do all quarks get annihilated?

I2004

Im aware of quark antiquark annihilation in a proton and have heard one opinon but want another. Do all the quarks get annihilated and just the number remain 3 remain constant in the proton or are the 2 up and 1 down quark the specific quarks that exist in the middle of the storm and they never go? how long do we think it takes for a full quark turnover?

is this also proved or just theory?

mpv_plate

This is proven by many experiments. If there were only 3 quarks in a proton, the collisions in LHC would look quite differently.

There are no specific quarks inside a proton that would be "untouchable": no quark is protected. Every single quark can be annihilated any time and other quarks are created in the meantime. There is just a small abundance of 2 up quarks above anti-up, and there is 1 more down on top of all anti-down. But again - the extra quarks are not special. You cannot say if a particular up is the extra one or not. All quarks are subject of the annihilation / creation.

Please also note that it is much more complicated. It's a messy inferno that cannot be easily described. It's impossible to even find out how many quarks there are inside a proton, because the number is changing extremely fast and actually it is uncertain. There are not only up and down quarks and antiquarks, but also strange quarks, gluons, etc., all of them moving at relativistic speeds, colliding, scattering, annihilating.

DiracPool

Is this correct? That the inside of a proton/nucleus is a teething swarm of energy manifesting and demanifesting "virtual particles" continuously? And that quarks and the other elementary particles are not static but transient states of a dynamical system? This is new to me. How does that view square with what I've learned about how most of the elementary particles were born soon after the big bang and the guy, I think it was Brian Cox, saying how every proton in the universe is identical to every other proton. What about all the free hydrogen in the universe? Is each one of those atoms and molecules undergoing a continuous metamorphosis?

mfb

There are no proofs in physics, but the quark model is extremely successful, and there is no similar alternative.

The typical timescale of QCD is ~400 MeV, this corresponds to 10^-23s.

It is just a model, of course, but I think it is quite good.

If you restrict this to protons, neutrons (in helium), photons and electrons, it is a good approximation.

This is right.

If you look inside the proton: Yes.
"Proton" is just a word for this mess of quarks and gluons.

Bill_K

Yes, it is uncertain, but no it is not "changing extremely fast." The picture that quantum states "seethe", that they "pop in and out of existence" is a popularized image of a quantum superposition, aimed at an audience where quantum superposition would be a difficult concept.

In fact the proton is a time-independent state. Its quark/gluon content does not change with time, let alone seethe. Rather the number of quarks and gluons present in a proton has a certain probability distribution.

K^2

It's hardly worth making a distinction. Time evolution of a proton is just a sum over all possible diagrams. In each particular diagram, there is quite a bit of "seething" going on. The sum over all possibilities gives you a steady state. It's not wrong to say that there are constant creation-annihilation processes going on within the proton. There are. They simply happen to be balanced to give you a particular state.

Bill_K

Suit yourself, but this is a sloppy way of thinking that I do not ascribe to. Especially, people with non-physics backgrounds deserve better information than this. It's not only a misconception, it's a harmful misconception, only adding to the impression that quantum mechanics can be understood as classical mechanics with the uncertainty principle tacked on. The sooner it's communicated that quantum mechanics is fundamentally different, the better.

One can tell from the followup questions we get how misleading things like this can be. We hear: charged particles shooting photons at each other. Angular momentum vectors wobbling back and forth, electrons dancing around the nucleus in well-defined orbits. Does Schrodinger's Cat "change extremely fast" between being alive and dead? Descriptions like these lead the novice to plausibly ask how fast, how often, what happens in between, and so on. Explaining them as simply examples of quantum superposition avoids the confusion.

The confusion extends, I'm afraid, to physics graduate students, who from taking a course in field theory get the impression that "diagrams" are the universal answer, and that time-dependent perturbation theory is appropriate for everything, including bound states. Or that the structure of protons can be understood from any kind of perturbation theory at all.

K^2

You can do non-perturbative QCD with diagrams. You just need infinitely many of them. :p That's, by the way, the only way I know how to actually do computations of proton structure. Approximations are made, and corners are cut, but it's not bad, all things considered.

But more importantly, we shouldn't be tailoring our jargon for the laymen. Scientists and medical doctors used to speak Latin specifically to exclude the laymen. Now, the fields have gotten complex enough for it not to be strictly necessary. (Though, I think we should have stuck with it.) It's very important to be able to quickly distinguish between scientists and people who try to pretend to be scientists.

When a student doing field theory asks about photon exchange in a scattering process, I can immediately tell a difference from someone who has no idea what they are talking about. In the later case, I'm going to try and lead the discussion towards a classical case, if it is possible. There is absolutely no reason to try and explain details of how QFT works to a person who doesn't know anything about QFT.

OP has been asking questions which amount to asking if we can pin-point a specific quark in a proton. Answer is no. And I have no better means of explaining it than saying that quarks are constantly created and destroyed. You think you can explain the full case, where number of quarks is a distribution to this person? You give it a shot.

bcrowell

It's inevitable that popularizations are going to create misconceptions. Ideally, everyone would go to grad school in physics. Realistically, that isn't going to happen.

A good way to deal with this kind of thing is to write a FAQ. We have FAQ sections in relativity and cosmology. It would be good to start building one for this subforum.

mpv_plate

I have read that significant amount of the proton mass is kinetic energy of the particles inside a proton. If proton is time-independent state, how can there be some kinetic energy inside it?

mfb

Simple: The kinetic energy is the same all the time.
You always have a superposition of many particles with different kinetic energy, and if you sum over all of those particles you get a constant value.

mpv_plate

Thank you for the answer, it is now more clear to me.

kurros

Note also that you could have asked a similar question of the electrons orbiting nuclei. These too are time-independent states, and yet have kinetic energy.

mpv_plate

That is right. I should have realized that.