The four-frequency of a massless particle, such as a photon, is a four-vector defined by

is the photon's frequency is a unit vector in the direction of the photon's motion. The four-frequency of a photon is always a future-pointing and null vector. An observer moving with four-velocity will observe a frequency

is the Minkowski inner-product (+−−−) with covariant components.

Closely related to the four-frequency is the four-wavevector defined by

is the wavelength) by

See also

• Four-vector

• Wave vector

References