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Why does a gamma ray change energy when it moves through Earth’s gravitational field?

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Figure 1: Spectrometer utilizing Mössbauer effect concept Effect is usually observed by measuring transmission of gamma rays from radioactive source through absorber containing resonant isotope. — Mössbauer effect Figure 1: *Spectrometer utilizing Mössbauer effect concept* Effect is usually observed by measuring transmission of gamma rays from radioactive source through absorber containing resonant isotope.

A gamma ray changes energy when moving through Earth’s gravitational field because it behaves as though it possesses an effective mass of $m = E / c^{2}$ , where $E$ is the photon's energy and $c$ is the speed of light.41

According to general relativity, electromagnetic radiation is subject to gravitational forces. As a gamma ray travels "up" away from Earth, it must expend energy to work against the gravitational field. Because the speed of light remains constant, the photon cannot lose energy by slowing down; instead, it loses energy by decreasing its frequency, a phenomenon known as gravitational redshift. Conversely, light traveling toward Earth gains energy, resulting in a frequency increase or gravitational blueshift.231

This effect was famously demonstrated in a laboratory setting by physicists Robert Pound and Glen Rebka, who used the high energy precision of the Mössbauer effect to measure the minute change in energy of gamma rays over a vertical distance of 23 meters.342