# Relativistic mass

mfb
Mentor
With c=1, the units match.
(Nearly?) everywhere where relativistic effects are relevant, unit systems with c=1 are used.

With c=1, the units match.
(Nearly?) everywhere where relativistic effects are relevant, unit systems with c=1 are used.
You can always set the proportionality constant to 1, in any equation; but commonly "mass" is specified in kg.
I wonder, why do so many people find it too much work to speak of "proper mass"? In fact it's really a non-issue: people should just be clear what they mean.

Last edited:
Certainly not in SR, and this is explained in the earlier given references.
I wrote it in order to remark this fact for those who can read these pages (by experience there are many people interested in these concept and who don't have a university or however a proper formation in the subject of relativity).
Nonsense - as the physics FAQ explains, "longitudinal" and "transverse" mass preceded "relativistic mass".
- http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html
But the meaning is what I wrote.
Obviously the units don't match, they are physically different concepts just as force is a different concept from acceleration.
No, because the "m" in F = m*a is certainly not a constant (hint: body 1 has mass m1, body 2 has mass m2, ecc.)
If one needs propaganda based on such bogus arguments, then the situation is very poor.
If you think just a litte bit more about it, you discover they're not.

mfb
Mentor
but commonly "mass" is specified in kg.
Not in atomic, nuclear and particle physics. Every particle physicist knows the masses of the fundamental particles to some approximation - but just in eV, which is a unit of energy. Sure, you can convert it to kg or say "eV/c^2", but I think you see the connection to energy.

I wonder, why do so many people find it too much work to speak of "proper mass"? In fact it's really a non-issue: people should just be clear what they mean.
"Mass" is clear, why should you call "mass" "proper mass" with exactly the same meaning?

Nugatory
Mentor
"Mass" is clear, why should you call "mass" "proper mass" with exactly the same meaning?

Although "mass" ought to be clear, there's a fair amount of observational data that suggests that it's not. We're standing in some of it right now:uhh:

And with tongue out of cheek, I find an appealing symmetry in being able to speak of "proper mass", the same way that I speak of "proper length" and "proper time" when there is any chance of confusion with the less fundamental frame-dependent length and time.

mfb
Mentor
Ok, more precise: The meaning of "mass" is clear for all particle physicists.
I think the same is true for nuclear physics and atomic physics and I would expect it in astronomy, too.

Any other fields which deal with relativistic effects frequently?
Solid-state physics can have very low effective electron masses, but I think those are invariant (and different from the electron energy), too.

Ok, more precise: The meaning of "mass" is clear for all particle physicists. [..]
That is imprecise. The different meanings of "mass" in the literature are clear for many physicists, but by far not clear to all. Some particle physicists use both meanings, just as is common for "time". It's because of unfamiliarity that erroneous statements are made of the kind that again appeared in this thread.

mfb
Mentor
Some particle physicists use both meanings
I never met one who called E/c^2 "mass".
I think "time" was always used as proper time (time in the frame of the particle), too. Event timestamps are in the frame of the detector, of course.