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So I have only just been introduced to the concept of rest mass in Special Relativity.

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?

Is the rest mass simply the "mass" of a particle in inertial frame of reference which has the particle at it's origin (assuming the particle is actually in an inertial reference frame)?

Qmechanic
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Adam Rubinson
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1 Answers1

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Rest mass (also known as the mass [1] ) is the Lorentz invariant absolute value of the particle's energy momentum 4-vector.

$$ m^2 = \mathbf{p}^2 = E^2 - \vec{p}^2 $$

If you don't use $c = 1$ units, that's $$m^2 c^4 = E^2 - (\vec{p}c)^2$$

Lorentz invariant means "the same in all inertial reference frames".

[1] Despite the continued use of the distinction between "rest mass" and "relativistic mass" in introductory texts particle physicists, cosmologist, and other professionals in relativity recognize only one mass--the one the intro tets call the "rest mass".

"Relativistic mass" represents a particular way to factor the energy of a moving particle, but is not otherwise particularly useful. We really, really prefer to use manifestly Lorentz invariant quantities.

dmckee --- ex-moderator kitten
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