Solution

Problem

$13.5.7.$ The light emitted from the surface of a star reaches the observer with a lower frequency than when it was emitted. The change in frequency is greater the more massive the star and the smaller its radius. What explains this effect? Why is it called redshift?

Solution

According to General Relativity, the gravitational field near massive objects curves spacetime and affects the behavior of photons. When light is emitted from the surface of a star, it loses energy while overcoming the gravitational pull. Since the energy of a photon is related to its frequency by $E = h\nu$, a loss of energy leads to a decrease in frequency and an increase in wavelength.

The frequency shift goes toward lower values. In the optical range, this means a shift of the emission spectrum toward the red region (wavelengths increase). That is why the effect is called redshift.