= Photon energy
{title2=$E=h\nu=\hbar\omega=hc/\lambda$}
{wiki}
The <energy> carried by one <photon> is proportional to its <frequency> $\nu$, with proportionality constant the <Planck constant> $h$. The equivalent forms use <angular frequency> $\omega=2\pi\nu$ and vacuum <wavelength> $\lambda=c/\nu$. Different observers can measure different <photon energies> because they measure different <frequencies>; the <photon> remains massless. In signature $(+---)$, an observer with <four-velocity> $u^\mu$ measures $E=p_\mu u^\mu$, where $p^\mu$ is the photon's <four-momentum> and $u^\mu u_\mu=c^2$.
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