Why do matter and antimatter annihilate?
Why do matter and antimatter annihilate?
From forming bound states to normal scattering, many possibilities abound for matter-antimatter interactions. So why do they annihilate?
From forming bound states to normal scattering, many possibilities abound for matter-antimatter interactions. So why do they annihilate? There’s a quantum reason we simply can’t avoid.
You're viewing a single thread.
I'm a smoothbrain, so I like to think about it as them simply canceling each other out. What I'm more curious about though, is why there's so much matter compared to antimatter.
You’re not alone; matter-antimatter asymmetry is one of the big open questions in physics. Most particle processes treat matter and antimatter identically, but there are a few areas where matter and antimatter have slightly different interactions. These occurrences are violations of Charge Parity symmetry aka CP Violation.
There must have been a certain amount of CP violation during the early phases of the Big Bang to explain our matter-dominated universe. But the known amounts of CP Violation are nowhere near enough to explain the asymmetry in matter and antimatter. There are some proposed mechanisms that would violate CP symmetry in sufficient quantities, but these haven’t been experimentally observed. There are ongoing searches to detect these processes, or related processes that would be possible if these existed. Neutrinoless double beta decay searches are one example of these detection efforts.
In summary, there’s a guaranteed Nobel Prize to whoever can answer your question.
I never thought I'd read the words "CP violation" and actually be interested and intrigued instead of disgusted.
Yeah it’s one of those terms that’s unfortunately been co-opted for another definition. Definitely made some of my google searches in grad school feel icky… The physics terminology came first though!
I work on a 0nuBB search doing detector R&D, this is spot on, but has 2 extra components. The three elements needed for explaining the asymmetry are:
- CP Violation
- Lepton or Baryon Number violation
- Interactions out of thermal equilibrium
These are the Sakharov conditions for Baryogenesis/Leptogenesis. #1 has been observed via the weak interaction but not in large enough quantities and is not observed via strong interactions, #2 is what proton decay and 0nuBB searches look for, and #3 can be, at least partially, explained by the expansion of the universe as a non-equilibrium interaction.
To get from leptogenesis to baryogenesis requires theretical physics I only barely understand using particles call sphaelerons that convert leptons to baryons.
Are there other theoretical interactions or consequences of interest if neutrinos turn out to be their own antiparticle? That idea is blowing my mind.
There absolutely are, but I’m not super familiar with all of the consequences of majorana neutrinos. /u/drail@fedia.io might be able to provide a better answer. My background is experimental nuclear physics, so I’m familiar a lot of experiments searching for beyond the standard model physics, but less so with the theory motivation.
One consequence of neutrinos being their own antiparticles is that it breaks lepton number conservation. This also breaks chiral symmetry, since all neutrinos are right-handed and anti-neutrinos are left-handed. This observation would also imply that neutrinos have mass - which is assumed but would be a really big deal to prove.
I heard the subway guy got in trouble for a CP Violation
That's what we genuinely don't know. Based on the standard model, it should be in equal parts.
Part of me is still half-convinced that there are whole galaxy clusters of antimatter that are simply too far away from other clusters to produce any noticeable gamma rays, and the reason they didn't interact near the beginning of the universe is the same reason the whole thing didn't collapse into a super massive black hole: we don't know yet, but probably along the same lines as dark energy. A lot of it did probably interact though and that's where a lot of the CMB comes from.
I'm definitely a lay person though, I'm sure an actual physicist can tell me that's definitely not the case, I just don't know why not yet.
My personal idea/hope is that there is some other dimension of spacetime over which the big bang had directionality, emitting matter and antimatter across different poles, and that's why. That'd also mean there's an anti-universe, which is why I like the idea.
In terms of the galaxies, I believe there's enough of an observable difference that I think we would be able to detect antimatter clusters, or similar, based on emission lines but I'm not 100% on that. Huge annihilation events from colliding galaxies and clusters would have massive energy signatures unlike anything else but the frequency of this would determine how likely it would be to see the evidence.
Don't colliding galaxies mostly not actually touch? I thought there's so much space between everything it's almost entirely gravitational interactions. I'd assume almost no huge annihilation events from that, or extremely low frequency.
The stars and planets, yes, but there is a lot of very diffuse gas that does collide
Despite space being “empty” there’s still a surprising amount of stuff streaming through it. There are protons, electrons, carbon nuclei, etc constantly slamming into the Earth’s atmosphere, producing showers of radiation. These cosmic rays are the reason so many sensitive physics experiments ( like dark matter and neutrinoless double beta decay searches) are located deep underground. The earth is a good shield against these cosmic backgrounds.
Even if there was an “isolated” antimatter galaxy, it would get bombarded with matter in the form of cosmic rays. The annihilation photons are a really distinct signal that would be hard to miss. There are a number of gamma ray telescopes in space that map out sources of gammas, and they would have detected an antimatter galaxy if it existed.
If the antimatter galaxies are so far away that they’re beyond the visible universe, then there’s still the big question of why there was a segregation of matter and antimatter early on.
Oops responded to the wrong comment
My personal idea/hope is that there is some other dimension of spacetime over which the big bang had directionality, emitting matter and antimatter across different poles, and that's why. That'd also mean there's an anti-universe, which is why I like the idea.
In terms of the galaxies, I believe there's enough of an observable difference that I think we would be able to detect antimatter clusters, or similar, based on emission lines but I'm not 100% on that. Huge annihilation events from colliding galaxies and clusters would have massive energy signatures unlike anything else but the frequency of this would determine how likely it would be to see the evidence.
There aren't. Even in "empty" space, there's about one atom per cubic meter, enough for a small amount of annihilation. We haven't detected any regions bordered by gamma ray emissions that would indicate a matter-antimatter boundary.
https://en.wikipedia.org/wiki/Baryon_asymmetry#Regions_of_the_universe_where_antimatter_dominates
so I like to think about it as them simply canceling each other out.
No, that's the result.
annihilation results in a large energy release. so nothing is actually disappearing. changing form maybe. I'm guessing at the big bang matter/anti-matter went opposite directions and we just can't see that half. not speculating about symmetry. just a large amount of anti-matter beyond observational light-speed limits. speculation