# Do we assume that the rest mass of a fundamental particle is constant in all inertial reference frames? i.e. is the rest mass of an electron if it is travelling at constant velocity c/2 (relative to the distant stars) the same as the rest mass of the electron if it is travelling at velocity 0 relative to the distant stars?

Do we assume that the rest mass of a fundamental particle is constant in all inertial reference frames? i.e. is the rest mass of an electron if it is travelling at constant velocity c/2 (relative to the distant stars) the same as the rest mass of the electron if it is travelling at velocity 0 relative to the distant stars?
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Farbwolkenw
Rest mass is the Lorentz invariant absolute value of the particle's energy momentum 4-vector.
${m}^{2}={\mathbf{p}}^{2}={E}^{2}-{\stackrel{\to }{p}}^{2}$
If you don't use $c=1$ units, that's
${m}^{2}{c}^{4}={E}^{2}-\left(\stackrel{\to }{p}c{\right)}^{2}$
Lorentz invariant means "the same in all inertial reference frames".