= Rest frame
{title2=$\mathbf p=0$}
The rest frame of a massive particle is an <inertial frame> in which its spatial <momentum> is zero. Its <four-momentum> is then $(mc,\mathbf0)$ and its energy is $mc^2$. Such a frame exists for a future-directed timelike four-momentum: the <Lorentz transformation> with boost velocity $\mathbf v=c^2\mathbf p/E$ makes the spatial momentum zero, and the <mass shell> relation implies $|\mathbf v|<c$. A massless particle has no rest frame, because $E=c|\mathbf p|$ would require a boost with speed $c$.
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