Mods, please move this if it isn’t right for GQ. I just wasn’t sure.
I have been thinking about particle physics a great deal lately. I have a nagging sense that there’s something we are missing or overlooking.
So, forgive me if this is a dumb question, but is it possible that a proton is something like a stem cell, in the sense of being made of material that is undifferentiated until it becomes necessary to specialize? That there isn’t one specific particle that confers mass, but rather that mass is a property of a whole proton, and the separate particles are simply tools that it “extrudes” in the course of various processes?
I’m not sure I understand your question, but I can say a few things that might grant some insight.
First, the proton is a composite particle, consisting of three quarks (two ups and one down).
Quarks cannot exist on their own, away from other quarks: Any grouping of quarks in the real world must be of a combination with no net color, and integer total charge (these two conditions work out to be equivalent). A proton certainly can’t emit free quarks.
All quarks of a given flavor are indistinguishable, as are all protons. By this I don’t just mean that they’re indistinguishable given current technology; they must be truly, completely indistinguishable in order for quantum mechanics to work in the way that we observe it does.
The three quarks that make up a proton each have mass, but the sum of those three masses is much less than the total mass of a proton. Most of the proton’s mass comes from the binding energy associated with the Strong Nuclear Force that’s holding the quarks together. The masses of the individual quarks are believed to be due to the Higgs mechanism, but the mass associated with the binding energy has no connection to the Higgs.
Yes, it helps, Chronos; but then where do all the other sub-particles come in?
As I understand it, in a collider, when protons hit, the collision disassembles the two protons into their composite parts (not rubble) which can’t be directly seen, but are each identified by a characteristic trajectory which leaves a detectible trace. The various traces each constitute a separate particle with a function, and a name. Is that correct?
Dammit, wiki’s servers are down at the moment; but all the recently-added particles are derived from the results of these collisions, then?
Yes, but you shouldn’t think of them all being packed inside the proton waiting to be broken out. E=mc^2. Energy equals mass. There’s a lot of kinetic energy contained within the protons when they hit each other. Some of that kinetic energy is converted into mass – i.e. new particles. The more energetic the impact, the higher the mass of the particles you can create.
Virtually all, if not all, the particles found in the last 75 years have been through this method. Remember “atom smashers”? Smashing particles into particles and observing what comes out is really the only method we have.
This isn’t rigorous, but essentially particles are formed from energy. Any energy source can form particles. The vacuum - space at its most stripped down - contains energy. QM says that virtual particles are formed in pairs, one matter, one anti-matter, and then almost instantly annihilate. These pairs can be any particles at all, since their mass is measured in terms of energy. (That’s why you see particles measured in terms of electron volts.) A proton has a mass of 938272310 ± 280 eV. The Higgs particle is estimated to be in the vicinity of 1.4 trillion eV.
Anyway, while the quantum world is considered to be a constant roiling boiling froth of virtual particles, we don’t see them unless something unusual happens and they fail to annihilate. Scientists need a better source of directed energy that can be used as the seed for creating particles. Accelerators are the best tool for that. The particles used in them can be accelerated to high speeds and this is the equivalent of high energy. The bigger the accelerator, the higher the energy, and the broader the range of particles that can come out of the collisions (which can be fine-tuned to produce specific energy ranges). It’s not that these particles are components of the accelerated particles; it’s that any particles can form out of the energy the collision provides. Only the latest versions have enough power to create energies as fantastically high as the Higgs particle is estimated to be.
Perhaps some of the confusion exists because scientists use energy to describe particles in two ways. One is as an equivalent of mass. An electron weighs 9.10938291(40)×10[sup]−31[/sup] kg. This is not 1 eV but 510998 eV. All the other energies in eV can therefore be put into kilograms.
But while the mass of a particle is a specific number, a particle can gain mass by acceleration and the resulting energy can also be expressed in terms of eV. The so called oh-my-god particle was measured as having an energy of 3×10[sup]20[/sup] eV, or around that of a thrown baseball. One particle. Nobody knows exactly what kind of particle it was, but that fantastically huge energy was a result of its relativistic speed, not because it was at its core a particle of a certain type.
I’m not sure where in all this your confusion lies. I think part of it comes from not understanding the terminology used to describe sub-atomic particles, which makes perfect sense to scientists but calls for careful reading by outsiders. But it’s really annoying to scientists to make a claim that “we’re” missing something. You are. They aren’t.
You can probably read up on it at Wikipedia, or maybe somebody better versed can explain it in better detail (this is a general overview, more of why than what):
Theoretical physics took a huge step forward when a good model of the weak force was developed. And not only did it explain that, but electromagnetism as well. This electroweak theory was so compelling that it almost “has to” be correct - every theoretical physicist loves it when theories get simplified*. The theory required something that would differentiate electromagnetism and the weak force (“symmetry breaking”), and one way to do that is with this Higgs mechanism. It is, as said, what gives those quarks their mass while photons, on the EM side, don’t have any.
It remains one of the only pieces of this nice theory that hasn’t been quite figured out experimentally - the Higgs boson would be excellent evidence if it is found. Or someone will figure out a way to do it that doesn’t require this.
*True to form for good scientists, it was not widely accepted until something it predicted, like the Z boson, was found experimentally.
And furthermore, the theory was also able to predict the ratio of mass of the W boson to the Z, and the ratio predicted was just right. I think that that mathematical relationship which predicted the mass is a large part of the “so beautiful it must be true” sentiment.
Is this the opposite of a nucleus where the total mass of constituents > mass of bound product? Wouldn’t this mean protons are less stable than lone quarks?
The OP’s idea of differentiation reminds me of quantum mechanics where the properties of a particle are only determined when (and how) you measure them.
By the time we can observe something and call it a proton, that object already has its life long characteristics, that have been established before we observe it.
You could say that sub-atomic particles are undifferentiated until something makes them become protons, etc, but the level of differentiation you refer to, is already established and not an unknown quantity.
If it were purely a matter of the energies, yes. But there are other criteria that determine stability. As I already said, isolated quarks (or anything else with nonzero net color or noninteger charge) are impossible, and it’s also nearly impossible to change the total number of quarks (with antiquarks counting as negative). The proton happens to be the lightest particle with a total of three quarks, so absent those extremely rare reactions which change the total quark number, there’s nothing it can decay into, and so it’s the next best thing to perfectly stable.
I thought the observation is what establishes it, or any subatomic particle? I’m not being smart-alecky here, but trying to get a sort-of philosophical handle on QM.
The proton’s “life-long” characteristics have been observed, analyzed mathematically etc., up the wazoo, so those characteristics are a given. Correct?
But there is always the chance of an “oh my God” proton, isn’t there? Just like there’s always a chance that an elephant could turn into a mosquito, given an infinite time-limit.
If it did something surprising, it wouldn’t be a proton. I suppose it’s conceivable that you might have some interaction which results in a particle that’s a superposition of a proton and some hyperon, to collapse into one or the other when observed, but I’m not sure what such an interaction would look like (it’d have to be something quite high-energy).
So, if I understand the above correctly, the object of colliders is not so much to simply disassemble the protons into their constituent parts, but to “pulverize” them to a sufficient degree that the resulting rubble re-groups into different kinds of particles – particles which are understood to not normally be present in an un-smashed proton. (except in potentia) So the protons are essentially grist.
That makes a little more sense to me.
Exapno Mapcase: Oh, to be sure, I have missed a great many things, starting with college. I am certain that you, and everyone who has responded here, are far better informed than I regarding QM and particle physics. Nonetheless, with all due respect, I reserve the right to an opinion on the matter, and I think that there is a perceptual stance we have that is holding us back from full understanding.
I’m confused here. Can’t a proton emit a positron and become a neutron, in the same way a neutron can emit an electron and become a proton? Isn’t that a well known nuclear reaction?
Or is that one of the exceptions you are talking about? How many quarks does a neutron have?
One is the opinion of an expert in the field, using years of research and understanding to give weight toward a certain position. The other is the general everyday sense, that anybody can have an opinion on anything, even if its totally worthless. You have the right to the second kind of opinion. You don’t have the right to have anybody else take it seriously. Lest you think this is an attack, I should point out that my opinion on any disputed subject in advanced physics is totally worthless as well. The only difference is that I assume that the world’s experts have more valuable opinions on those subjects than I do.
Well, plus a neutrino, too. Yes, that’s possible, but only if there’s energy coming from somewhere other than the proton. A proton just sitting by itself will never turn into a neutron plus other things, because a neutron (by itself even, even before you add the positron and neutrino) has more mass than a proton, and that mass has to come from somewhere.
The very rare exceptions I refer to are so rare that we’ve never actually seen them occur. As in, if you waited for somewhere north of 10[sup]34[/sup] years, the proton you’re watching might decay into something (most likely either a neutral pion and a positron, or a pi+ and a neutrino). 10[sup]34[/sup] years is of course far larger than the age of the Universe thus far; we can only push the bound that high by watching a great many protons at once.
The best scientific idea behind a “stem-cell particle” is probably string theory. It’s popularity waxes and wanes, at least partly because it’s got no experimental evidence. Whether the math has any merit is up for debate.
Anyway, the (vastly oversimplified) idea behind string theory is that all particles are made up of strings of vibrating energy. The way they vibrate determines the properties of the particle - “mass vibrations” give mass and “charge vibrations” give charge. If true, it means that a proton (or anything else) could be turned into something else by changing the underlying vibrations. The strings themselves are undifferentiated except for the way in which they vibrate
Of course, the String Model is still subject to all the same conservation laws (charge, mass, etc.) as any other particle physics model. They’re probably recast as momentum conservation laws or something similar, but you still can’t violate them.