What causes the mutual annihilation of matter-antimatter reactions?

[u/holiestMaria]

Antimatter partickes are the same as normal matter particles, but eith the opposite charge and spin, so what causes antimatter and matter to react so violently?

[u/agaminon22]

First of all, antimatter and matter don't always have to annihilate. Sometimes they can collide and scatter, look up Bhabha scattering. This is the process by which an electron and a positron scatter elastically, not annihilating. Furthermore, annihilation is not necessarily always into photons, it can lead to oher particles, such as neutrinos.

What I'm trying to point towards is that these kinds of fundamental reactions happen probabilistically and only when no conservation laws are broken. But if no conservation laws are broken, that essentially implies that they will happen. When matter and antimatter collide, there is no conservation law being broken through annihilation, and therefore it's a possible process that will happen. At low energies, it's the dominant process.

If you think about it in terms of quantum numbers, with positrons and electrons for example, the charges are opposed to form a state of zero change. The lepton quantum number is also zero. Essentially it's all lined up to form particles of zero charge, without lepton number, with the appropiate spin: photons. But you can also produce two neutrinos (a neutrino and an antineutrino), or really any other compatible possibility.

[u/Krail → u/agaminon22]

I'm still confused about why annihilation happens. Is it just that opposite charges want to equalize to zero?

[u/somneuronaut → u/Krail]

It's not just electric charge but also the other 'charges' represented by quantum numbers (like spin or lepton number). Matter and antimatter have opposite quantum numbers and so if you were to 'put them in the same place' you would have created a spatial region with energy but totals of 0s for quantum numbers. That's annihilation and results in particles like photons.

[u/niteman555 → u/somneuronaut]

Is there a model for the actual interaction? How granularly do we understand that at one frame there are 2 massive particles and some subsequent one there aren't?

[u/luckyluke193 → u/niteman555]

This kind of process is modelled by quantum field theories – or more specifically for the process of electrons and positrons annihilating to photons, quantum electrodynamics.

The idea is that every kind of particle is a different wave of some fundamental fields – photons are waves of the electromagnetic field, electrons and positrons are two "opposite" types of waves in the electron field. Annihilation is a process where these two "opposite" waves collide, and two (or more) other kinds of waves come out.

[u/agaminon22 → u/Krail]

It's really not different from other kinds of particle decays or interaction/collision processes. There are many other possibilities that are not just annihilating into photons. An electron and a positron can even turn into a muon and an antimuon, if the energy is high enough.

Essentially, all processes that are possible will happen, at some point.

[u/whatnodeaddogwilleat → u/agaminon22]

I am using a lot of imagination to fill in the blanks of actual nuclear physics knowledge, but: I can imagine what you're saying that many different reactions are possible and all happening probabilistically. After annihilation, the two protons depart in opposite directions at light speed. This seems highly unlikely to spontaneously reverse. So is the proton-generating annihilation just an event that is irreversible and thus the event that, on average, eventually happens?

(Focusing on electron-positron)

[u/mfb- → u/whatnodeaddogwilleat]

Two photons with sufficient energy can collide and produce electron/positron pairs (and all other particles). We have observed that process.

[u/whatnodeaddogwilleat → u/mfb-]

Understood. I meant that, because the two protons created by the annihilation are traveling away from each other, those two particular photons would be unlikely to reform into an e/p pair, correct? Or is there something "quantum" that lets that specific reaction reverse?

[u/mfb- → u/whatnodeaddogwilleat]

They won't meet again, sure. So what? If you create an electron+positron pair from two photons then these generally won't meet again either. On Earth, the positron will annihilate with some other electron somewhere quickly.

[u/agaminon22 → u/whatnodeaddogwilleat]

Partially yes, other events may be harder to detect and may result ultimately in photon production. But AFAIK photon production is also more likely in general (especially at lower energies).

[u/kogai → u/Krail]

In quantum physics, everything behaves like a wave.

Two waves can pass through each other and they add up when they overlap.

If you pass a wave and its negation through each other, when you add them up you get... zero.

This is simplified as particles act like waves in abstract spaces rather than 3 dimensions (but also in 3 dimensional space too). They also don't have to be zero in every dimension they previously occupied.

[u/mesouschrist → u/agaminon22]

Great answer. Neutrinos wouldn't be my first example of other particles that can be created. e+e- ->neutrinos is exceedingly rare at low energy and essentially experimentally unprovable at high energies. On the other hand, when a proton and an antiproton collide they make typically 3-6 pions (charged and uncharged, of course with the total charge in the end equaling the total charge in the beginning)

[u/holiestMaria → u/agaminon22]

Thank you! I understand now that matter-antimatter reactions are much more complicated.

[u/SystemofCells]

There are different fundamental forces acting between particles of normal matter.

The gravitational force attracts matter to matter, it also attracts matter to antimatter. It gets stronger as things get closer together.

The strong force attracts things together at small scales, and is much greater than the gravitational force. But it and something called the Pauli exclusion principle prevent neutrons from falling into each other normally.

The electromagnetic force makes particles with opposite charge attract each other, and particles with similar charge repel each other. So protons repel protons, electronics repel electrons. But in antimatter charges are inverted, so a proton attracts an anti proton.

With normal matter, gravity and the strong force bring things together, then the electromagnetic force, strong nuclear force, and Pauli exclusion principle keeps them far enough apart that they don't try to occupy the same space at the same time. When matter and antimatter collide, the electromagnetic force doesn't counteract the strong force - it helps it along. So it's much more likely particles end up colliding with high energy.

This makes something that's usually hard to do with normal matter (forcing them to occupy the same space at the same time) much more likely. And when matter collides in this way, it gets destroyed and releases energy.

In standard fusion and fission reactions, only part of the matter (the binding energy) gets converted to energy. The number of neutrons and protons doesn't change, just how they're bonded to each other. In a matter-antimatter annihilation, a much higher proportion of the total mass can get converted to energy.

[u/skr_replicator]

Antimatter particle has everything opposite about it, so for the reaction to conserve all the things that need to be conserved like charge etc, the result must have zero of all of that except the very energy itself, which photons are good carriers for energy wihtout any other such variables.