The four-frequency of a massless particle, such as a photon, is a four-vector defined by N a = ( ν , ν n ^ ) {\displaystyle N^{a}=\left(\nu ,\nu {\hat {\mathbf {n} }}\right)} where ν {\displaystyle \nu } is the photon's frequency and n ^ {\displaystyle {\hat {\mathbf {n} }}} 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.
The four-frequency of a massless particle, such as a photon, is a four-vector defined by N a = ( ν , ν n ^ ) {\displaystyle N^{a}=\left(\nu ,\nu {\hat {\mathbf {n} }}\right)} where ν {\displaystyle \nu } is the photon's frequency and n ^ {\displaystyle {\hat {\mathbf {n} }}} 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.
Source: Wikipedia "Four-frequency" · CC BY-SA 4.0
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