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It's a Christmas time and so I hope I'll be pardoned for asking a question which probably doesn't make much sense :-)

In standard undergraduate nuclear physics course one learns about models such as Liquid Drop model and Shell model that explain some properties of nucleus.

But as I understand it these models are purely empirical and most importantly incompatible. Perhaps it's just my weak classical mind but I can't imagine same object being described as a liquid with nucleons being the constituent particles floating freely all around the nucleus and on the other hand a shell model where nucleons occupy discrete energy levels and are separated from each other.

Now I wonder whether these empirical models are really all we've got or whether there are some more precise models. I guess one can't really compute the shape of the nucleus from the first principles as one can do with hydrogen atom in QM. Especially since first principles here probably means starting with QCD (or at least nucleons exchanging pions, but that is still QFT). But I hope there has been at least some progress since the old empirical models. So we come to my questions:

  1. Do we have a better model for description of a nucleus than the ones mentioned?

  2. How would some nuclei (both small and large) qualitatively look in such a better model? Look here means that whether enough is known so that I could imagine nucleus in the same way as I can imagine an atom (i.e. hard nucleus and electrons orbiting around it on various orbitals).

  3. What is the current state of first-principles QCD computations of the nucleus?

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nice question, will be awaiting the answer as well. –  Cem Dec 23 '10 at 20:38
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Why don't you ask Santa? –  muntoo Dec 23 '10 at 21:11
    
So who does Santa get presents from? Just asking... –  Glytzhkof Jun 29 '11 at 2:26

3 Answers 3

up vote 3 down vote accepted

I can't give you a real answer but I figured this may be better than nothing:

  1. I don't know of any better model, although I'm not an expert in nuclear physics so I can't be sure one doesn't exist.
  2. I suspect that any better model would not give a nice neat physical picture of the nucleus, the way we have a nice neat physical picture of the atom. In particular, I wouldn't expect to see a model that predicts concentric wavefunctions for nucleons, the way quantum mechanics describes electrons. Protons and neutrons are complex composite particles with many internal (and external) interactions, so their wavefunctions will tend to be much more localized than those of electrons. This is one case where the classical particle model actually is not too bad.
  3. From what I've heard (from people who know this business), the current state of the art in lattice QCD computations is calculating the mass of the proton with some accuracy. They're a long way away from being able to actually simulate a whole nucleus.
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Thanks David. So if I understood you correctly, you are saying that I shouldn't expect any positive answer to my question. Still, I'd like to think that we can solve (numerically at the very least) small neuclei by assuming certain structure of neutrons and protons (I think their mass/charge distributions are pretty well known?) and certain effective residual strong force between them? –  Marek Dec 24 '10 at 9:39

Relevant link - Nucleons in platinum nucleus seem to move in a coordinated fashion

One more link - One-neutron halo nucleus Be-11

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Thanks, I'll read through it. But after reading first paragraph I have to say I dislike the article for having a sensational feel to it and it seems that it is written at the level of usual science journalism (read, complete crap). –  Marek Dec 24 '10 at 14:51
    
@Marek, I agree with you, but the article is based on real scientific work. –  voix Dec 24 '10 at 14:56
    
yes, it would appear so; references therein are definitely useful, so thanks again. –  Marek Dec 24 '10 at 15:27
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Halo nuclei, from what I've heard, were a sensation when they were discovered in the 80's, and they're still very relevant. The possibility of their existence sets a restrictive demand for theoretical nuclear models, a demand able to discard, refine or confirm the models. I partook in an experiment at ISOLDE earlier this year, mapping primarily Be-{10,11,12} to give more experimental data for theorists. The Be-11 paper you linked mentions some current nuclear ab initio models, and also the intricacies of naming one model "correct", since all models work pretty much equally well in this case. –  Daniel Andersson Dec 26 '10 at 1:19
    
@Daniel: thanks for giving this info. Now, if you cared to elaborate on these (and related) topics in a separate answer, that would be great and I'd probably give you an up-vote. –  Marek Dec 30 '10 at 16:46

When you get down to these length scales, you have to be careful with words such as "look". In my mind, the "answer" for this question requires a roundabout in the philosophical area.

"Look" - what does it mean in physics? Even more relevant: "is". When a physicists uses the word "is", s/he is really using a shortcut for saying "behaves as it would be", that is, it behaves according to some model (this is a realization that took a couple of years for me to reach. Of course, I might change it in the future as I continue my physics intake). This is not only true for quantum physics, but in the macroscopic world, the distinction is often thought of as not as important. No one can deny the following though: a rod is a model for a certain object, and we can prescribe this model to real-life objects and calculate. In some cases, the physical object that we one day called a rod might be called a spring, depending on what we want to do. What "is" the object? In the physicists eyes, it is both a rod and a spring, and there is no incompatibility, as long as one knows what one is doing. What does it "look" like? It depends on the length scale and the application.

In the quantum world, de Broglie says that the constituents are both particles and waves. Contradiction? No.

How does a nucleus "look"? Which model is "right"? It depends. How I would intuitively imagine it in my mind is as a continuous domain of a charge (strong/weak/EM) cloud. Is this "correct"? It is one possible model, that works well in some applications. In other applications, it is just a hard sphere/wave/point/oscillator...

Sorry about the flaky amateur-philosophical mumbo-jumbo :-) Question to think about: what does "is" mean for a physicist (or a mathematician, or many other people).

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The size of the atom is not defined - it depends on the model. Do we look at electrical charge? Mass? The radius is different. I think you still have some thinking to do concerning models and reality, not to be crude. –  Daniel Andersson Dec 24 '10 at 0:13
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I think you are being harsh Marek. Electrons dont orbit on precise paths, as you say they are probablities. That mental model is a fudge - the 'look' is actually incorrect as the electron could be anywhere. I think Daniel didn't do a bad job at defining the problem between defining something micro with macro terms like 'appearance'. The appearance is inherently random. It is probably fair to say that the two theories in your OP are right, depending on how you interact with the nucleus. The nucleus 'looks' like both of them. I find the shell model more satisfying to visualise myself –  SoulmanZ Dec 24 '10 at 0:28
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Not a bad explanation, really. But it may be disappointing for most non-physicists –  belisarius Dec 24 '10 at 2:12
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I used to think I'm a uber-hippy, but this answer proves me wrong. -1 @daniel. Even seen as a philosophical observation, your answer makes little sense as an answer to @marek's question. –  user346 Dec 24 '10 at 8:37
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"The shell model of the atom is not a good picture overall. It explains some things from a macroscopic perspective, but it fails hard at atomic distances." Ok. I have no idea what you're talking about. "If you accept the shell model for the atom as the "true" intuitive model, you might just as well accept the point model of the nucleus, is what I mean. They both work well. If we probe deeper and deeper, do we find strings? Who knows." –  user346 Dec 24 '10 at 8:37

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