How can something (ie. light) have volume and energy, but no mass?

Question

I’ve watched multiple YouTube videos and read a couple things online and now I have a headache. It still doesn’t make sense to me.
If photons have volume, then there can only be a finite number of photons in a given space, right? And once that limit is reached, why can’t I squeeze in one more photon? What is stopping me, the “walls” or “shell” of the photons? What are the walls/shells made of?

Every source I’ve looked at agrees that light is BOTH a wave AND a particle. I can understand why waves don’t have mass, but then what the hell is a “particle”? Every other elementary particle like quarks have mass, right?

In: 27

Answer 1

There are a few misconceptions to clear up here.

1. Photons are massless point like particles that have no volume.
2. Volume and mass are not correlated. Electrons are point like particles that have mass. They take up 0 space and yet exert gravity on other objects.

What you’re thinking of as volume is the interaction of quarks, gluons, and electrons. All of these are point like particles with mass that exert forces on one another. We can package them into something that has a “volume” (i.e a proton or an atom) but the constituent parts have zero volume in and of themselves.

Quarks and electrons are something called “fermions” and fermions have a rule where you cannot have two fermions in the same “state”. In practical terms that means as I push fermions closer together there’s a limit under which I cannot push anymore and they will resist this.

Photons however are bosons. This means they don’t follow this rule. You can push as many photons you want into the same state. They don’t push back. So you can have a many photons as you like “smushed together”. They have no volume.

An interesting side tidbit is that because photons move at the speed of light this implies that they have no mass. It turns out that things that move at the speed of light have no mass. Why? Because Einstein’s special theory of relativity says so and any other situation would be a violation of those equations. And at this point we have little evidence that those equations are wrong and mountains of evidence that they work.