# Is “Invariant Mass” a Misnomer? Should We be Saying “Rest Mass” Instead?

Invariant mass increases when a photon is absorbed, or is caught between two mirrors, or a clock is wound up without any other change, or its temperature increases due to absorption of energy, or if it is set rotating about an axis through its center of mass.

On the other hand, if its (linear) velocity increases, the rest mass does not increase, and neither does the invariant mass.

In fact, it is widely acknowledged that “invariant mass” is just another term for “rest mass”.

I propose that we start saying “rest mass” a bit more, and “invariant mass” a bit less, especially when the intended audience are laymen or physics undergraduates.

“Invariant mass” is such a confusing term. “Rest mass” is clearly understood.

Furthermore, I suggest that, in the context of relativity, we don’t shorten “rest mass”, or “invariant mass”, or “relativistic mass”, to “mass”, as this is very, very confusing. In relativity mass has two meanings, “rest/invariant mass” and “relativistic mass”.

Also, I propose that rest mass be represented by m subscript nought, and relativistic mass (if used) by m subscript v. I would say that, especially in popular works or undergraduate textbooks, in the context of relativity plain “mass”, and plain “m” by avoided, because it isn’t clear what kind of mass is meant, and laymen and physics undergraduates tend to get confused as a result.

Believe it or not, there is a fashion among particle physicists (who are the main professional users of relativity theory) and many other professional relativity experts to just say “mass”, and by that to mean “invariant mass” (which they tend to insist is different from rest mass, although I’ve never understood exactly how). Furthermore, they use the letter m to represent it, and not m subscript 0.

Lewis Epstein’s “Relativity Visualized” is awesome, and I highly recommend it. By “mass”, he means “relativistic mass”. My one complaint about the book is that it doesn’t warn the reader that many top experts think “mass” means “invariant mass” and don’t bother to clarify that they don’t mean “relativistic mass”, which is what most laymen and physics graduates mean by “mass”. I was confused for years when reading about relativity, and even talking to professional physicists didn’t allow me to realize that “mass” had two meanings. It seems that physicists simply assume you know what they mean. Thus I have debated whether the photon has mass, saying it does, while someone has insisted it doesn’t, with, seemingly, neither of us realizing that we are talking about things.

According to the “mass means invariant mass” school of thought, you should never speak of relativistic mass, and Einstein was wrong to have spoken of it. They say that energy does not have mass, and that E = mc² doesn’t mean what most people including most physics graduates think it does. They say that mass does not increase with velocity. They say photons have no mass, while conceding that they have energy and momentum.