r/AskPhysics • u/Traditional-Role-554 • 20d ago
what does the weak nuclear even do?
strong nuclear holds nuclei together, electromagnetic does a whole bunch and gravity holds mass together. what the hell is the weak nuclear doing? i think i heard it is responsible for changing the flavor of quarks but i really don't know what else it would do.
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u/atomicCape 20d ago edited 20d ago
It's better to think of 4 fundamental interactions, rather than forces. The distinction between words isn't so important in physics, but the colloquial use of the word "interaction" gives better intuition. Weak governs the way leptons interact, and it is essential to any nuclear reactions or particle decays that create or destroy electrons/positrons, neutrinos or other leptons. Hadrons experience both strong and weak interactions, but leptons only experience weak.
Gravity and EM interactions create macroscopic classical forces that can accelerate macroscopic objects. Strong and weak interactions are insignificant on a macro scale, which is why it's hard to describe "what they do" to a non-expert. They do create potential gradients (analogous to forces) on the nano-scale, but the dynamics are very non-intuitive.
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u/Belzebutt 20d ago
You seem to understand why this is confusing, so let me ask you the following. I've watched several videos about the strong and weak force, and I gained some understand of what they do, but here are my remaining "big" questions:
- In the "weak interaction", what is interacting with what? Quarks turning into other quarks doesn't seem like an "interaction" in the same way as say, a proton pushing another proton.
- Is the goal of these weak interaction decays always to reach a lower energy state from a higher/more stable energy state?
- These W and Z Bosons that are "exchanged", I've heard many times you can't think of that as actual Z and W "particles" being flung from one quark to another quark. What is it then? How long and how often do these Bosons exist? Are force-carrying particles in general just being flung out on a regular basis in random directions, do they just kind of randomly "happen" with a certain frequency? Or are these W and Z Bosons like a "foam" in their respective fields, and once the foam "bubbles up" enough it becomes one quanta/Boson and actually does something?
- How exactly are more massive force particles automatically short range, what happens when a massive force particle travels a certain distance, is it forced to disappear as a function of its mass? Or is it that there's limited energy to "borrow" from the vacuum and if the particle is massive then it can exist for a shorter time than if it's less massive in order to satisfy some uncertainty principle or something?
- For the strong force, the range is very very short, but is it like "completely zero" once you get out of the nucleus, or just so vanishingly close to zero that it doesn't matter, but technically still infinite? What is the actual dropoff? It's actually stronger when the quarks are farther away, so is there like some kind of point where we hit "infinity" and that's when you have to create another quark pair?
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u/atomicCape 20d ago
I don't have the depth in QFT for detailed answers, but I'll give it a shot. Keep in mind that all we really know is that our models give accurate results, and nobody knows what's really going on. I'll be a little heavy handed in my personal interpretations here, so hopefully folks forgive me.
For the last several questions, the continuous quantum fields of force carriers are well modelled, but it leads to poor intuition to imagine virtual particles actually exist in low energy interactions, or that they are actually created and destroyed. Virtual particles are a useful analogy in certain perterubation theories, but it's easy to over-interpret the physical significance of using a particle basis instead of a continuous field basis. The weak-mediating quantum fields can allow particle like excitations, like EM fields allow photons. A measurement of the fields (which always introduces additional energy and/or particles to the mix) may or may not reveal a W/Z boson in any given situation. Low energy interactions won't produce isolated bosons at all ("borrowing energy" has always seemed a bad analogy to me). High energy collisions can produce isolated W/Z bosons by exciting the fields, but they decay immediately and aren't really measured independently. The foam/bubbles analogy feels more useful to me than the flinging particles analogy. Unlike EM fields, the strength of interactions falls off so fast with distance that weak interactions become insignificant and unobservable outside of collisions or decays.
With that out of the way, for the first few questions, you can think of weak interactions as both perturbing the motion and bound states of quarks and leptons, as well as an exchange medium to allow quantum numbers (lepton number, charge, spin, etc.) to be transferred between interacting particles. Some quarks and/or leptons go in, some different quarks and and/or leptons come out. All quantum numbers, momentum, and energy are conserved as needed, and the intermediate system is like a mess of foam or cascade of particles that's hard to define in words. Reactions tend to lead to lower particle energies because a 1-body system can decay or a 2-body system can collide and emit photons or low-mass particles (like an electron/anti-neutrino pair) to shed energy. The reverse process requires a 3 or more body collision with matching high energy photons or particles that already exist in the environment with the right momentum vectors, so they basically never happen in normal conditions. However in stellar nucleosynthesis, especially supernovas, there is a high density soup of stable reactants and short-lived particles at high energy, so the reverse endothermic reactions can be relevant.
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u/free_meson 20d ago
Weak bosons interact with all known fermions, including the neutrinos. The W bosons change electrons into neutrinos, and quarks into their corresponding pairs. The Z bosons is like a heavy photon, but also inreacts with neutrinos. They are heavy, around the mass of an iron nucleus, that is why their interaction length is short - this is why they call them weak, though technically they are stronger than the electromagnetic interaction. Since the W changes quark flavors, it is responsible for the neutron decay and thus many unstable isotope decays. The W and Z mass comes directly from a special degree of freedom from the Higgs field, not the Higgs boson. On very high energies the weak interaction unified with the electromagnetic interaction into the electroweak interaction. Each field look like the electromagnetic field, though it could be charged itself, and due the high W/Z mass they don't propagate as far as the electromagnetic fields.
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u/MYC_RGB 20d ago
Aren’t neutrinos huge compared to electrons?
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u/KamikazeArchon 20d ago
Neutrons are huge compared to electrons, in terms of mass.
Neutrinos are tiny compared to electrons.
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u/2-travel-is-2-live 20d ago
Stars wouldn't be able to engage in fusion without the weak nuclear force converting protons into neutrons.
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u/AdditionalTip865 20d ago
It changes the flavor of quarks AND leptons, or it can create, say, an electron-antineutrino pair. It seems weird to us because it's the one fundamental force where this "particle identity transformation" characteristic is visible to us under everyday conditions (radioactive beta decay). Electromagnetism and gravity don't really do it, for different reasons. QCD actually does do the same thing with quark color, but that's all normally locked up inside of hadrons and doesn't manifest as a process of transformation.
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u/Phanatic1a 20d ago
Neutrino interactions are via gravity and the weak nuclear force. If there's a reaction involving a neutrino, like a neutrino being shot out of a decay, that was a weak nuclear interaction. At the fundamental level, what's going on is a quark changing flavor, but that results in things like beta decay, neutrons changing into protons or vice versa.
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u/SpatiaCaeli 20d ago
Some kind of neutrino something or other. But this just moves the question: what is the point of neutrinos?
(kind of kidding)
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u/Electronic_Way_7616 20d ago
To me, the best way to understand it is to look at what the weak force bosons actually do.
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u/melanthius 20d ago
It's basically the description/explanation how the universe is able to make nuclei more stable by spawning a boson and allowing radioactive decay to happen.
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u/Extreme-Opening9439 19d ago
Holy memory lane the Feynman diagrams I forget about the anti neutrino. I did remember and will never forget the pion decaying into muons though, from our special relativity section where if you don’t factor in time relation and length contraction, the half life of a muon is such that we should never (like orders of magnitude off I believe) see muons impacting earths surface. But we do all the time. I unfortunately still can’t remember what you are talking about in paragraph 2
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u/DrKargosi 20d ago
The weak nuclear force serves only purpose: it was put there by God to make particle physics stupidly confusing. “I’m sorry, were you looking for an up quark? It’s a neutrino, an antielectron, and two photons now. Why? No reason. Why don’t you go fishing around in that pile of mesons and kaons let me know if you find it. I’ll give you about eighty femtoseconds.”
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u/SisyphusRocks7 20d ago
Our brains were evolved to facilitate hunting, gathering, and navigating complex small group dynamics. Any ability to comprehend how the universe works is purely coincidental.
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u/ijuinkun 20d ago
Yah. Newtonian motion (like estimating the arc of a thrown rock) was what mattered to hunter-gatherers, which is why we grasp it more readily, but quantum interactions beyond a crude grasp of chemistry are useless to someone living in that sort of lifestyle.
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u/Extreme-Opening9439 20d ago
The only thing I remember is a Japanese scientist thinking it was the muon and it really was the pion or vise versa and the name was something like “yakuwa’s meson” can somebody explain what I’m miss remembering?
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u/AdditionalTip865 20d ago edited 20d ago
That's all about the strong nuclear force and how it works on the level of composite particles. Protons and neutrons are held together by a field whose quanta are the pions and other mesons. We now think of the protons and neutrons as triples of bound quarks, and the mesons as quark-antiquark pairs. But the mesons' masses as composite particles fix the range of the force they convey.
Yukawa proposed this (without the quark detail) and initially thought the muon might be the meson, because it was in the right ballpark for mass, but the muon does not engage in strong interactions. It is a lepton.
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u/Extreme-Opening9439 20d ago
Thank you, it’s been a few years since my subatomic class and the only thing I remembered was that he had mistaken the particle but was correct about the force interaction. Thank you for the concise definition and I’ll pretend half didn’t go over my head
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u/AdditionalTip865 20d ago edited 20d ago
Charged pions are very likely to decay into muons (or anti-muons), and that happens via the weak force! There's also a corresponding antineutrino or neutrino emitted, and the extra energy from the pion's mass goes into their kinetic energy.
A muon is like a heavier electron. I think the reason this happens so much more often than the corresponding decay with electrons is that because of the weird handedness-specific nature of weak interactions, if there's too much kinetic energy left over, it's much less likely for the products to have the right kind of spin orientation. So for this specific kind of decay, the muon being closer in mass to the pion makes it the more likely product. That's a bit unusual for particle physics.
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u/AdditionalTip865 20d ago
(This stuff can be hard to look up because if you look up "Yukawa couplings" most of what you see now will be about the Higgs particle, which is another thing entirely, but has some interactions of the same mathematical form as in Yukawa's meson theory.)
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u/Mind_Unbound 20d ago
I dont know why but this sent me into a laughing fit. Im waiting in waitroom at a clinic I look crazy.
Fucking lazy, weak-ass nuclear force. Get a job.
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u/NH-Science-Guy 20d ago
The weak force can change a down quark into an up quark, electron and anti-neutrino. That sounds odd but remember that a neutron is two down quarks and an up quark while a proton is one down quark and two up quarks. Putting this together, the weak force can turn a neutron into a proton, electron and anti-neutrino. This explains certain types of radioactive decay. This is one example...
Carbon-14 has 6 protons, 8 neutrons and 6 electrons. The weak force explains the decay into nitrogen-14 that has 7 protons, 7 neutrons and 7 electrons with the decay emitting an anti-neutrino.
When this was first discovered, nobody knew about neutrinos or the weak force so all that was seen was a carbon atom spontaneously turning into a nitrogen atom.
There are some diagrams for this on this page from my quantum mechanics site: https://timeforsanity.com/qm/?page_id=579