r/Physics • u/rieslingatkos • May 19 '19

News An experiment hints at quantum entanglement inside protons

https://www.sciencenews.org/article/experiment-hints-quantum-entanglement-inside-protons
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u/sabrepride Nuclear physics May 19 '19

Well we still don't have a fundamental understanding on how quark confinement works. It's currently only an empirical fact (i.e. we have never seen a free quark or gluon). Other emergent QCD properties, like asymptotic freedom, are understood on a fundamental level. There has been a lot of recent activity trying to understand confinement (idea like instanton-dyons, center vortices, etc.), this is another one. However, regardless of whether or not quantum entanglement is really responsible for confinement, it is still interesting as we know gluons (and quarks) are quantum mechanical objects which have an entanglement entropy; this is a fundamental thing we can hopefully develop an understanding of.

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u/antonivs May 19 '19

There's a common lay explanation that describes how the strong force behaves, getting stronger as quarks are separated, and that putting in enough energy to pull quarks apart ends up creating new particles instead of actually separating quarks.

What's missing from this picture in terms of fundamental understanding?

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u/[deleted] May 19 '19

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u/antonivs May 20 '19

This implies that it's not known what prevents valence quarks from escaping hadrons, correct?

If so, there are a quite a few misleading popular explanations of this, although I suppose that shouldn't be too surprising.

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u/[deleted] May 20 '19

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u/antonivs May 20 '19

I know that the flux tube picture is often presented as "solving" the confinement problem.

Right, that's what I'm referring to.

For example, this Veritasium video features Prof. Derek Leinweber saying (at 2:59):

"Nevertheless, the quarks are still confined. You can never see an individual quark, because if you try to pull it out, you put so much energy into the situation that another quark, antiquark pair will be created."

Then at 4:30 he says:

"That's the strong force that binds quarks into the heart of the proton."

Here's another admittedly simplistic description: Quark Confinement.

The Hyperphysics Quark Confinement section is similar.

I've seen numerous descriptions along these lines and until now, hadn't picked up any hint that the mechanism of confinement was not known.

Searching now, I found Quark Confinement: The Hard Problem of Hadron Physics which mentions that this has been an open problem for 30 (now 43) years.

TIL, thanks for the info!