In physics and general relativity, gravitational redshift is the phenomenon that electromagnetic waves or photons travelling out of a gravitational well lose energy. This loss of energy corresponds to a decrease in the wave frequency and increase in the wavelength, known more generally as a redshift. The opposite effect, in which photons gain energy when travelling into a gravitational well, is known as a gravitational blueshift. The effect was first described by Albert Einstein in 1907, eight years before his publication of the full theory of relativity. Observing the gravitational redshift in the Solar System is one of the classical tests of general relativity.