Why do neutrons repel each other?

Question

I can understand why 2 protons will repel each other, because they're both positive. But there isn't a neutral charge is there? So why do neutrons repel (do they even or Have I been misinformed?)

EDIT the reason I ask is because I've just been learning about neutron stars and how the neutrons are forced(as in they repel) together according to my teacher (he's a great teacher btw what I just said doesn't make it seem so though) So I wondered why they have to be forced by gravity and not just pushed.

Answer 1

Neutrons have spin 1/2 and therefore obey the pauli exclusion principle, meaning two neutrons cannot occupy the same space at the same time. When two neutrons' wavefunctions overlap, they feel a strong repulsive force. See http://en.wikipedia.org/wiki/Exchange_interaction .

Answer 2

Neutrons (and protons) being spin 1/2 fermions, must fit antisymmetric wavefunctions. This "wavefunction" doesn't always involve waves, though. For nucleons - the generic term for neutron or proton - this wavefunction for the pair is a produce of (1) a spatial part, (2) a spin part, and (3) an isospin part.

The isospin part is a clever way to describe charge possibilities of otherwise identical particles. We regard neutrons and protons as being in a sense identical. Just as a spin 1/2 particle can be "up" or "down" along some chosen axis, so is an isospin 1/2 particle can be "up" or "down" along an abstract mathematical axis - it's exactly the same SU(2) math as spin - but it plays out in physical reality as charge. For nucleons it's not +1/2 and -1/2 charge but with an offset, so we have +1 (proton) and 0 (neutron). This idea is from Heisenberg in 1932.

Now, how can the overall wavefunction of a pair of particles be antisymmetric? There are three factors - right away we can imagine three possibilites: any one factor being antisymmetric with the other two symmetric. We could also have all three antisymmetric at the same time.

An antisymmetric spatial wavefunction would have a node, like an atomic p orbital, like the electric potential around a dipole antenna. This is a higher energy state than a simple spherical blog, a Gaussian. Given the range of internuclear forces, this nodal antisymmetric wavefunction has more energy than if the two nucleons just stayed apart. This is a matter of radial or angular kinetic energy has to be either "zero" or some quantized value that exceeds "escape velocity" So forget that part of the system wavefunction being antisymmetric.

BTW, we don't have separate spatial wavefunctions for the two nucleons - whatever one does, the partner does the exact opposite, like a two-body celestial mechanics problem. They orbit a common barycenter.

The spin part could be antisymmetric. This is a bit tricky. If particle #1 is up and #2 is down, we can write "UD". There is also "DU". We form the spin part of the wavefunction for the pair as UD-DU. We could instead choose UD+DU but note this is symmetric. So are UU and DD. Just how UD-DU differs from UD+DU may mystify beginners in quantum mechanics, but it's important, and it's how physical matter works whether we humans like it or not. (You might also see where the 'u' and 'd' quarks got their names. The quark idea came along years after isospin.)

Neither D nor U is really a wave or a function; they're at most just rows and columns in matrices if you must represent them in familiar math. Otherwise quantum physicists just deal with these symbolically. Still the jargon is "wavefunction" - we silly humans and our primitive scientific language!

The same math applies for isospin. But the physics differs. We've concluded that the spatial part of the system wavefunction must be symmetric, so it's up to either the spin part or the isospin part to be antisymmetric. But not both! If spins are symmetric - they're parallel. This is experimentally the case - the Deuteron (obtain by distilling "heavy water" from water) - so we deduce that the isospin part is antisymmetric. That is, we must have one isospin "up" and one isospin "down" - neutron and a proton, not two neutrons or two protons.

Just why must the spins of the two nucleons be parallel? The strong force holding them together - the exchange of pions, kaons and other mesons - works better in that case. To explain that takes deeper analysis than I can go into here. When the spins are antiparallel, there's not enough force to keep the nucleons together.

This would be the case though, were to try pushing two neutrons together. They'd be both isospin "up" therefore symmetric isospin part of the wavefunction, therefore requiring an antisymmetric spin part, which leads to the pions and their buddies not getting as good a grip on the neutrons, which drift off going their separate ways.

Answer 3

I think this may be a summary of other answers, but there are a couple things going on here. First, neutrons are electrically neutral, so an obvious force is the Van der Waals force. However, due to quantum mechanics and the Pauli exclusion principle (as noted above), neutrons' wave functions cannot (ish) overlap and so they are subject to the Neutron Degeneracy Pressure. I think if you understand these two concepts, then you should have a fairly good grasp on what is going on between neutrons in a neutron star.

Answer 4

Neutrons consist of quarks that are electrically charged, so when two neutrons get close enough to each other the higher electrical multipole moments will become relevant and cause repelling. But also note the magnetic moment and the strong force Cedric mentioned plus the Pauli exclusion mentioned by nibot.

Answer 5

They repel because of the same reason why, say, atoms or molecules repel: because of Pauli principle which is the basis of elastiсity and solidity of matter. Why you cannot move through the walls? Why the dish holds food? Because all these objects are composed of fermions, the particles of substance.

This is opposed to say, light which can go through another light ray without any interaction. Because light is composed of bosons - the quants of radiation.

Answer 6

As they are not charged, they only interact through their magnetic moment (which at very low energy could lead to kind of Van der Waals forces, the strong force and the weak force.

If you try to create a nucleon-nucleon system and that you do some QM calculations using an Hamiltonian taking into account the nuclear force, the electric force and the spin 1/2 of the nucleons you will have different contribution to the total energy, some being positive, other negative.

For example, in the proton-proton case you cannot have a bound state because the electric repulsion is stronger.

You also have a contribution that depend on the [projection of the] spin of the nucleons. If they are equal the contribution lower the energy of the system and you can have a bound state.

That's the case of the deuton (a proton-neutron bound stage, very weak).

But in the neutron-neutron state, both being of spin 1/2, you cannot have the same projection, and thus you cannot obtain a bound state.

If you interpret that as "they repel each other" then you have your explanation.

Answer 7

Ah, given your edit, I think you are really asking, why are neutron stars so much denser than other objects (regular stars, planets, etc)?

Gravity acts to clump stuff together in the universe. Objects like stars and planets are formed and held together by gravity.

The reason planets and stars aren't compressed down to an infinitesimal point by gravity is because the stuff they are made of eventually begins to resist further compaction.

Normal matter is made out of atoms, which consist of a tiny nucleus surrounded by a cloud of electrons (which themselves have no physical size at all). It's really the electron cloud that makes things seem solid. The electrons in the atoms in my feet repel the electrons in the floor, so I don't fall through it.

But despite this repulsion, most of the space taken up by atoms is empty space. It is fair to say that the Earth, made of atoms, is also mostly empty space. It just seems solid because of the electron repulsion.

Neutron stars, however, are just made out of neutrons, which can nestle up right next to each other. There is none of this empty space inside the cloud.

Essentially all of the mass of an atom is in the nucleus (the protons and neutrons). Neutron stars can be vastly, vastly denser than stars and planets made out of atoms, because they lack the empty space inside the electron clouds.