GRAVITATIONAL REDSHIFT: TESTING RELATIVITY THEORY
In his general theory of relativity Einstein showed that for a distant observer an object in an intense gravitational field would be contracted, have gained mass, and have slowed down its clock time. Consequently, if the object is an atom, time dilation should also play a role when it emits a photon. Relativity predicts that the wavelength of the photon is lengthened, or shifted to the red, by an amount that depends on the strength of the gravitational field. The relative change in wavelength (del lambda / lambda) is proportional to the mass of the attracting body divided by its radius.
This effect, known as the gravitational redshift, has practical astronomical interest because it occurs when a photon of light escapes from a star. If the star's gravitational field is sufficiently intense, we can measure the change in wavelength. We cannot easily measure this effect for the sun, but for a white dwarf of solar mass and small size (where the mass divided by the radius is large) the gravitational redshift is of measurable
size. It has been observed for several white dwarfs in binary systems; this is made possible because, by using the companion's spectrum, we can differentiate between the gravitational redshift of the spectral lines and the Doppler shift produced by the system's radial velocity. The measured redshifts agree satisfactorily with those predicted by theory. And the gravitational redshift has been verified with even greater accuracy in a laboratory experiment.
