What I like of the right-hand, left-hand split is that it emphasizes the mystery of neutrinos.
We know neutrinos have a mass, we don't know how much. We don't know how to incorporate the neutrino mass into standard model. Other fermions come in left-handed and right-handed forms but we only see left-handed neutrinos.
One idea is that right-handed neutrinos do exist but are highly elusive. In fact they are a good answer to the dark matter and other physics mysteries
> We know neutrinos have a mass, we don't know how much.
How small is the possible range? Just last week I read - but possibly misunderstood - that the W-boson mass could not be directly measured so the researchers were using the masses of other decay particles to measure it. One of those other decay particles was the neutrino.
That the total mass of neutrinos is commensurate with / exceeds all the hadronic mass is rarely mentioned. With so many interactions emitting neutrinos, does everything end up as neutrinos, finally (neglecting expansion)? Or does something consume neutrinos and yield net hadronic stuff?
Particles like protons and electrons can never turn entirely to neutrinos because electric charge is conserved. Protons and bound neutrons seem to be the only stable configuration of quarks & gluons, ultimately there is quark charge that is conserved that prevents quarks from going away unless protons really do decay... which would have to happen at a very low rate if it does.
As a layman I never understood how the stability/decay of a neutron could be worse when it is unbound than bound. Does it even make sense to call a neutron a "particle" if its internal stability becomes worse when there is nothing around it? Is there any layman explanation of this that might vaguely resemble reality? The only way I can imagine this making sense is to just imagine neutrons and protons as dissolving together (liquid drop model?) rather than as being individual particles. And even then, I thought that model is inaccurate for explaining other phenomena.
This is a massive oversimplification and anthropomorphizes elementary particles, but hopefully satisfies your curiosity. The reason for the worsening stability is because a free neutron has more potential energy than one bound to a proton. In the same way that a muon will "preferably" decay into an electron by interacting with a W-, if you were able to have a down quark floating in space by itself (you can't, for unrelated reasons,) that down quark would "want" to decay into a much lighter up quark by exchanging a W-. In both of these cases, the elementary particle loses mass through the interaction.
Inside the free neutron, that's what happens, too. A down quark interacts with a W- to transform itself into an up quark; the neutron is now a proton, with the W- carrying away some of the mass and electric charge.
However, neutrons and protons are more complex than just quarks. They're also big bundles of gluons, exchanging colour charge amongst each other. These gluons have mass and you need energy to create them. They actually account for most of the mass of a neutron/proton. A bound neutron and proton are exchanging these gluons amongst each other, too.
Suppose you had a bound neutron and proton. If the neutron were to decay into a proton, the electric charge of the particles (now both +1) would cause them to repel; you can imagine either of two things might happen: the protons will now move apart _or_ they'll stick together. In the former, you need kinetic energy and in the latter, you're going to need more gluons to hold the whole thing together. Both of those cases requires more energy (==mass) than the neutron would "lose" by converting to a proton.
So, if you can imagine the potential energy landscape, the bound neutron is at a local minimum (lower than a free neutron)
Thanks a lot for the explanation! I see what you're saying. It raises some follow-up questions for me, if you don't mind:
Why can't the W-'s kinetic energy be smaller than whatever the neutron would lose by converting to a proton? There's no quantization preventing that here, right? My guess here is that it would not have (for the lack of a better term) sufficient "escape velocity", meaning that it would fall back into the neutron again, but wouldn't that imply (a) this is more like a dynamic equilibrium than a static one, where neutrons turn into protons and back into neutrons repeatedly, and (b) quantum uncertainty (Heisenberg, tunneling, etc.) should still mean that bound neutrons should still decay once in a while, and perhaps (c) a neutron next to a proton might randomly "swap" places once in a while if a W- from one gets pulled into the other one? (And if any of these is the case, then where do they draw the line to declare that a bound neutron is "stable"?)
I think I see what you're asking, but I'm not sure I can give a satisfactory (and correct) answer, here but I think you're on the right track:
> Why can't the W-'s kinetic energy be smaller...
That W- is a virtual particle - it can have any energy (according to some probability distribution) and you're correct that it doesn't _have_ to observe conservation of energy as long as that violation only happens for a very short period of time, which is precisely Heisenberg - there are other numbers it must conserve, such as electric charge.
> (b) quantum uncertainty (Heisenberg, tunneling, etc.) should still mean that bound neutrons should still decay once in a while
We do see this; this is what beta decay is.
> (c) a neutron next to a proton might randomly "swap" places once in a while
In general, this is correct. With limitations on what quantum numbers need to be conserved, these events are all constantly happening all the time. When writing out the equations for the system, you don't really have a term describing a proton and a term describing a neutron; you have a complicated mess of _all possible_ interactions and each elementary particle is described as a sort of "mix" of all the things it could be. So, in that sense, a bound neutron and proton is it's own thing - there's plenty of examples of these quasiparticles throughout physics and the definition of "stable" is quite application specific.
The neutron isn't really a particle at all, but a seething mass of quarks and gluons crawling over one another, some trying desperately to get free and the others even more vigorously clawing them back, with fur (color-charge) and blood (fractional electrostatic charge) flying.
Fun idea! But space keeps on expanding. So at some point most mass will be in black holes, but every black hole will be so far from every other black hole that even light will travel forever without ever being able to reach another.
and far far away more of them are being created and inflating, once you far enough way you are in something that looks like our universe, somewhere out there inflation is stopping and the big bang is happening there.
It's easy in my mind looking at the classical black hole picture and wondering what happens around the singularity to wonder if inflation gets re-started inside a black hole and maybe in that old picture where you can go into a rotating black hole and come out in the "asymptotically flat spacetime" it is a universe made by that mechanism.
Of course I think the classical black hole picture might be "not even wrong" and you might run into more than one firewall (such as the inflation zone) on the way there.
Is this model actually taught in high schools at this point?
I remember being taught about electrons with their valence shells, protons, and neutrons. That's it. I didn't hear about a boson or a neutrino until well into adulthood.
The Standard Model was covered in A Level Physics syllabus in the late 1990s - I remember being taught it. The top quark was experimentally confirmed in 1995, so this was exciting stuff.
Most people surely take two years to do their A-levels? Or have they got easier since the early seventies when I did my three, or have the students become better at studying?
In the ACT at least, years 11 and 12 are called ‘college’, and the NT, years 10, 11 and 12 are ‘senior secondary’. It’s confusing, but each state has their reasons.
Yep, at A level (2007 ish) we had a college trip to visit CERN while it was being built. It was cool to see ATLAS before they joined up the beam lines.
My college physics department was otherwise very underwhelming and I'm amazed they had the imagination to plan it.
It's the other way around, the Higgs field is the field introduced to break the EW symmetry. It can do this because it is particular in that it has a non-zero vacuum potential, that in essence picks out a "direction" in EW-space at each spacetime point that becomes special.
It's like the underlying EW theory has an internal space and the equations don't care about the "origin" in this space, if you made a simulation of this and moved the origin everything would look the same. But as soon as you introduce another all-encompassing field that interacts with EW that has a specific absolute value, the equations lose the symmetry - you can't move around the origin anymore as in effect you have now fixed the numbers.
Why does it seem like the further they explore into these particles and the particles that make them the model gets more and more complicated? It seems they are getting further away from the unified model and still have far to go. In my mind I would think that as they got closer to this unified grand theory of everything the "fundamental" particles and their model would be simple, not complex. Am I totally off base here?
why is the absolute overwhelming majority of stuff in the universe (and 100% of things outside of special lab settings) made up from 1st generation matter?
why is it that all stuff made from charm, strange, top, and bottom, quarks decays right away?
> why is the absolute overwhelming majority of stuff in the universe (and 100% of things outside of special lab settings) made up from 1st generation matter?
The first generation have the lowest mass, and there are interactions between generations. Since physical systems like to explore the local energy space and find the lowest one, it follows that more energetic systems will quickly stabilize to lower energy configurations.
You might rightfully have two follow up questions at this point:
1. Why do the first generation fermions have the lowest mass?
2. Why do physical systems like to find the lowest energy configuration?
I don't believe anyone has the answers to those - so far as we can tell this is just the way the universe is. Maybe some day someone will figure out why.
Indeed, 1. is a mystery. But, 2. is a consequence of 2nd law of thermo. Specifically, the lighter the mass of a decay product, the larger the number of possible final states in the kinematical phase space (in general). For instance, if there is just enough energy to make a pair of particles, then only one final state is possible: particles sitting at rest. On the other hand if energy is “left over” in addition to rest energy, then there are many possible final states corresponding to the pair flying outward in different directions. Hence, the decay to the lighter products are entropically preferred. This is mathematically encapsulated in Fermi’s Golden Rule.
The first generation ones don't come with a little number "!" on them. They are "first generation" because they're the ones we found first. Why did we find them first? Because they're the most common ones. And why are they the most common? Because they're the ones with lowest mass.
I mean, you could still ask why the different generations have different masses at all, but if they do, it follows that the "first generation" has the lowest mass.
Does thermodynamics not suffice for (2)? Or does that just restate the question, somehow?
I learned many things about the SM from this presentation I had never before retained.
Wondering now to what degree string theory seems to exactly require all of this, vs. merely be apparently compatible with it, insofar as it can be "solved" at all. Where it is too hard, maybe it is not known whether certain SM features are compatible, and everyone just hopes?
By thermodynamics: energy is conserved globally so when a protein molecule relaxes to a minimum energy configuration the energy gets transferred from the protein molecule to its environment. Total energy stays the same but it is more spread out. There are dramatically more possible ways the energy could be spread out than be bunched together: so if it starts out in a bunched out state it will spread out unless there is something, usually a "conserved quantity" or quantum number that makes it impossible for the state to roam randomly across the phase space.
We know why SM particles come in "whole generations": a mathematical consistency condition known as anomaly cancellation [0]. We also know that with only 2 generations we cannot get violation of charge-parity symmetry [1] via the Cabibo angle [2] alone; to get CP violation we need at least a 3x3 CKM matrix [2]. CP violation is one of the Sakharov conditions [3] that is required to get matter/antimatter asymmetry from early universe dynamics.
This is not an a priori reason, per se, but it does waggle its eyebrows up and down in a sort of anthropic way.
The article mentions that antimatter is explicitly excluded to keep the diagram/model simpler. It sounded like it would be basically another whole copy of the same shape, bit I have no idea how it would be connected, which would be interesting!
We know neutrinos have a mass, we don't know how much. We don't know how to incorporate the neutrino mass into standard model. Other fermions come in left-handed and right-handed forms but we only see left-handed neutrinos.
One idea is that right-handed neutrinos do exist but are highly elusive. In fact they are a good answer to the dark matter and other physics mysteries
https://arxiv.org/abs/1303.6912v3