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Why do neutrons repel each other?

What I mean is that the neutrons are attracted to one another via gravity, so what force keeps them from collapsing to form a "neutron black hole"? Considering that they have no electric charge...

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lee
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It's well explained at wikipedia:

Neutron stars are very hot and are supported against further collapse by quantum degeneracy pressure due to the Pauli exclusion principle. This principle states that no two neutrons (or any other fermionic particles) can occupy the same place and quantum state simultaneously.

qoqosz
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  • i red it, but i couldn't understand what force is it. i mean is it an actual force that keeps them apart or a quantic phenomenon? or is it the thermal movement... – lee Jun 06 '12 at 22:22
  • It's a Fermi gas pressure. Similar to air pressure but neutrons here obey quantum Fermi–Dirac statistics unlike air particles for which Boltzmann statistics is more appropriate. – qoqosz Jun 06 '12 at 22:48
  • @lee It comes from the Pauli Exclusion rule for particles with half-integer spin (called fermions)---they can not occupy identical quantum states---so, yes it is intrinsically quantum mechanical and it is also a "actual" force. – dmckee --- ex-moderator kitten Jun 06 '12 at 23:39
  • @dmckee if it's an actual force, what are the force carriers in this case, gluons? – Michael Jul 31 '14 at 22:10
  • @Michael Hmmm...it's been a while since I wrote that. I have to admit that I have waffled over time on how to answer that question. There is no force carrier, but the effect can be unambiguously and correctly written in terms of a pressure. Alternately the combination of gravity and the residual strong forces (carriers graviton and light mesons) have pushed the Fermi energy too high for further compression to be energetically favored. – dmckee --- ex-moderator kitten Jul 31 '14 at 22:20
  • In all the neutron stars that have been observed so far, the force that stops the neutrons getting closer together is the strong nuclear force. Ideal neutron degeneracy pressure is insufficient to support any neutron star more massive than $0.7M_{\odot}$. So, whilst NDP is important, it is not what supports neutron stars. – ProfRob Mar 22 '15 at 12:48