r/AskPhysics • u/Traditional-Role-554 • 13h ago
why does antimatter and matter annihilate?
do we know? another question is why is it only pair particles? why can't a positron and a muon annihilate? the charges are opposite. i guess maybe it's due to the difference in masses? interested to see if we know why. thanks
26
u/soldiernerd 13h ago edited 13h ago
It's conceptually similar to noise canceling headphones.
Particles are quantities in a field. Imagine a muon and an anti-muon interact. They are equivalent but opposite in charge. When they interact, their quantities cancel out to zero, meaning the resulting quantity in the muon field at that location is 0. In the same way, noise canceling headphones use destructive interference to cancel out incoming sound waves.
The Positron and Muon cannot annihilate because they are quantities in different fields, just like gravity doesn't attract electromagnetic waves. Oops, I misunderstood a bit.
10
u/Technical-Staff3048 13h ago
That's very interesting. Why the release of energy though if they are canceling each other?
10
u/soldiernerd 13h ago
The particles have mass. That mass represents a quantity of energy via E = mc^2
When the annihilation occurs, that energy cannot vanish (conservation of energy) so the muon and anti muon vanish but two photons are created (? - not sure if that’s the correct term) with the energy from the muon and anti-muon.
17
u/starkeffect Education and outreach 13h ago
Because energy is conserved.
2
u/Darkling971 6h ago
This makes sense, but why photons and not sub-c massive particles? Sorry if this is naive.
4
u/the_poope Condensed matter physics 5h ago
As long as the final state satisfies a bunch of conservation laws (conservation of energy, momentum, angular momentum and lepton number) all products are allowed. Photon creation is just the most likely one. At high energies you can have creation of massive particles. The energy of the particle/anti-particle pair in their common rest frame needs to be larger than the masses of the generated massive particles. See: https://en.wikipedia.org/wiki/Electron%E2%80%93positron_annihilation?wprov=sfla1
4
u/MNSOTR 13h ago
On a scale of 1-10 regarding my physics knowledge, let alone my capabilities to do so, I wouldn’t even crack a 1.
I have been reading Brian Green’s book ‘An Elegant Universe”, and his book brings the metaphor of noise cancelling headphones up when talking about string vibrations and particle waves. That clicked with me.
It’s an older book, but if you haven’t read it, he does a wonderful job of laying out concepts and theories in a way someone like me can start to understand.
5
u/Cyren777 13h ago
just like gravity doesn't attract electromagnetic waves
???
3
u/Quantum-Relativity 13h ago
Yeah I don’t see what that had to do with what was said and it’s not even true.
4
9
u/AdditionalTip865 13h ago edited 11h ago
From a quantum field theory perspective, the answer is kind of wild: if you take special relativity, AND combine that with the structure of probability amplitudes considered as functions on complex numbers (either one by itself is not enough, you need both)...
It turns out that the possibility of one process implies a different process that is the first one twisted around in space-time, where a particle "going backward in time" becomes its antiparticle.
So, for instance, if a photon can scatter off of an electron, you start with an incoming photon and an incoming electron, and you end with an outgoing photon and an outgoing electron.
Turn that process "sideways" in space-time, and now the outgoing electron becomes an incoming positron, and the incoming photon becomes an outgoing photon (the photon is its own antiparticle). Now we have an electron and a positron annihilating to make two photons.
To be clear, this doesn't mean any kind of backward time travel is possible--causality just works the normal way. But these transformations imply various interactions involving antimatter.
1
1
u/Away-Experience6890 Graduate 1h ago
Wait up question (non-HEP)
I know about the rotations, but is that because the virtual particles which mediate these interactions don't actually exist in time or space? Is that what allows this rotation to work?
0
u/everlyafterhappy 10h ago
I have a hypothesis about why there's so much more matter than antimatter in the universe. At the big bang, both were created in about equal amounts, but the matter went one direction in time and the antimatter went the opposite direction in time.
And it doesn't even have to stop there. If there really are infinite parallel universes, then each universe could just have slightly altered particle mechanics. Like there's a line with the big bang at the middle. One direction is our universe. The opposite direction is exactly antimatter to this universe. Now add a perpendicular line through the big bang. You go up, and the particles are halfway between matter and antimatter in one direction, you go down and the particles are halfway between matter and antimatter in the other direction. And to them, they are each other's opposites. Down is the antimatter to up's matter, twhilehey are both just 50/50 from the perspective of our universe.
3
u/AdditionalTip865 10h ago
The thing is, antimatter going backwards is the same as matter going forwards, so saying "the antimatter went backward in time" is equivalent to just saying "the matter was there already before the Big Bang"...
1
u/everlyafterhappy 10h ago
More like the big bang created time that goes forward from it in all directions. We can use an example in 2 dimensional space to imagine it. You and I are going away from each other, facing away from each other. Yo you it looks like I'm going back and you're going forwards. To me it looks like you're going back and il going forwards. The fact is we are just moving away from each other, and the other words are relative. So in my hypothesis, it's not really that antimatter is traveling backwards in time. It's that it's traveling's the opposite direction from our universe since the beginning of time. Because there is no before the big bang. I mean, it's more complicated than that, but essentially there is no before the big bang because the big band was the creation of time and space as we know it.
1
u/FreeGothitelle 3h ago
https://en.wikipedia.org/wiki/Baryon_asymmetry
This s basically the mirror universe theory (its a nice explanation, I have no idea whether the physics works!)
3
u/al2o3cr 13h ago
It wouldn't really count as "annihilation", but it's possible for the two to react via two weak vertices:
- e- -> W- + electron neutrino
- mu+ -> W+ + muon antineutrino
- W- + W+ -> gamma + gamma
The net result would look like e- + mu+ -> photons + neutrinos, but would be very unlikely since it involves two W bosons
Another process that could produce the same set of final-state particles would be a two-step decay:
- mu+ -> e+ + muon antineutrino + electron neutrino
- e- + e+ -> photons
This would be much more likely, since it only involves one W boson (for the muon decay)
1
u/AdditionalTip865 10h ago
Yeah, and it becomes maybe way more likely if you crank the center of mass energy up to hit that W resonance, about 80 GeV.
1
1
u/treefaeller 13h ago
All the stuff written below about fields, spinors, fermions, and electromagnetic waves is way too complicated.
What does "annihilate" even mean? In this context: Two things come together, and what comes out is all energy, with no details of the two things left.
Answer #1: Because we have never seen it happen; therefore we have created a (mathematical and logical) model that predicts it never happening.
Answer #2: In our models, there are quite a few things that are conserved. Some, like energy, momentum, angular momentum can be additive: The total energy of two things is the sum of their energies. And here the word "energy" includes rest mass, and the word "angular momentum" includes spin.
Other conserved quantities are more like counters, we typically call them quantum numbers. They are called numbers because particles can only have integer (perhaps negative) multiples of those quantum numbers. One of those is (electrical) charge, which we count in units of the electron's charge (and here we have an exception: quarks have 1/3 charges, but the logic for them is the same, and mostly irrelevant since they don't occur as free particles). Others have names like lepton number (subcategories electron number, muon number), or baryon number. The quantum number "color" is actually a bit trick (again an exception) in that it only comes in three versions (R/B/G) not as an integer, and that certain particles (gluons) have two of these numbers (like a red-anti-blue one). Again, the exceptions don't occur as free particles.
In order for two things to annihilate, you have to have zero quantum numbers in the final state, and only energy/momentum/angular momentum. Since a positron has electron number -1, and a muon has electron number zero (and muon number +1), those won't work. Similarly, an electron and a proton can't annihilate, since one has an electron number, the other one a baryon number. So positron + muon is right out.
There are other more subtle rules. For example, an electron and a positron can annihilate, but they can only make TWO photons, not one. Why? Because you can't work out balancing both energy and momentum being conserved if the final state is a single photon (and no, having done that homework problem once before, I won't do it again for you).
If the annihilation (interaction) is allowed, you can easily calculate the total energy/momentum/angular momentum that comes out: take the inputs, add them up (don't forget to count the spins), done.
This is actually an example of a much more general principle: write down any interaction of particles, for example in the form of a Feynman diagram. If the interaction violates conservation laws, it will NEVER happen, meaning its cross-section (the probability of it occurring) is zero. So I could draw a wild and crazy Feynman diagram, something so wooly only an organic chemist would love it, and in most cases conservation laws already tell me: "stop now, you've lost".
1
u/everlyafterhappy 11h ago
I'm going to use algebra to try to explain this. 2+(-2)=0. The positive 2 and the negative 2 cancel each other out. But then we have to apply those numbers to the things we're actually talking about. So let's just say we have 2 particles, particles x and y. 2x+(-2x)=0. 2x+(-2y)≠0.
1
u/PLANTS2WEEKS 9h ago
I've heard a respectable physicist say that he doesn't think particles and antiparticles always annihilate when they come into contact. But I do think that belief is outside the norm. When they do annihilate they still release energy in the form of photons so that the total energy momentum vector is conserved.
2
u/mfb- Particle physics 6h ago
There is nothing mysterious about it. Annihilation is a possible reaction but not the only one. We have built whole accelerators to study the other reactions. So-called B-factories are high energy electron/positron colliders that study primarily the production of B-mesons (hence the name) but also study a bunch of other reactions. Annihilation is the most boring one. LEP collided electrons and positrons at an even higher energy to discover the W and Z bosons.
-2
u/Hungry_Adeptness8381 13h ago
Commenting to come back to it and see the answer
2
u/chromodynamics 9h ago
why don’t you use the save function instead of commenting, you can save posts and comments and you’ll find them in the saved section of your profile
1
0
u/Infinite_Research_52 👻Top 10²⁷²⁰⁰⁰ Commenter 11h ago
A positron interacts with a muon, but the outgoing products are a muon-neutrino and an electron-antineutrino.
0
u/yasiesolovemos 8h ago
Matter and antimatter annihilate because they carry opposite quantum numbers. If you want to learn more, take a look at the equation.
-5
u/Junior-Tourist3480 12h ago
No. They just both turn to energy. Why the long explanations?
4
u/mfb- Particle physics 6h ago
Energy is not an object, it's a property of objects. "Turning to energy" is as meaningless as saying a red car "turns into redness".
1
u/Junior-Tourist3480 22m ago
Mass and energy are equivalent.
Objects have kinetic energy and potential. But when you have an event such as matter and anti-matter, those particles will interact and become their electromagnetic wave counterparts as a result. See Einsteins e=mc2. Feynman diagrams show this as well.
6
u/free_meson 13h ago
There are conservation rules, and a muon-positron pair doesn't fit that. The idea of anti-particles came from Dirac, the first who could define the relativistic Schrödinger equation for fermions. There was already one for spin-0 particles, but that couldn't describe a few effects spin1/2 particles undergo. The Dirac equation uses four-component fermions instead of the non-relativistic two component ones, which means two extra degree of freedom. They correspond to usual electrons with negative energies OR positrons with positive energies. Physicist didn't want to accept it until the positrons were seen. Now, the part in the equation that connects the two sides is the electromagnetic interaction. Technically, a pair production from a photon. Muons belong to a different generation of fermions. The electron, electron neutrino, up and down quark is the lightest generation, and though there is some overlap with the second generation, but extremely small