I’d like to hear that too. Like you, I’m an interested amateur, but nothing more.
One of the shames about the shrinkage of SDMB, and the growth of other online outlets for expertise, is that although we have a deep bench of professional experts on a lot of topics right here right now, it’s a pale shadow compared to what it once was. As well more of us retire than begin our careers each day. So slowly we’re all becoming former practitioners in our respective fields. Which means both some forgetting of some of what we once knew, and some increasing amount of never learning about what’s changed since.
If you posit that for whatever reason at the very outset there was some excess percentage of one kind over the other, then over time the two kinds of matter would meet and annihilate one another. With no mechanism to make more of either, then after sufficient time there would be zero, or nearly zero, of one kind. Plus whatever was the primordial excess of the other for whom there isn’t, and never was, an annihilating partner particle. And that primordial excess would be pretty much all that remains in the whole universe for us to see now.
If that process of burning the minority component down to zero was still in progress someplace or everyplace, we’d see it happening. Because it’s plenty energetic. We don’t see that annihilation happening nearly anywhere, much less everywhere. IOW, to a whole bunch of decimal places, all we see is matter, matter everywhere. So the mutual annihilation process has (almost entirely) run its course.
The big mystery AFAIK is why there was, immediately at the time of the Big Bang, or maybe at the end of Inflation, that slight excess of what we call “matter” over what we call “antimatter”.
As well, as vast and as massive as the Universe is, it’s darn near empty percentagewise. So the excess was a small amount percentagewise. I don’t know whether there’re any theories or educated guesses on how much matter and antimatter there was at the beginning. IOW, if annihilation wasn’t a thing, would the universe now be 1% more dense with all the matter + antimatter still present, or 1 jillion percent more dense?
I know I don’t know. But somebody might have some good ideas.
The definition of a quantum-scale “particle” is kinda fuzzy up at our macro level. Which also means the idea of “contact” is kinda fuzzy too. IANA expert even a little bit, but AIUI, the “particles”, which are spatially tiny, annihilate on “contact” and not before.
IOW, it’s not something analogous to bringing two subcritical radioactive masses slowly together where they’re exchanging ever more energy ever more energetically on a macroscopic scale until something big happens.
You mostly won’t get gamma rays from heavier particles, and of the ones you do, they’ll mostly be… 0.511 MeV. Electrons and positrons annihilate neatly directly into photons, because the electromagnetic interaction is the strongest interaction they’re subject to. For something like protons and antiprotons, though, they’re also subject to the Strong Interaction, which is going to overwhelm anything that happens via the Electromagnetic. So the first thing that’ll happen is your proton and antiproton will re-arrange themselves into three pions, which will either be three neutral pions (1 in 3 chance), or one each of positive, negative, and neutral (2 in 3 chance). The neutral pions will then eventually decay into gamma rays (more energetic than electron-positron gamma rays), but the charged pions will mostly decay into muons (and neutrinos), and the muons will then decay into electrons or positrons (and neutrinos), and finally any electrons and positrons you end up with will eventually find each other, and you’ll get electron-positron annihilation again. End result is that most of the energy ends up in neutrinos, not photons at all, and most of the photons will be at electron-positron energy.
And yes, of course we’ve looked for this. I’ve read plenty of papers that talk about this search having actually been done.
Another big mystery, IMHO, is why the matter versions of hadrons and leptons seem to have survived in equal amounts. Why is it that protons outnumbered anti-protons and electrons outnumbered positrons in the same amounts? I’ve read about various speculative hypotheses, but none with any solid evidence.
One is that matter and anti-matter really were formed in equal amounts, but that for whatever reason protons were more stable than anti-protons and we’re just waiting for all the protons to decay into positrons and pions over a time period of something greater than 10^34 years. At that point the pions will decay rapidly into neutrinos and the positrons and electrons will annihilate, and we’ll be left with a universe full of photons, neutrinos, dark matter, and dark energy.
Another is that in the very early universe, something like within the first few million Plank times from 10^-44 seconds to 10^-36 seconds, that the hypothetical quantum field for axion dark matter particles (assuming that’s what dark matter turns out to be) was rotating one way rather than the other, which favored matter over anti-matter.
FWIW, I certainly don’t check the board nearly as regularly as I did in past years (past decades?) due to life circumstances, but my DMs (with email notifs on) are always open if anyone ever wants to summon thoughts from this specific physicist.
There are a lot of directions here. I’ll sketch some of them, and I’m happy to go into more details on any.
Annihilation products. This is the one you were exploring above, namely that the interface between matter and antimatter domains would produce photons. As you expect, this is indeed well-trodden territory. No excesses in suitably redshifted photon fluxes are seen, and based on observed distributions of matter in the universe and constraints on matter densities in the intergalactic medium, current data is sufficient to exclude the presence of matter/antimatter boundaries in the observable universe.
Cosmic rays. Our little pocket of space is bombarded by cosmic rays. These come from numerous processes at solar system, galactic, and intergalactic scales. Most of these cosmic rays are boring things like electrons and protons, but antimatter can also appear. Barring new physics, this antimatter comes from collisions of ordinary cosmic rays with other regular matter in the universe or in violent processes like supernovae and gamma ray bursts. The more complicated the antimatter gets, though, the harder it is to make using mundane physics. Antiprotons? Not that hard to make. Antideuterons? Sure. Antihelium-3? Pretty rare. Antihelium-4? Very rare.
The space-based experiment AMS is designed to measure cosmic ray fluxes. Among its science goals is measuring the antimatter cosmic ray fluxes near Earth to look for anomalies. An important note, though, is that estimates for the rates of the “rare” stuff are hard to build, and different attempts at this lead to widely varying predictions. It’s just a very hard problem given available astrophysical data and our current understanding of the most violent processes and regions in the universe. Some of the AMS measurements are within the realm of “looks normal” and others are kind of weird, but nothing definitive. The first piece of potential new physics that would enter the fray is dark matter, as dark matter annihilation can lead to additional antimatter fluxes, so a lot of literature has been produced trying to tie AMS observations to dark matter theories.
There is a lot more to be said here, but I’ll pause except to note that a new balloon-based experiment called GAPS will make similar measurements soon.
Early universe dynamics. If you try to separate matter and antimatter into distinct regions, you need a mechanism to do that. There is a great corpus of cosmological data that gives insight into the dynamics of (anti)matter in the early universe. Cosmic microwave background radiation, large-scale structure (e.g., superclusters, filaments), and more have well-measured detailed patterns that match exceeding well what would be expected if the random, entropic processes in the early universe, together with the structure-forming influence of gravity, proceeded as expected. In other words, there is no evidence that anomalous large-scale dynamics were present. One specific conclusion is that, if there were antimatter domains, they would have to still be comingled with matter domains at least until a time well after recombination, and they would be quite noticeable then.
Baryon/photon ratio and abundances of light elements. These closely related quantities measurable through cosmological observations point to two key conclusions: there is a massive deficit of baryons relative to photons in the universe, indicating significant early (anti)baryon annihilation, and there was not “late”-time antimatter present to mess up the balance of primordial light elements, for which observations match expectations quite well.
More “how” than “why”: We know the Standard Model has all the qualitative mechanisms needed to take a hot soup of equal parts matter and antimatter and turn it into a soup with a slight excess of matter. However, the Standard Model cannot get the job done quantitatively.
In particular, quarks are the only particles so far known to violate charge-parity symmetry, but they don’t provide enough of this crucial ingredient to get to the level of imbalance needed in the early universe (about one part per ten billion). Many particle physics experiments are searching for new sources of charge-parity symmetry violation to fill this quantitative gap.
As to “why”: the anthropic principle could be invoked here. If an asymmetry weren’t present, we wouldn’t be here asking the question.
Matter/antimatter annihilation is very well studied in the lab, as it’s not hard to make antimatter for study. The details of any particular annihilation will depend on the particles involved and how “hard” they’re hitting each other. Using your language, it’s definitely more the “sudden upon contact” picture than any sort of fizzing sensation upon approach. If instead of individual particles you wanted to contemplate a hypothetical block of anti-material in empty space smacking into a regular matter block, then the first moments of contact would release enough energy to vaporize the two blocks entirely, halting any further macroscopic annihilation.
The process doesn’t work in this tidy way, unfortunately. The valence quarks don’t just regroup. A mess of annihilation happens in tandem with the highly stochastic process of hadronization, and the resultant particle list and corresponding momenta can be pretty much anything allowed by conservation laws. You might get two daughter particles, or you might get eight. You can get pions, kaons, rhos, omegas, etc. Your example of the \pi^+\pi^-\pi^0 final state happens in only 7% of annihilations, for example. The most common cases are those with lots of pions present, since they are the lightest mesons and since you can freely add neutral pions without any charge conservation concerns. With 2 GeV of energy to get “spent”, it’s actually sort of hard to only get three particles coming out.
Or … if there actually are additional spatial dimensions curled up … really close but traveling differently through those dimensions than regular matter so virtually never the twain be met, or observed … except maybe by gravity I guess, since in that imagining gravity travel through those dimensions?
So close but still effectively more distant than we can see …
So far as anyone can tell, charge conservation really is absolutely absolute. So a surplus of positive particles of one type had to be balanced by a surplus of negatives of some other type.
Assuming we’re talking about a low-KE annihilation (where the only source of energy is the rest mass of the protons themselves), wouldn’t pions be the only option, and not more than a handful of them?
I assume the genesis of matter and antimatter at the Big Bang is a known quantity amongst Astrophysicists and Quantum Cosmologists, but is there any interpretation where particle/antiparticle pairs was working at the universal scale?
By that, I mean, could matter be assigned it’s own universe…ours…and antimatter have a universe of it’s own?
Two universes separated by a void so no annihilation takes place?
Yes, I meant “why” in the sense of mechanistic cause = process, not “why” in the sense of epistemic cause = motivationed reason for that process existing. Sloppy wording on my part. “How” is much better.
Thank you overall. It’s refreshing to read the crisp prose of somebody who really knows their stuff deeply and so can explain the simplified gloss neatly.
Suppose you had 2 100# sacks of flower, black flower and white flower. Once activated they would annihilate one another. Then you mixed them together and activated them inside of a large sphere in Outerspace. They immediately start the annihilation process. What would the distribution of the survivors look like after things settled down?
You get all the energy out that you put in, and that includes the rest mass of the particles (times c2, because our units are stupid) plus their kinetic energy. So, if you didn’t have to spend any energy to make the bulk masses of those particles and/or get them up to whatever speed they have, then you “win”.
We’ve been down that road, though. If you want to revisit such speculation, I’m happy to, but as a starting point you might want to revisit the problems we discussed last time. To summarize that discussion (wow, ten years ago): one can’t arbitrarily manipulate language in a story-like fashion and have it make sense as a model of physics. We understand the properties of antimatter extremely well. We make it everyday and study it everyday. It’s not something that acts in any quirky way. It’s acts just like boring ol’ matter, aside from one nuance in the weak force of the Standard Model that means matter and antimatter interaction rates can, in some specific cases, differ.
Yes, I was referring to low-KE (essentially at rest) annihilation. The proton and antiproton together represent 1.9 GeV of rest mass. A pion has a rest mass around 0.14 GeV. In principle there’s enough oomph in the annihilation to make thirteen pions, though such a crazy final state would be heavily disfavored for phase space reasons. More typical is half a dozen or so, with some energy left over for outgoing pion kinetic energy. But there are loads of others mesons light enough to be produced in the annihilation, though of course without as many at once.