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My intuitive understanding is that although atoms are mostly empty space, the nucleus is solid. Thus a neutron or a neutrino can collide with it and do things like deflect or cause a nuclear reaction.

But does this intuition reflect the underlying reality? Since we usually think of a solid in reference to substances comprised of atoms, how does the concept of solid extend into the subatomic realm?

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At least on current physics theories, there is nothing "solid" in the sense you mean. In most models of physics elementary particles are zero dimensional, which means, their size is a "point", whatever that means. The nucleus, in particular, is pretty "empty". It consist of soup of quarks and gluons, that is mostly energy; and both quarks and gluons have no size (at least in the standard model of physics). There are other theories where both quarks and gluons are made of still smaller things, but the story repeats. In the end, there is nothing such as a "solid" particle. All fundamental particles have infinitesimal size. Or, if space is discrete, a minimum size dictated by the quantum size of space itself. PS: the deflection of a particle by a nucleus is not because it is "solid", but because of the strength of the forces that a particle feels when is close to the nucleus.

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  • $\begingroup$ "Or, is space is discrete, a minimum size dictated by the quantum size of space itself." Is this supposed to be a claim or a question? In the former case, I'd like some evidence and/or references. $\endgroup$
    – Danu
    Commented Mar 5, 2014 at 6:37
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    $\begingroup$ I mean "if". I corrected it thanks. What I meant is that if space itself is made of "units", or quanta, so that there could be nothing smaller than these units. $\endgroup$
    – user16007
    Commented Mar 5, 2014 at 6:40
  • $\begingroup$ Danu, dot get angry, I will erase my earlier comment on your answer, I was drunk. Deal? $\endgroup$
    – user16007
    Commented Mar 5, 2014 at 6:43
  • $\begingroup$ Ehh? It's fine, don't worry about it. What state you're in isnt't any of my business. $\endgroup$
    – Danu
    Commented Mar 5, 2014 at 6:58
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In a similar way that atoms are mostly empty space the same is true for nuclei and nucleons.

The nucleus is composed of nucleons and is held together by the spill over strong forces that hold together the quarks within the nucleons ( neutrons and protons are made up of quarks and the energetic interactions between them).

The reason a table top is solid is because the outer electrons in the atoms of your hand are repelled by the outer electrons of the atoms of the table. It is the effect of forces.

Scattering with elementary particles, with energies higher than the binding energies that hold the electrons around the nucleus will allow examining the composition of the nucleus of an atom, and it has been established in nuclear physics experiments that there are protons and neutrons bound by the spill over strong force whose fields will interact and scatter the incoming particle. Thus below certain scattering energies the nucleus appears "solid" and above transparent showing up the composition.

When the energies get higher as in the LHC the composition of protons and neutrons has been studied and it consists of point particles called elementary particles.. These react like point particles, and in the nucleon ( proton or neutron) a "cloud" of quarks and gluons cover the dimensions of the nucleon, pictorially:

colliding protons and quark gluons

where the dots are quarks antiquarks and the squiggles gluons in the sea connecting the three constituent quarks ( the ones which build up the mess).

It has to be kept in mind that this pictorial form mimics the probability of finding a point particle , quark or gluon, within the nucleon. As the elementary particles are point particles again one can can say that at any instant photo, most of the space is empty of particles, and what presents a "solid" form during a scatter , are the interaction forces acting between these point particles and an incoming one ( for example a photon) .

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