Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.

Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within baryons or mesons.

This sentence makes me very nervous: Due to a phenomenon known as color confinement

This sentence is Like I want to prove something spurious to save the subject (quark).

share|improve this question
add comment

5 Answers

A point of view that is missing from all the answers so far is that particles are best viewed as excitations above some background (usually called the "vacuum"). This means that the particle content depends on the phase (or "material") that you're studying. For example, in a crystalline solid, you don't "see" atoms, but only vibrations of the entire crystal lattice --- a picture which becomes pretty useless once that crystal melts.

In the low-temperature, low-pressure phase that the majority of the observed universe seems to be in, quarks are confined and do not appear as the elementary excitations. However, at high temperature and pressure quarks and even gluons can become deconfined, to the point where they can be regarded as elementary excitations (this is still being probed and the consensus view on this is evolving). Incidentally, an amusing (but astrophysically realistic) case is high-density but low-temperature; in that case the current (as far as I know) view is that one gets a colour superconductor. Indeed, that phase may be the "same" as the normal phase (in that there exist smooth paths which connect them), and leads to the identification of baryons as quarks and mesons as gluons, appropriately dressed by screening charges.

From a theory side, they exist in so far as they are the fundamental fields used in the description, and that description has been shown to be remarkably good in a wide range of situations. Maybe one day we will discover substructure or something, but that theory must reduce to QCD in the appropriate limits, and in those limits we will still be talking about quarks and gluons.

share|improve this answer
add comment

This question can be answered by analogies:

Thermodynamics was the pinnacle of the mathematical formulation of physics for a long time. Then statistical mechanics was experimentally discovered to hold, due to the particulate nature of matter with a whole new tool box for calculations. Did this make thermodynamics "false"?

The quark model and its theoretical formulation is based on solid experimental evidence, as Ron Maimon points out in his answer. If, and it is a big if by itself, an underlying theory is ever found which will be necessary to explain new, unknown to us now, data, which may change the way physicists explain the Standard Model accumulation of data, the quark model will still hold, in a similar manner as thermodynamics still holds, though it is emergent from quantum statistical mechanics.

So no, the quark model cannot turn out to be false.

share|improve this answer
add comment

Your position on quarks, that they are made-up constituents, was the mainstream view in physics from 1964, when Zweig and Gell-Mann independently proposed quarks, to November 1974, when the charm quark was discovered and everyone else decided it was the right idea. In this intermediate time, the idea of a permanently confined particle was considered suspect, so the theories of the strong interaction were not allowed to speak about hypothetical pointlike consituents. The quark model was still very strongly supported by indirect evidence, but in a sense it is good that strong interaction community rejected these ideas, otherwise we might never have string theory.

The phenomenon of confinement is not so mysterious, it was already understood on 1 dimension by Schwinger in the early 1960s. In 1 dimension of space, the electric force doesn't spread out as it does in 3 dimensions, and the force doesn't get weaker with distance. This means if you pull apart an electron and an anti-electron in 1d, at some point, you do enough work to make new electron anti-electron pair from the vacuum, and this pair-production neutralizes the two particles. This means that the only finite energy states are neutral composites. This was extended to the non-abelian case by 'tHooft, and the notion of confinement is completely understood in 1+1 dimensions.

The idea Gell-Mann indirectly promoted, and which was established by many people in the 1970s, is that something like this happens with quarks, that they are linked by a flux-string that makes the force constant, as if it were effectively 1 dimensional. This flux-tube idea was not well established in the 1970s, but you can't reject it anymore. Aside from lattice simulations, which show the flux tube in static-force calculations (the force between two quarks is constant with distance when they are far apart, just as in 1 dimension), there are also known exact dualities between string theories of infinitesimal flux lines and certain gauge theories that are similar enough to QCD that one can get a handle on how confinement qualitatively happens.

So the basic answer to your question is "no". It is as impossible for quarks to be wrong as it is for there to be no such thing as an antiproton. The evidence for quarks now comes from heavy-quark physics, where we can see spectroscopically the heavy quarks bound in non-relativistic bound-states with other heavy quarks. These charm-charm, bottom-bottom systems behave in just the way expected from nonrelativistic particles bound by a gauge force.

There is separate routine evidence from high-energy inclusive scattering. When you smash protons, you see jets, which are showers of particles in certain directions, and the jet emissions are correlated, so if you see three jets emerging, you can figure out the momentum of the objects which came from the collision point, and the probability distribution of the jet energy and angle can be calculated from QCD. The QCD calculations are in complete accord with experimental data, so much so that one has to go to high order of peturbations to match the distributions in complex multi TeV-energy scattering.

share|improve this answer
Ron, would you say that confinement in higher dimensions is due to the very same mechanism as in 1+1 dimensions, but acting within the flux-tube? –  Mitchell Porter Sep 11 '12 at 3:28
@MitchellPorter: This is well accepted. The problem is showing how the strong field gets squeezed into 1 dimension. The basic idea is 'tHooft's, that the strong vacuum can't sustain a weak chromoelectric flux, like a superconductor can't sustain a weak magnetic flux. In a superconductor you squeeze flux into a flux line. If you have a weak chromoelectric flux, it orders the vacuum too much, the strong field is Euclidean random at large distances, so instead of adding a small bias over all space, you compress the bias into a flux line. This is the standard idea, but it's not quantitative yet. –  Ron Maimon Sep 11 '12 at 3:59
... but it can't be wrong, because you can see the flux tube emerge in lattice QCD, and it's also completely clear from the long-distance independent randomization that this is what happens. –  Ron Maimon Sep 11 '12 at 4:00
@Ron Thank you for these insights into the dimensional aspects. I wonder if you might be able to take a look into my open question. Perhaps I could draw an understanding of the lattice structures interactions. –  Garet Claborn Sep 16 '12 at 22:59
@GaretClaborn: It's not an open question--- it's a nonsensical question. The Higgs field doesn't have a "shape" and it is no more top anti-top as bottom anti-bottom or up anti-up. It's not a quark bilinear, this is clear, but it might be a bilinear of something else. The quarks have direct strong interactions, while the Higgs gets strong interactions only through loops. I don't have any more to contribute to the answer, because "shape of the Higgs" doesn't make sense. –  Ron Maimon Sep 17 '12 at 7:34
show 4 more comments

The idea of quarks wouldn't ever be false, but the categories of the Standard Model are based on a certain unspoken assumption: that the physical ontology stays constant at such a small scale.

share|improve this answer
Where does the word "ontology" appear in the standard model? The ontology is the path integral fields, and the calculations and experiments show that it is present at nuclear scales, not any philosophical prejudice. –  Ron Maimon Sep 11 '12 at 21:26
add comment

It is believed for good reason that confinement isn't just some made up thing. The good reason is from Lattice QCD. While it is not analytic proof, simulations of QCD show that QCD is indeed confining naturally. That is, confinement is already hidden inside QCD and happens naturally. Furthermore, for high enough temperatures QCD is deconfining, and the result is a quark-gluon plasma, where the two run around like a plasma. It is unfortunate that as of yet little experimental evidence exists for a deconfining phase of QCD, at least to my knowledge. But computational work on QCD sheds a great deal of insight into the genuine physics of QCD. As a note, deep inelastic scattering provides evidence for quarks, as the experiment resembles Rutherford scattering, which phenomenologically helped develop an accurate model of the atom eventually. Funny enough the "planet model" of the atom is an inaccurate interpretations in certain circumstances.

That being said, our interpretations of physics and physical systems changes (period). So almost inevitably, while quarks will probably stick around, how we view them will change.

If you are interested in constraints put on QCD by computational simulations look into Lattice QCD and Gauge Fields on a Lattice. It has been an important step in tapping into non-perturbative QCD.

share|improve this answer
A question for you: is anyone approaching the continuous confinement problem from the perspective of conformal limits of lattices a la Stanislav Smirnov's approach to percolation and other problems? –  Ryan Thorngren Sep 10 '12 at 23:14
@user404153: Everybody is doing that, that's the only approach. –  Ron Maimon Sep 10 '12 at 23:40
@RonMaimon Haha, thank you. I admit I cannot imagine any other way to tackle it. –  Ryan Thorngren Sep 11 '12 at 0:25
@GaretClaborn if you consider the atom as a little solar system with electrons orbiting the nucleus, then you end up having a continuum of energy states for any energy greater than the rest mass of the nucleus. This isn't the case, since we know that electrons in atoms only orbit at select energies. In other words there is gap in the energy an atom has where there is no electron orbiting the nucleus, and the lowest possible energy observed with an electron orbiting the nucleus, i.e. there are "orbitals". Schrodingers equation predicts these energy levels. –  kηives Sep 16 '12 at 23:01
@GaretClaborn Most circumstances. But, famously, if you fire electrons at an atom, sometimes it just goes through, and other times it scatters, and scatters backwards sometimes. This demonstrated that the charge distributions in the atom are not smeared uniformly everywhere, since the electron is affected differently for each shot. This clumping of charge distribution is attributed to the idea that the atom is built up of constituents, each of which has a charge or is neutral. –  kηives Sep 16 '12 at 23:40
show 3 more comments

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.