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?
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Photons don’t really have volume, not the way you think of it. To take one definition of volume, there is no box so small that a photon wouldn’t fit in it. In another sense, which is more meaningful in quantum mechanics, you can define the size of the area that a photon is “usually” found in. This idea turns into volume as things get big, but strikes me as different when small.
>If photons have volume, then there can only be a finite number of photons in a given space, right?
Not really. If you get enough photons packed right enough, they’ll start interacting in weird ways, interfering with each other and very, very eventually making a black hole, but in for all practical purposes, you can pack arbitrarily many photons into any given space.
>Every source I’ve looked at agrees that light is BOTH a wave AND a particle
No. Rather, light is something that sometimes behaves like a wave and sometimes behaves like a particle. “Particle” and “wave” are physical/mathematical descriptions which are meaningless on a “philosophical” level, but useful day-to-day.
>I can understand why waves don’t have mass
Things that act as waves absolutely can have mass. For example, an electron in an atom acts like a wave, and it has mass (9.1 x 10^-31 kg, plus a relativistic adjustment having to do with the energy it has, as though it was moving really fast, though “speed” and indeed “moving” don’t really make sense in this case – qmech can be unintuitive).
>but then what the hell is a “particle”?
“Something that acts like a particle”. Useful, right? Generally, a particle is something that has a well-defined, specific location (and hence also stuff like a well-defined velocity), a well-defined volume, a well-defined mass (even if this mass is zero), a well-defined shape, a well-defined elasticity (which defines how it acts when it bumps into other particles), etc. etc.
Note that “well-defined” doesn’t mean “we know it”. For example, while we have some ideas of the radius of an electron (it’s probably no smaller than 10^-22 m and no bigger than about 3 x 10^-15 ), we don’t really know it precisely.
>Every other elementary particle like quarks have mass, right?
Most of them, though gluons don’t, and electron neutrinos were also predicted not to have any – though experiments suggest that they do have a very, very, very small but non-zero amount of mass.
We haven’t addressed the other question in your title, which is how light can have energy. Light has no resting mass, but it also moves at the speed of light. As something’s speed gets closer and closer to the speed of light, its effective mass gets greater and greater (this is a consequence of Special Relativity). For a photon, you find its momentum and kinetic energy through a process that basically takes zero times infinity…and out pops some well-defined, finite number, like 1.6 x 10^-19 J and 5.3 x 10^-28 kg m/s . Crazy, eh?
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