The equation E = pc holds true

The relativistic relation between energy $E$ and momentum $p$ for a particle of rest mass $m_0$ is given by

$E^2 = p^2c^2 + m_0^2c^4$

For a photon, the rest mass $m_0 = 0$. Substituting this in the equation, we get

$E^2 = p^2c^2 \Rightarrow E = pc$

For an electron, the rest mass $m_0 \neq 0$. Therefore, $m_0^2c^4 > 0$, which gives

$E = \sqrt{p^2c^2 + m_0^2c^4} > pc$

Thus, the equation $E = pc$ holds true for a photon but not for an electron.