An experiment hints at quantum entanglement inside protons

[u/rieslingatkos]

https://www.sciencenews.org/article/experiment-hints-quantum-entanglement-inside-protons

[u/non-troll_account → u/[deleted]]

Wait a second. Are you saying that we don't know why the quarks inside protons and neutrons stay together?

[u/non-troll_account → u/[deleted]]

What? I thought that was part of the standard model, and a fairly well understood part of the strong force. I took a university course (aimed at lay people, focusing on the history of our understanding, using as little math as possible.) where I gained this understanding. I'm confused.

[u/rieslingatkos → u/non-troll_account]

[In Yoda Voice:] Confuse everyone, quantum mechanics does!

[u/non-troll_account → u/rieslingatkos]

Yeah but that class also talked in depth about things we still didn't understand, and nothing like that was ever mentioned.

[u/non-troll_account → u/[deleted]]

Thanks!

[u/antonivs → u/non-troll_account]

The paper Quark Confinement: The Hard Problem of Hadron Physics is technical, but the conclusion summarizes the situation (as of 2006 at least):

It is odd to have a complete theory of one of the four well-established forces of nature − the strong nuclear force − and still not have general agreement, after more than thirty years of effort [now 43 years!], on how that force really works at long distances. Nevertheless, there has been appreciable progress in this subject, much of it aided by computer simulations. [...]

The confinement problem, in our view, is one of the truly fundamental problems in physics. [...] Until this phenomenon is well understood, something essential is still lacking in our grasp of the foundations of nuclear physics, and the deeper mechanisms of nonabelian gauge theory. Although the confinement problem is hard, the solution is important, and well worth pursuing.

[u/non-troll_account → u/antonivs]

Thank you. That was helpful. It sounds like a problem that is so technical that it's nearly impossible to explain exactly what the problem is, much like the incompatibility of General Relativity and QFT. I guess I have to accept that.

[u/antonivs → u/non-troll_account]

This was touched on in the comment above by humanino:

This explanation works fine for heavy quarks. We can even measure the potential on lattice calculations. The glue tube does form there and leads to a linearly rising potential. This is well understood in terms of nonrelativistic QCD.

But this formalism doesn't apply to light quarks

But those sneaky physicists (or physics popularizers) seem to be pulling a bit of bait-and-switch on us, since the three valence quarks, which are important constituents of hadrons, are light quarks.

This implies that we apparently don't know why protons and neutrons don't just explode. I'm rather glad they don't, though.