The heat treatment caused the cobalt to diffuse into the iron to a depth of about 300 nm or 1000 atomic spacings. The source was then mounted on the cone of a loudspeaker driven at 10Hz to sweep the source velocity in a sinusoidal variation. The detector was a thin sheet of iron about 14 micrometers thick which was also annealed. The heat treatments were found to be crucial in obtaining high resolution. | Index General relativity ideas References | ||

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## Scout Rocket ExperimentIn 1976 the Smithsonian Astrophysical Observatory sent aloft a Scout rocket to a height of 10,000 km. At this height, a clock should run 4.5 parts in 10^{10} faster than one on the Earth. During two hours of free fall from its maximum height, the rocket transmitted timing pulses from a maser oscillator which acted as a clock and which was compared with a similar clock on the ground. This result confirmed the gravitational time dilation relationship to within 0.01%. | Index General relativity ideas | ||

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## Gravitational Time DilationA clock in a gravitational field runs more slowly according to the gravitational time dilation relationship from general relativity
This time dilation is about 1 part in 10
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## Time Dilation on EarthThe gravitational time dilation expression has such a small second term in the denominator that excessive numerical accuracy is required to evaluate it directly. Using a binomial expansion : so that the first approximation to the time expression is The numerical values were calculated using g = 9.8 m/s | Index General relativity ideas | ||

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