Non-physicist question:
What purpose do neutrinos serve in the complex world of elementary particles?
Are they "just" a side effect of radioactive decay, or do they facilitate some other reaction?
(edited to change the original question from "what are neutrinos for?" to the current question which was suggested by a grandchild comment)
Neutrinos were originally a kind of mathematical placeholder to allow for conservation of lepton number (electrons, muons, etc). Turns out that they're real! More to the point, you have six basic leptons (and their antiparticles) if you count the types of neutrinos and six basic quarks (and their antiparticles). It's an interesting parallel.
They react with very little, as it turns out. Chargeless, and so they care not for electromagnetic forces. Strong nuclear force is also a miss. Originally, it was thought that only the weak nuclear force was the way they could interact with matter, but with confirmation that they have a non-zero mass, they can also interact with gravity. This makes them both hard to detect, but also an excellent way to peer through things like clouds of interstellar dust, or through the Earth.
Even zero mass particles interact with gravity. Anything with energy does.
Having non zero mass means that at least in theory we could slow them down to get a better look at them. But so far we have no idea how, which means that we don't even know what the mass actually is. We just know from the way from the types of neutrinos observed that they can change type, and they couldn't do that if their mass were zero.
> Having non zero mass means that at least in theory we could slow them down to get a better look at them. But so far we have no idea how...
How about cosmic expansion? Of the neutrinos emitted early in the universe, shouldn't most still be around, given how weakly neutrinos interact? And, given how much everything in the early universe is receding from us, shouldn't they be slowed down in our frame of reference? If they were emitted when the universe became transparent to neutrinos, what Z would that correspond to? What velocity would we observe in the local frame of reference? (Does it depend on how close to c their velocity is? Do we know?)
What would we expect the density of such neutrinos to be? Enough that we could observe it? (One "gotcha" is that slower-moving neutrinos might have a smaller interaction profile than fast-moving ones, and so be harder to detect.)
Wikipedia says decoupling was at 1 second after the Big Bang, and that neutrinos from that era have energy of 1e-4 to 1e-6 eV (compared to current neutrinos that may be as much as 0.8 eV).
I don't know any answers here, but this is an awesome question. I too am now super-curious about the Cosmic Neutrino Background.
https://en.wikipedia.org/wiki/Cosmic_neutrino_background
...And I suppose there are probably good reasons for this to be impossible, but wouldn't it be wild if a "mechanism" for things like the "randomness" of beta decay were that a really slow/low energy neutrino from the big bang interacts with a neutron, causing it to decay into a proton, and an electron, and the neutrino gets a boost in energy as well.
Antineutrinos can cause inverse beta decay, so maybe neutrinos can cause inverse fusion?
Edit: apparently it just causes transmutation.
https://en.wikipedia.org/wiki/Homestake_experiment
There are various kinds of neutrino interactions:
https://www.vivaxsolutions.com/physics/feynman-diagrams.aspx
...(scroll down for the nice animated diagrams).
Sorry, I was being brief. I usually say "anything with a non-zero rest mass-energy" for anything for a photon, photons never being at rest. Briefly, some had thought that neutrinos might be little more than floating carriers of lepton number. Later that was amended to merely "massless" (in the sense that they would at least be like photons). I've never been in that crowd. My thoughts are that mass-energy is the coin of existence and gravitation the inevitable consequence, but I do not speak for everyone, just trying to give a kind of non-exhaustive overview of the history to respond to the for part.
Neutrino detectors essentially slow them down - a neutrino hits an electron, transferring momentum, and then the high speed electron gives off Cherenkov radiation.
Oversimplifiying, they are chargeless electrons.
If you have an electron(negative) and a proton (positive), then the charge may jump and you get a neutrino(no charge) and a neutron(no charge).
It's more common in the other direction. If ypu have a neutron(no charge) it splits into a proton(positive), an electron(negative) an an antineutrino(no charge).
Neutrinos must exist to satisfy conservation of lepton number. Specifically, the weak interaction can change flavor. In beta decay (for example), a down quark changes into an up quark, which converts a neutron into a proton and emits an electron. Conservation of lepton number implies that another particle must also be emitted with the properties of a neutrino.
They preserve the spin quantity in particle interactions. E.g. in beta decay a spin-neutral atom has a neutron that decays into a proton and an electron. Neutrons, protons and electrons have spin 1/2, so an extra 1/2 spin came about. An anti-neutrino has spin -1/2 which is emitted to balance the spin quantity.
- "do they facilitate some other reaction?"
They are not carriers of a fundamental force, if that's what you are thinking about.
https://en.wikipedia.org/wiki/Force_carrier
Neutrinos can interact with atoms, producing high-energy charged particles.
They serve the same purpose as every particle field, a store of energy as it moves between forms in this waterfall of existence towards maximum entropy.
Do they have purpose to us / our technology?
Possibly the military can use them to communicate with submarines.
> What purpose do neutrinos serve in the complex world of elementary particles?
To utterly confuse physicists, no other purpose detected
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A more charitable reading of the question could be, “what purpose do neutrinos serve in the complex world of elementary particles?”
Everyone benefits when physics and physicists are more approachable
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The rules of physics have a very clear purpose: to predict how and when some natural phenomenon occurs. So the question can be re-phrased as: why did we come up with the notion of a neutrino, what phenomenon did it explain?
We certainly didn't just happen to see one colliding with with something, we knew they have to exist far before the first one was ever detected.
Way to totally miss the point. I wasn't talking about the rules we create; I was talking about the actual rules in the physical world. These exist independent of us. They function, they don't predict.
The notion that those actual rules have some sort of purpose reeks of a medieval mindset, where the world is the creation of God and has His Purpose, which it is the function of natural philosophers to figure out.
> I was talking about the actual rules in the physical world. These exist independent of us. They function, they don't predict.
Nobody knows what those are though; mere humans have to get by with prediction.
In fact there's no airtight reason to think that you can even in principle make the leap from valid predictions to knowing what the actual physical rules really are. This is the whole "problem of induction" thing:
https://en.m.wikipedia.org/wiki/Problem_of_induction
No amount of observing the universe can ever conclusively prove that our ideas about how it really functions are true, because we're stuck inside it and can't directly inspect the clockwork, if there is any.
I find this sort of nihilism completely tedious. Yes, we can know there are neutrinos there. This physical reality is not identical with our theories about them.
Do you not recognize that "There is no purpose to neutrinos. Your question makes no sense." is itself tedious nihilism?
So you reinterpreted the OP's question in the most meaningless way you wanted to, and then declared "How does this question make any sense?".
Nice contribution to the discussion! /s
Yes, I interpreted "neutrinos" to mean "neutrinos", instead of something else.
How utterly terrible of me.
“Who ordered that?” is a pretty famous question in particle physics. I think the commenter is just asking “who ordered neutrinos”? It’s a good question, even if it’s not formally rigorous.
https://www.aps.org/archives/publications/apsnews/201711/his...
That (which was about muons) was about the muon not being predicted. The neutrino was predicted (to explain missing energy in beta decay).
Great. So we have the following:
Q. Who ordered neutrinos?
A. They're necessary because they carry energy away during beta decay.
Is the Q there a made up historical quote? Why, yes it is.
Elementary particles do have a purpose, they are for explaining a measurable physical phenomena.
https://cds.cern.ch/record/677618/files/p115.pdf
See the above paper, but my memory is that they are involved in weak force and beta decay? Having a particle that's not needed would be like having high dimensional strings that may or may not exist.
Elementary particles don't explain anything. Theories of elementary particles explain things. Don't confuse the map and the territory.
The person I was responding to clearly wasn't asking "what's the purpose of theories of the neutrino" but rather what was the purpose of the particles themselves.
"Elementary particles do have a purpose, they are for explaining a measurable physical phenomena."
Don't confuse what you think was said with what was said. I didn't say neutrinos were the explanation or that they were doing the explaining.
If you can explain the theory, without the neutrinos, then you have a new theory. The theory is itself used as an explanation of experimental results, and I'd argue that the actual theoretical explanation is carried out by humans for other humans to not confuse the theory map for explanation territory. Frankly, I also think the dogmatism and distinction are silly, but don't confuse what I said.
Once again, you are confusing a thing with a theory about the thing.
Please provide a theory about (explaining) a phenomena, which does not presuppose the existence of the things (not the theory and not the phenomena) used to explain it. I think it's going to be a boring tautology of an unfalsifiable theory.
The photo electric effect is a theory of energy emission of light interacting with a metal. Quantized photons and energy levels are things used in the theory to describe experimental results. If you accept the theory, you accept it's elements, it's things. If you want to build a new Photo Electric theory, it will probably also involve some things you propose existing.
Explain Beta decay using the standard model without presupposing neutrinos exist. Note that beta decay was observed and the theory about it (a different thing) were developed long before neutrinos were directly measured (but those things must exist for the theory to be true). In fact predicting the existence and properties of particles before their measurement based on theory is the basis of most particle physics.
You seem confused, because humans who propose theories give purpose to their elements, because by definition those theories (their explanation) cannot be true without the existence of those elements. You seem to want inanimate objects to have desire, which is dumb.
I don't understand why you are so aggressive to a layperson asking a question like this. They simply want to better understand what implications the existence of neutrinos has on our understanding of the world, what they actually do, etc. I doubt they were wanting some sort of philosophical understanding of a greater Purpose for neutrinos.
I'm very annoyed by teleology.
> what are neutrinos for?
I'm not sure I understand your question, sorry!
> Are they "just" a side effect of radioactive decay
They are (as fas as we know) elementary particles as several other elementary particles.
> do they facilitate some other reaction?
Again, I'm not sure I get the sense of your question, but I'd say no, as they are the most elusive particles we know.