Let’s Stop Saying that Photons are Massless and Instead Say that they are “Restmassless”.
Laymen and a good few physicists are confused about whether the mass of an object, such as an electron, proton, or a spacecraft, increases with speed. They are also confused about whether a photon has mass, and whether energy has mass, and what E = mc² means.
According to what I’ll call the “invariant mass” (“IM”) school of thought, photons are “massless” or “have no rest mass” or “have a rest mass that is equal to zero”. A photon cannot be at rest, so it’s not clear to me what the latter statement even means.
The reason given by non IM-ers for why an object cannot reach the speed of light is that its mass would have to become infinite in order for that to happen, and that an infinite amount of energy would be needed to accelerate it to that speed.
After years of being confused when reading what physicists had written, and even when talking to them, a few months ago I found out that there is the IM school a of thought, which is a “fashion”, if you like, primarily among particle physicists, or at least led by them, and to some extent among other professional physicists who use relativity in their work, to say that “mass” means “invariant mass” (AKA rest mass AKA proper mass), and that mass does NOT increase with speed. If only “rest mass” were substituted for “invariant mass” there would be a lot less confusion, because “rest mass” is a much more widely known term.
According to these IM school physicists (AKA “professional relativists”, a term that is popular on Stack Exchange but not so much elsewhere), Albert Einstein was wrong when he said that mass increases with speed. Not wrong about how the world worked, but wrong about how physicists should talk about mass. Wrong about what the word “mass” should refer to. Einstein did seem to switch back and forth in his later years between being anti and pro IM school, it must be said.
Most particle physicists are in the IM school. To the average particle physicist, it has become almost a sin to say that mass increases with speed, or that a photon has mass. They say it’s not a good way to look at it. It’s not a matter of what experimental results you will get, there’s no disagreement on that.
There are however quite a few physicists who take a different view, some of them extremely respected. Richard Feynman is one, and Lewis Epstein (author of my favorite relativity book, “Relativity Visualized”, is another. They always disagreed emphatically with the IM school of thought.
As far as I know, no one besides me has pointed out the purely terminological issue of whether it would be good idea for these IM school particle physicists to say “invariant mass”, or “proper mass”, or “rest mass”, rather than the confusing and ambiguous “mass”, which is almost always understood to mean “relativistic mass” by nonexperts.
I propose that whenever the context involves relativity, we avoid the ambiguous “mass” and use “rest mass” or “restmass”, because its meaning is well-understood by everyone, and because it’s shorter than the alternatives, if that is what is meant. The could be m subscript nought. It could also be called “m sub nought”, “m nought” or “emnought”.
If “relativistic mass” is being referred to, then that is what it should be called. The symbol could be m subscript v. It could be called “m vee” or “m sub vee” or “emvee”.
The photon and other particles that are presently, by most IM school particle physicists, said to have momentum but not mass, and to be “massless”, could be instead said to be “rest massless” or “restmassless”, or “emnoughtless”. Instead of “masslessness” one could say “restmasslessness”.
As things stand, it is extremely difficult to find out about the existence of this almost political dispute within physics, because members of each group tend not to mention the opposing view, and in many cases don’t even know that there is an opposing view. A professor or graduate of physics knows only the way that they were taught. Once you know of its existence, you can easily find out about it, it must be said. On the other hand, it’s not easy to make sense of the IM school’s position, unless you know advanced math. It’s easy to see the drawbacks of the IMS position when one is talking in English about relativity. It’s as if making the calculations easier is all the IMS cares about, and they would rather not talk about how difficult it makes talking about relativity and thinking about it in English. Or so it seems to me.
Many people are confused, including a good few physics professors about the meaning of “mass”. Many believe both that mass increases with speed, and that a photon has no mass, which is to switch from one definition of mass to another in mid sentence, and to switch from IM to non IM. Only relativistic mass increases with speed (rest/proper/invariant mass does not) while a photon only has zero rest/proper/invariant mass (its relativistic mass is not zero).
Others, I expect, think that mass does not increase with speed, while photons have mass. This confusion is probably less common than the one described in the previous paragraph.
If my proposals were implemented, it would greatly benefit the Earth’s hundreds of millions of intelligent laymen and millions of physics graduates who are confused about the meaning of “mass” and therefore about whether mass increases with speed, and whether a photon has mass, and related matters. What would the cost be? The thousands of scientists that use relativity would be slightly inconvenienced, perhaps, by the hassle of saying exactly what they mean, and uttering an extra syllable once in a while, and writing an extra symbol once in a while. Another idea is for IM-ers, when talking non colleagues to substitute “rest mass” for “mass”, or at least clarify that by “mass”, “rest mass” is meant.
Another likely benefit is that by choosing either “rest mass” or “relativistic mass” you are getting the issue out in the open, while saying “mass” ambiguously, sweeps the issue under the carpet. If it really is better, as most particle physicists claim, to banish all thought of relativistic mass, then what better way to spread the word? As things stand, nearly all laymen, and a vast majority of physics graduates have no clue that the experts in relativity have a problem with the concept of relativistic mass.
The reason the particle physicists might think you should wash your mouth out with soap if you say that mass increases with speed, or even mention “relativistic mass”, is not easy to understand. It seems to be that there is something called a Minkowski diagram, that is a sort of spacetime diagram that represents a geometrical explanation of relativity (both the special and the general theories) that suits a particle physicists for two reasons. The first reason is that it makes their calculations easier, and the second is that they think it is an elegant way to understand relativity. I would tend to take their word for it that it makes for easier calculations. I’m not convinced that it really is elegant, or if it is, whether that alleged elegance outweighs the somewhat unintuitive and difficult nature of Minkowski diagrams. I can’t help wondering whether it is a case of making a (perceived) necessity into a virtue. Furthermore, it is alleged that Minkowski diagrams work better without relativistic mass.
Lewis Epstein relegates an explanation of Minkowski diagrams to an appendix in his “Relativity Visualized”. Epstein uses throughout his book, another kind of diagram, sometimes called “spacepropertime diagrams”.
I have only one complaint about Relativity Visualized, and that is that nowhere in the book, not even in an appendix, does Epstein mention the controversy about what the word “mass” should/does mean. He uses mass to mean relativistic mass, and makes no remark on that. This omission caused me to be severely handicapped in my reading and conversations regarding physics, for may years. Epstein had successfully explained the basics of relativity, and for that I am eternally grateful, but really should have let me know that “mass” was a word with two meanings out there in the world.
It may be mathematically convenient to banish relativistic mass, and even be mathematically elegant, and maybe even conceptually elegant, or not, I don’t know. But to me at least it is very strange that the IMS says a photon has no mass, but when trapped between mirrors, it contributes to the mass of the system, and when absorbed by one of the mirrors (if not a perfect a mirror) it likewise adds to the mass of the system. It is also strange that when a photon is emitted, it carries away mass with it. The weight and the inertia and the gravitational field (curvature of spacetime) of a moving object increases if it’s in a gravitational field, but not it’s mass. The IMS also claims that a photon has momentum but it doesn’t have mass. To most physics graduates, momentum is mass times velocity, so this is a very unintuitive idea. Non IMS says a photon has both momentum and mass.
Appendix
This article follows on and likely contains everything from this one from a month ago plus some other stuff. As you can see it was and then deleted from SE (or rather “hidden” which means you can see it only if you have the link to it. It is something like an unlisted video on YouTube, I think):
