In short: charge is not the only thing that defines a particle. Although the charge of the neutrinos is zero their spin differs.
A longer form of this answer is here. The answers go a bit more into detail on why “zero charge” isn’t precisely correct either but I’m not sure if that goes too deep for what you’re interested in!
I’m going to copy-paste the exact relevant bit here:
For each neutrino, there also exists a corresponding antiparticle, called an antineutrino, which also has no electric charge and half-integer spin. They are distinguished from the neutrinos by having opposite signs of lepton number and chirality. As of 2016, no evidence has been found for any other difference.
I knew about the chirality difference, that there are no right-handed neutrinos nor left-handed antineutrinos (or something along those lines, breaking what was thought to be a fundamental parity or symmetry), but what puzzled me was that I thought the charge difference was the one big fundamental difference between matter and antimatter, and suddenly tonight the neutrino question popped into my head. At the very least I knew that it’s not a mass/negative mass type of difference.
Now as for that bit that says “opposite signs of lepton number”… I’d never even heard of this concept or characteristic, until right now.
Lepton number is an observationally conserved quantity. As far as I know there’s no fundamental reason for it to be conserved (and indeed there are searches for physics beyond the standard model that would violate it) but it’s been found to generally be conserved in reactions so far. Lepton particles have a lepton number of +1, lepton antiparticles have -1.
There’s a similar conserved quantity known as the baryon number, with a similar definition. Protons and neutrons (baryons) have values of +1, anti-protons and anti-neutrons are -1.
An example: consider the beta- decay of a neutron, baryon number +1 and lepton number 0. It emits a proton (baryon number +1), an electron (lepton +1), and an electron anti-neutrino (lepton -1). Total lepton number of the decay products is 1-1=0, so the value is conserved.
On further thought, this is really strange.
I can visualize a negatively charged electron and a positively charged positron making contact and annihilating, how the minus and the plus cancel each other.But what is it about neutrinos and antineutrinos that make them cancel out when they come into contact? What is it about their positive and negative characteristics that can make them go “poof!” in a burst of photons?
A visualization you could try (this obviously isn’t going to match the physical reality necessarily) is what would happen if you had two vortex phenomena (like tornadoes or whirlpools) spinning in opposite directions and they collided?
Imagine if you dug a hole in the ground. In order to dig the hole deeper, the soil needs to be piled up somewhere else. Then, imagine if you decided to move the dirt pile on top of the hole - what would happen? The soil would fill the hole, and you’re left with nothing. We are simply returning to the original state of things.
That’s the core idea of particles and antiparticles. At the very crux of things, there is only energy. But sometimes, the energy is able to disturb a quantum field and that produces a particle and antiparticle. The fact that the charge of the particle/antiparticle pair is opposite is not the central property of this pair. Rather, the central property that distinguishes them is more fundamental. They are fundamental opposites, and as a consequence of that fact, then they have opposite charges. They also have opposite spins for the same reason. To put it more briefly, they aren’t opposite because they have different charges. They have different charges because they are opposite.
When a particle and antiparticle touch, because they are fundamentally opposites, they will cancel each other out, and the energy that went into creating them gets released.
In the way you describe it, electromagnetic charge is kinda easy to visualize. But when we get into Weak Force interactions, that’s when these other, much more abstract features come into play.
For example, when you say “Spin” you don’t mean regular ol’ Angular Momentum, I’m guessing, but other weird types like Isospin or I-don’t-know-what.
This is all fascinating stuff, truly. And way beyond my pay grade, lol!
I found that this is one of the few areas where aphantasia is a strong advantage :D
I can’t help you finding better metaphors beside “it’s like charge but different” as I have “accepted” the whole quantum topic as math that for some random reason can be used to make predictions which accidently correlate to our reality…
Maybe a helpful visualization is one of the precursors to quantum field theory, Dirac’s sea.
The idea is that you can think of a particle as sitting on top of the surface of the “sea” while an anti particle is represented by a hole in the surface, large enough to fit one particle. When a particle encounters such a hole, it naturally drops down into it and settles there. This essentially “destroys” both the particle and the hole (the anti particle).
So essentially the opposite charge, spin, etc of a particle and anti particle are a consequence of their opposition in their fields, not the cause for the annihilation.
(Not a scientist, grains of salt and all that).
I live under the impression that we don’t conclusively know, although some headway was made. There is a chance that neutrinos are their own antiparticles. I think the right term to start a search on the topic is Majorana particles. This theory was featured in Project Hail Mary by Andy Weir, BTW.
I apologise, I don’t have time for a more exhaustive explnation, I would have to study it again first. If you want, I can try to have a look at it later.
If that is so, if neutrinos really are their own antiparticle, would that theoretically mean that there is no such thing as neutrino annihilation?
Like photons, which are their own antiparticle, and don’t annihilate on contact with each other, but those are bosons with a completely different spin, and also have zero rest mass, unlike neutrinos/antineutrinos, which DO have mass, but seem to somehow draw it out from something other than the Higgs Field.Huh, I never really thought about boson antiparticles, thanks for driving me to it. I did a little digging and I’m happy to report that what I wrote seems to be accurate, it isn’t known whether neutrinos are their own antiparticles or not. The term Majorana particle only applies to fermions, which I didn’t know. As for photon-photon annihilation, why do you think it can’t happen? Annihilation is when 2 particles collide and produce a bunch of other particles, often photons, but not necessarily. Does that not happen to photons? For possible neutrino-neutrino annihilation, my quick uninformed search suggested that possible neutrinoless double beta decay may be interpreted as annihilation of neutrinos. The wiki particle says it would require change of the neutrino to a right-handed one, which seems like a requirement for annihilation anyway? I don’t know, I really barely know anything about this stuff. But it seems that if neutrino is its own antiparticle, its annihilation with itself is not obviously out of the question. I had no idea we don’t know where they take their mass. That’s very, very interesting, thank you!
Photons can interfere creatively or destructively with each other, but they don’t annihilate on contact, they do not interact with each other that way. I am also pretty sure the same applies for gluons, and I have no idea what the deal is with the two Weak Force bosons, which have mass even though they are bosons!
The Weak Force is a very, very strange, abstract, lovely thing, and if you have a coin with Electromagnetism on one side, the Weak Force on the other side, and flip it with enough energy, that’s when they realized that at higher levels it’s actually the Electroweak Force.
Would you mind defining what an annihilation is? What I read (which isn’t much, admittedly) sounded like it’s just a particle and antiparticle interacting in a way that makes them disappear and other particles appear, while conserving a momentum and charge of the whole overall interaction. How is it fundamentally different from, let’s say, two high-energy photons colliding and creating an electron-positron pair? I’m not saying it isn’t, I’m just curious why and how.
All I know is that when matter and antimatter particles annihilate, what that usually means is that they become photons, so that their rest mass - what we usually mean when we say matter itself - is gone, having turned into pure energy, mainly gamma rays I believe.
The other part that you allude to, has to do with how at the quantum level, processes are time-symmetric or time-reversible, look exactly the same if you view their behavior forwards or in reverse, you cannot tell which way it’s going. Antimatter behaves just like matter, but from our perspective like an egg un-breaking, or a car un-crashing, or an ice cube un-melting.
What’s puzzling me is how photons, other bosons like gluons or majorana particles are supposed to be their own anti-particle, how does that affect their time-related behavior and interactions with themselves and other particles, I have no idea… at least not yet.
In fact, I hadn’t even thought about this strange question until just now, and I love it!
