As far as I’m aware, our understanding of the strong interaction and strong nuclear force is pretty good. We can explain how particles move and stick together with good models. But from what I can find, I gather we do not have a mathematical theory that explains this in a precise manner. For example, for electromagnetism and gravitation, we have well established models that predict the phenomena precisely. My question is, is there anything lacking, such as a big hole in our knowledge, or is there any big problem or issue that has stopped us from establishing a mathematical model for the strong interaction?

  • $\begingroup$ I gather we do not have a mathematical theory that explains this in a precise manner. This is wrong. There is a precise mathematical model of the strong interaction between quarks. It has existed for about 50 years and is well-confirmed. The strong interaction between nucleons is a messy side-effect but is in principle explained by the same theory. $\endgroup$ – G. Smith Feb 26 '20 at 0:45
  • $\begingroup$ @G.Smith anywhere I could find it then? :) $\endgroup$ – Melvin Feb 26 '20 at 6:23
  • $\begingroup$ The green box here has the QCD Lagrangian for a single kind of quark. All six kinds of quarks are the same in how they feel the strong force; they just differ in mass (and electric charge, which is not relevant to the strong force). The entire theory flows from this expression! $\endgroup$ – G. Smith Feb 26 '20 at 7:23
  • $\begingroup$ If that kind of Yang-Mills Lagrangian is meaningless to you, then you should start by learning the quantum field theory of scalar fields, then QED, then QCD. If the entire concept of a Lagrangian is meaningless to you, you need to study advanced Newtonian mechanics. $\endgroup$ – G. Smith Feb 26 '20 at 7:23
  • $\begingroup$ I see in your profile that you are a high school student. Relativistic quantum field theory is generally graduate-school-level physics and is not easy to learn. If you want to eventually learn it, for now I suggest learning Special Relativity, classical EM, advanced classical mechanics, and nonrelativistic QM. If you try to plunge in to QFT without understanding these first, you will probably be lost by page 2 of a QFT textbook. $\endgroup$ – G. Smith Feb 26 '20 at 7:35

Most of the calculations in quantum mechanics are made using the perturbation calculus. That is, we assume that the theory with no interactions that describes a free particle is an approximation of the full theory, take the classical solution of the free equations, and then add corrections. There are usually infinitely many quantum corrections that need to be calculated, but they often form a series in which every next term is much smaller than the previous one, and calculating just few initial terms gives a good approximation of a full result.

The problem with strong interaction is that the interaction is so strong we cannot reasonably approximate the real physical states with free-particle states. That means we fail at the first step, and we cannot use the perturbation approach that works in other cases.

Anther problem with QCD is that gluon fields interact directly with itself, making even the classical equations are non-linear, as opposed to linear equations of QED (in which electromagnetic field does not interact directly with itself, but only with charged matter). And we don't have the right mathematical tools to fully solve non-linear equations. This is similar to Generla Relativity, which also has non-linear equations. Because of that in GR we only know very few exact solutions. QCD has the same problem, and because of that we need to relay more on the simulations than on analytical solutions.

  • $\begingroup$ well and good,but I would like to draw the attention on the ampliduhedron which is another calculatonal view of the dynamics of strong interactions , not perturbative ,arxiv.org/abs/1312.2007 and arxiv.org/abs/1810.08208 $\endgroup$ – anna v Feb 25 '20 at 13:00
  • $\begingroup$ Dear @annav, sometimes you are spot on, sometimes you miss, but always with plenty references to back up your point. These smack of AI behavior. Please, please tell me that you are indeed human, or at least partial human. $\endgroup$ – MadMax Feb 25 '20 at 15:23
  • $\begingroup$ @MadMax as I am turning 80 in april we can settle on the partially human :). Because of my age I do not trust my memory and look up as much as possible for references to what I think is a correct answer/comment , then it is a waste not to link what I found. $\endgroup$ – anna v Feb 25 '20 at 15:48
  • $\begingroup$ Thanks @annav. Superb job! I am impressed. You 80, Joe 77, Mike 78, and Bernie 78. "Do not go gentle into that good night". $\endgroup$ – MadMax Feb 25 '20 at 15:55
  • $\begingroup$ @MadMax + anna v :) aww meep. ❤️#meep. (that's meant to be a soft warm Good sound to wish soft Warm feels from my little Warmheart.) $\endgroup$ – The_Sympathizer Feb 26 '20 at 3:00

Wikipedia has mentioned that in the high energy range which is the domain of particle physics the perturbation theory works fine which can result in sort-of-analytical form of predictions which match the experiment results with good precision which is the main success of the theory of strong interaction come from for now.

But in the low energy range when quarks bind to together to form hadrons and hadron bind toether to form atomic nucleus, the perturbation theory fails and the only thing valid is sort-of-pure-numerical-framework solutions called lattice QCD.

With LQCD and enough computing power you can predict properties of hadrons and atomic nucleus and even nuclear reactions with high precision from the theory of strong interaction. But for now due to limited computation power LQCD can only predict hadron with some acceptable precision.

Due to the near to the end of the Moore's Law, maybe only small atomic nucleus can be predicted finally in the future without model/algorithm/hardware breakthrough. Another breakthrough may come from Quantum Computing.

Another route is phenomenological model like the many models of atomic nucleus which has little to do with the theory of strong interaction.

For example, for electromagnetism and gravitation, we have well established models that predict the phenomena precisely

This is because the classical model - the Maxwell' equation which deal only with EM field in vacuum or simple material which can be treated as a continous bulk with some properties like dialetric constant which can be used in the equation. Once you begain to deal with "true EM problem" - like predicting structures and properties atom/molecule/liquid/solid from quantum mechanics, it soon become not that precise too, a lot of trade off and approximation is needed to make it computable on current generation of computers.

For gravity, there is not even a "true gravity"(quantum gravity) theory for now and you treat star/galaxy/Universe as a continuous bulk using GR, like in classical EM.


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