Gravitational redshift
This video looks at the idea of gravitational redshift and how it describes the change in frequency of a light signal using the scalar product of a four wave vector with the four velocity of an observer who is located at the point of emission or reception. It shows how this equation can also describe the energy of a particle or photon in both special and general relativity and goes on to use this result to determine the ratio of the frequency of a light wave at the points of emission and reception in various space-times where the emitter and receiver are moving or stationary.