# Clarification of rest mass

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)?

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 – Qmechanic♦ May 18 '12 at 23:53 Okay, from what I gathered, it is invariance under change of reference frame. I am not "interested" in this, I just need to know for an exam in 2 days so I can't spend time researching this stuff. hence the question. – Adam Rubinson May 19 '12 at 0:12

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.

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ah, I was thinking wiki meant it was form-invariant, but it says it is Lorentz invariant... – Adam Rubinson May 19 '12 at 0:34
@dmckee: As I said elsewhere, it is not reasonable to call rest-mass "mass" unless you already understand that "c=1" so that you can call the "relativistic mass" the "energy". Otherwise, you get confusions about the mass of an electron going fast in a circle in a magnetic field, which you can deflect by a force perpendicular to the circle, and find the Newtonian mass has increased. When you think mass and energy are different things, then you need different names for the energy when it is considered as energy, and when it is considered as mass. It is considered as mass more often than not. – Ron Maimon May 19 '12 at 2:35
I think it will be useful to add that the rest mass is the analogue of the length in a three dimensional vector. In three dimensions the length is invariant under translations and rotations, and similarly in these four dimensions the rest mass is invariant under Lorenz transformations. – anna v May 19 '12 at 3:55
@Ron: I disagree. "Relativistic mass" is a kludge to maintain the form of certain Newtonian equations, and its modest convenience is overshadowed by the confusion it engenders. Those electrons are not Newtonian, and it does not help to pretend you can fix it by treating $\gamma m$ as a property of the particle when it mixes a particle property with a frame of reference property. Better to simply say that $F = \frac{\mathrm{d}\vec{p}}{\mathrm{d}t} = \frac{\mathrm{d}}{\mathrm{d}t}(\gamma m \vec{v})$ which gets the same result. – dmckee May 19 '12 at 19:01
I think this would be the point at which any further discussion of the issue goes to Physics Chat. – David Zaslavsky May 20 '12 at 2:55