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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!