Einstein shift

According to the theory of relativity produced by Albert Einstein, photons leaving a massive light source such as a star will be retarded by the light source's gravity and lose energy in the process. This causes an increase in the light's wavelength λ, shifting the color of the light towards the red end of the spectrum, this effect is also known as gravitational redshift. For a star of radius R and mass M, the fractional increase in wavelength is:

<math>\frac{\Delta\lambda}{\lambda} = \frac{G}{c^2} \cdot \frac{M}{R}</math>

In which G is the gravitational constant and c is the speed of light. The coefficient G/c² = 7.414×10^-29cm/g. For the Sun, M = 2.3×10³³g and R = 1.394×10¹¹cm, so Δλ/λ = 1.23×10^-6. In other words, each spectral line should be shifted towards the red end of the spectrum by a little over one millionth of its original wavelength.

The gravitational redshift of the Earth can be detected using the Mossbauer effect. In addition, observation of much more massive and compact stars such as white dwarfs have shown that Einstein shift does occur and is within the correct order of magnitude.