Are quarks shared between nucleons?

[Devils]

Homework Statement

This is not a homework question, I'm asking it here to avoid being shot down elsewhere.

Are quarks tightly bound to each nucleon? Is the nucleus just a bunch of anonymous quarks held together by the strong nuclear force, or is each proton and neutron separate? When particles collide, to they "stick" to their "mother" nucleon?

3. What have you tried already?

http://physics.stackexchange.com/questions/184220/what-is-the-motion-of-quarks-inside-nucleons

but it didn't help much.

 

[mfb]

Quarks (of the same flavor and color) are not distinguishable. You cannot label them and ask if the same quarks are in the same nucleons later, or even if those quarks still exist.

 

[Devils]

So its just an accounting mechanism. Pictures of a nucleus being made of individual protons & neutrons is a simplification, its just a bunch of quarks and one force to bind them all?

I would think delocalised electrons are similar eg in benzine the electrons are not bound to a particular hydrogen nucleus.

 

[Jeff Rosenbury]

An elementary particle only exists when it is being "observed". Further the observation interacts with the particle changing it. ("Observed" means interacted with in a way we can test, such as covalent bonds in molecules.)

Between observations it ceases to exist -- at least from a scientific viewpoint. No observation implies no science.

You are welcome to decide for yourself, as a matter of faith, what "really" happens. But science is about what is observable.

And yes, this is a model/accounting trick. It's called the standard model. It allows predictions to be made with great accuracy, so it seems to be worth a lot.

 

[mfb]

Between observations it ceases to exist -- at least from a scientific viewpoint.

Only in some interpretations of quantum mechanics.

But the pictures of nuclei made out of nucleons that have three quarks inside are a massive oversimplification, independent of the interpretation.

 

[Jeff Rosenbury]

Only in some interpretations of quantum mechanics.

Interpretations are matters of faith, at least initially. They can motivate thinking and help to develop new, testable hypothesis, so they are far from useless. But until and unless they can be tested, they are opinion. (Well informed and relevant opinion, but opinion.)

 

[mfb]

Stating things as fact when they are true in some interpretations only is misleading, and it is certainly not a scientific viewpoint.

 

[Vanadium 50]

They can motivate thinking and help to develop new, testable hypothesis, so they are far from useless. But until and unless they can be tested, they are opinion.

I disagree that they are far from useless. They are useless. The definition of an interpretation is that it gives the same answer, so it cannot - by definition - lead to anything testable. If it makes a different prediction, it's a different theory.

 

[Jeff Rosenbury]

I disagree that they are far from useless. They are useless. The definition of an interpretation is that it gives the same answer, so it cannot - by definition - lead to anything testable. If it makes a different prediction, it's a different theory.

Yet more fully developing two different interpretations can lead to subtle, testable differences. Of course they usually don't, but new ideas need to come from somewhere.

So perhaps only "mostly useless"?

 

[nikkkom]

Yet more fully developing two different interpretations can lead to subtle, testable differences.

By definition, something which gives testably different prediction is not an interpretation, it's a different *theory*.

 

[mfb]

Can we please stay on topic here? This is not a thread about interpretations of quantum mechanics.

 

[RGevo]

I can give you my opinion on the thread, though it's just an opinion since I lack the knowledge of nuclear physics.

Protons and neutrons have quantum numbers (isospin), and the constituent quarks/gluons which we interpret in perturbative QCD inside the nucleons obey momentum sum rules etc.

It's not clear to me whether such statements are true at the level of protons and neutrons within bound nuclei. I suppose overall the entire system of protons and neutrons conserves charge (electric/colour/isospin/spin..).

But does it make sense to ask about the individual protons and neutrons within a bound object, and then further ask about constituent quarks? I don't think so.

I should say, I know there collective effects which alter the observed distributions of quarks and gluons within a nucleon (proton/neutron) within bound atoms as compared to free protons and neutrons. They quarks aren't 'shared' among nuclei, but are certainly affected by the presence of other nucleons in the atom

 

[Vanadium 50]

This thread is kind of a mess. Lots of blind alleys and wrong directions.

There are two questions wrapped in here. One is "Is there any way to say that quark used to be in that other nucleon?" The answer is no. Quarks with the same quantum numbers are indistinguishable and as such the question cannot be answered. The other is "Is there any reason to describe a nucleus as a collection of nucleons and not just a collection of quarks" (e.g. can Ca-40 be described as 20 protons and 20 neutrons or does it need to be described as 60 up quarks and 60 down quarks) and the answer to that is yes. Indeed, the very existence of Ca-40 comes about because 20 protons and 20 neutrons is a particularly stable configuration.

 

[FieldTheorist]

Are quarks tightly bound to each nucleon? Is the nucleus just a bunch of anonymous quarks held together by the strong nuclear force, or is each proton and neutron separate? When particles collide, to they "stick" to their "mother" nucleon?

There are multiple answers to this question, because (like all non-technical questions) this is vague. A nucleon is a (meta)stable bound state of quarks and gluons. That means that you take mixtures of odd numbers of quarks, they create a strongly-coupled gluon field around them, which binds them together (i.e. a bound state) for a significant amount of time (protons theoretically are stable, meaning they never decay, but single neutrons outside of an atom will decay after about ~10 minutes).

Quarks do leave their nucleons, in a sense, during the propagation of the residual strong nuclear force. The residual strong nuclear force is when two nucleons exchange pions (the carriers of the residual strong force, which are composite particles made out of two quarks), and it's what counteracts the electromagnetic force of the tightly bound positively charged protons. It's also why you need more nuetrons as you increase the periodic number (i.e. number of protons) on the periodic table, because they provide more residual strong nuclear force. However, getting back to your question, because all quarks of the same type (i.e. same properties: charges, generation, and chirality) are all identical, it's meaningless to say "Quark A stayed in the nucleus, and Quark B got exchanged with the nucleon."

An elementary particle only exists when it is being "observed". Further the observation interacts with the particle changing it. ("Observed" means interacted with in a way we can test, such as covalent bonds in molecules.)

Between observations it ceases to exist -- at least from a scientific viewpoint. No observation implies no science.

You are welcome to decide for yourself, as a matter of faith, what "really" happens. But science is about what is observable.

And yes, this is a model/accounting trick. It's called the standard model.

This is not a mainstream scientific perspective (those follow more along the lines of the Copenhagen interpretation or the various decoherence interpretations). I would suggest that non-experts take the above post with a grain of salt.

As a personal comment, I consider this ultra-positivist philosophy to be incoherent nonsense. I'm a scientific realist and a proponent of the confirmation holism, as are almost all major physicists that I've ever met or read (e.g. Weinberg wrote a well-known article on this, called "Against Philosophy." I disagree with parts of his premise, but the attack on positivism is definitely spot on.)

I disagree that they are far from useless. They are useless. The definition of an interpretation is that it gives the same answer, so it cannot - by definition - lead to anything testable. If it makes a different prediction, it's a different theory.

It's nearly useless to explain this to crackpots. I once got into a nearly ~100 post, year long discussion with a Lorentzian aether theorist over this. (It wasn't a total waste of my time, I learned a valuable lessons on how to deal with these people.)

Anyways, I'm not saying anyone on here is a crackpot, but the crackpots I've met before are surprisingly ignorant and recalcitrant on this point.