# Insights Do Photons have Mass? - Comments

1. Aug 24, 2015

### ZapperZ

Staff Emeritus
2. Aug 24, 2015

### Staff: Mentor

I never saw the relativistic mass used in a recent (not decades old) professional environment.

3. Aug 24, 2015

### ZapperZ

Staff Emeritus
Just as a clarification, Greg has been graciously reposting my old Blog entries, and a few other FAQs that I had made, to the Insight section. So unfortunately, many of these require a bit more "refinement", especially on the typos, grammatical errors, etc...etc, of which I'm too darn lazy to make right now.

So this is why I am not sure why this FAQ appears in the High Energy Physics section. It probably belongs in the General Physics section or even Relativity forum.

But I would also like to include a post that I've written on the issue of "relativistic mass". There is already a FAQ on this, but I want to include references on why the term "relativistic mass" should not be used anymore, and why many are starting to shy away from it.

Zz.

Edit: It was moved. Thanks!

4. Aug 24, 2015

### Staff: Mentor

Good job on the article. Maybe you could help clarify a point for me.

You said, "The invariant mass of a particle is defined as the total energy of the particle measured in the particle’s rest frame divided by the speed of light squared."

In a recent PF thread, (https://www.physicsforums.com/threads/mass-of-an-electron.826015/#post-5187800) I learned that an atom with electrons in an excited state has (slightly) more rest mass than the same atom in the ground state. From that, I leap to the conclusion that a hot object has more mass than when it was cold, a spring gains mass as it is stretched, a molecule has different mass than its consituents, any chemical reaction must absorb or release energy and therefore does not conserve mass, and any solid structure has different mass than its constituents. The unifying principle is that the rest mass energy is any energy (regardless of type), that remains with the object when momentum is zero.

I recognize that the mass differences I'm talking about are tiny; almost too small to measure.

Thermal energy is tricky because it has to do with motion. But thinking of F=ma, if I accelerate a hot object i must accelerate its thermal energy with it.

I guess my question is this. Is there any difference between what you called "total energy of the particle measured in the particle’s rest frame" and what the others called "internal energy" in the other thread?

A second related question. For purposes of gravitation, all forms of energy gravitate equally, correct? That includes kinetic energy and the energy of massless particles. If the mass of a black hole was converted to massless energy in the singularity, we wouldn't be able to tell in terms of the external gravitational field, correct? F=mMG/R*R should be more properly written in terms of energies.

5. Aug 24, 2015

### Staff: Mentor

They are notable for nuclear reactions, they might become accessible for chemical reactions within the next decade or two.
If they move in the same way, yes.

The composition of the interior of black holes is not known, and irrelevant in general relativity.

6. Aug 24, 2015