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I wonder if there existed a complete theory describing the behaviour of quarks in nucleons and other baryons and mesons and the behaviour of gluons. Does QCD alredy describes it all in perfect detail or is it a unclear subject and poorly understood? The standard model of particle physics seems already to describe every interaction other than gravity, but is it really complete and completely valid?

Oh yeah and do we know what color charge is physicaly?

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  • $\begingroup$ No theory in physics is complete and completely valid. That's not the requirement on physical theories (or scientific theories in general). The requirement is that they can explain and numerically correctly describe a wide range of phenomena on some scales and for some range of phenomena. QCD is very good at giving us the right description of known hadrons, but it is not the right tool to describe nuclei. One can say the same thing about the Schroedinger equation and molecular physics. One simply doesn't get very far with it (alone) past hydrogen. It doesn't even work well for helium. $\endgroup$ – CuriousOne Dec 21 '15 at 2:50
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    $\begingroup$ "what color charge is physicaly"...what do you mean by that? What is electromagnetic charge, "physically"? What is the weak hypercharge, "physically"? Also, how does this question related to your other question? $\endgroup$ – ACuriousMind♦ Dec 21 '15 at 2:52
  • $\begingroup$ Its a bit off indeed but I just wondered and you are right I also wondered about those other questions. But is there anyone trying to know what they really are? $\endgroup$ – user97166 Dec 21 '15 at 2:53
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    $\begingroup$ @Dibs You would need to define what you mean by that before anyone can answer it, and I've never met anyone who actually can define it. Most people seem to mean an explanation that feels intuitive and physical to them, but when examined closely those explanation are no more real than the ones they object to. $\endgroup$ – dmckee --- ex-moderator kitten Dec 21 '15 at 3:40
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    $\begingroup$ No, it's not "just invention". Energy in particular starts from a particular integration of Newton's 2nd Law (the Physics 101 concept) and goes on to find a deeper explanation as the Noetherian current associated with invariance in time. That second notion is part of a deeper structure than is apparent from staring at Newton's Laws, but it isn't a mental picture of gear and other things that you understand intuitively, it's a mathematical abstraction of a high order. $\endgroup$ – dmckee --- ex-moderator kitten Dec 21 '15 at 12:16
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All good comments above but @Dibs seems a little hazy about what constitutes the standard model and the regions of validity of its components. Without getting into too much technical detail, here is my attempt to clarify the situation.

QCD is the theory of the strong interaction. It is unusual among physical theories in that it seems to work better for high energy processes than low energy processes. What I mean by "work better" is that it is easier to perform the computations and compare the results with experiments. For low energy interactions between hadrons, for example, we cannot calculate with precision the interaction between two nucleons and so we are forced to employ effective meson exchange models if we wish to study nuclei. It is far from a theory that "describes it all in perfect detail."

QED (the quantum theory of the electromagnetic interaction) is the closest we come to a "really complete and completely valid" theory. It can be used to calculate certain quantities to ten significant digits (which matches the current limits of experiment). This theory could be pushed further should experimental improvements justify the effort.

QED has been unified with the weak nuclear interaction (the theory of beta decay) to form the electro-weak theory and this has been unified with QCD to form what we call the standard model. The standard model is not complete, however, because it does not incorporate gravity and, furthermore, has no explanation for dark matter, or for the big bang itself. Some might also include dark energy in the list of excluded phenomenon, but see my answer to this question: Are there experiments taking place right now that might show evidence for or falsify dark energy or dark matter? for a possible explanation for dark energy that falls within the standard model.

A final comment: The basic difference between physics and mathematics is the level of precision that is attainable. We can know with certainty the value of pi to millions of significant digits but the best that can be done with physics will always pale in comparison.

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  • $\begingroup$ Interesting stuff, Lewis. Particularly since electron capture is the flip side of beta decay. $\endgroup$ – John Duffield Jan 3 '17 at 13:42