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
[deleted]
>If photons have volume, then there can only be a finite number of photons in a given space, right?
That’s wrong, actually. Photons are a type of particle called a boson, and two (or more) bosons can occupy the same space.
The counterpart to a boson is a fermion. Fermions obey the Pauli exclusion principle, meaning they cannot occupy the same space or energy state.
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?
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.
One way to shift your thinking is that light is a wave.
Waves can pass through eachother, sit on top of eachother. The reflect, split, interfere and bend.
Waves are the transfer of energy through a medium (for light this is the electro-magnetic field).
When a physicist says light is a wave and a particle what is really meant is light is a specific thing, a “wavicle” that has properties of each. There is no macroscopic, common sense analog to this thing.
A singularity can have mass but no volume, right?
Don’t be mislead by the term “particle”. It’s giving you a picture of a photon of light like a tiny little ball, which doesn’t really work (for anything in the quantum realm, really).
Particle in this case more means that photons are a packet of energy that can only interact all at once. It’s still a vibration in the electromagnetic field even when it’s interacting as that packet. and just like a water wave, two photons can pass through the same location at the same time without an issue.
> If photons have volume, then there can only be a finite number of photons in a given space, right?
Photons don’t seem to touch each other, that is to say they seem to lack the property of touch with respect to each other. Take two water hoses and point them at each other and they will bounce and splash. Take two laser pointers and cross the beams and nothing happens.
> BOTH a wave AND a particle
Actually they BEHAVE like a wave and particle, photons aren’t a wave and particle.
Calling light both a wave and a particle is a tricky proposition. One person could tell you it’s both a particle and a wave and another could tell you that it’s neither. With added information, neither claim would be incorrect.
Here is the necessary information to make sense of it:
At the deepest level of accepted physics, it is an excitation in a quantum field. That’s all it is. That begs the question – what the hell is an excitation in a quantum field?
Fortunately, that question doesn’t need to be directly answered to understand the particle/wave relationship with light. All you need to know to get your head around this question is that both wave-like and particle-like behavior emerges from the dynamics of quantum interactions.
Thinking of light as a wave or particle is useful, but not universally applicable, so it doesn’t work as a description of light at its most fundamental level. It is still highly descriptive of how light works at other levels.
So is light a wave, a particle, both or neither? Yes.
Latest Answers