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At the center of neutron stars are tightly packed neutrons with almost no space between them. Also, proton and electron have all merged (electron capture) forming as much neutrons as possible. When this star exceeds the Tolman–Oppenheimer–Volkoff limit, the result would be a black hole.

Out of curiosity, if neutrons in extreme sense could ever merge or undergo a $n+n$ reaction?

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  • $\begingroup$ What would be the output of an n-n reaction? $\endgroup$
    – Ofek Gillon
    Commented Sep 22, 2021 at 7:44
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    $\begingroup$ see en.wikipedia.org/wiki/Hexaquark $\endgroup$
    – anna v
    Commented Sep 22, 2021 at 8:03
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    $\begingroup$ The discussion of the dineutron here may be of use: it has been observed, but is a transient resonance rather than a bound state. $\endgroup$
    – J.G.
    Commented Sep 22, 2021 at 9:27
  • $\begingroup$ $n + n \rightarrow e + p + \bar{\nu}_e$? $\endgroup$
    – Jojo
    Commented Sep 23, 2021 at 11:59

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The existence of quark stars has been considered. These are hypothesized to be more dense than neutron stars and to consist of quark-gluon plasma. In such a plasma the boundaries between nucleons would no longer exist. Many open questions exist about quark matter, but the existence quark-gluon plasma is considered to be confirmed. enter image description here

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    $\begingroup$ I thought the existence of QGPs (but not quark stars) was pretty well-established experimentally by now? $\endgroup$
    – llama
    Commented Sep 22, 2021 at 16:48
  • $\begingroup$ You are correct. I edited my answer accordingly. $\endgroup$
    – my2cts
    Commented Sep 22, 2021 at 17:47
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Wikipedia says:

The dineutron, containing two neutrons, was unambiguously observed in 2012 in the decay of beryllium-16.[8][9] It is not a bound particle, but had been proposed as an extremely short-lived resonance state produced by nuclear reactions involving tritium.

It references this article, “First Observation of Ground State Dineutron Decay: 16Be”.

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    $\begingroup$ This sounds more like a "nucleus" without a proton than two neutrons that actually "merge" in any sense. $\endgroup$
    – Guntram Blohm
    Commented Sep 23, 2021 at 8:34
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As far as I understand, the nuclear force has a potential curve that looks similar to the Van Der Waal forces. Notice how the potential goes up exponentially when nucleon separation is approaching zero.

potential

So even if neutrons are neutral and there is no Coulomb repulsion, you will have to overcome the nuclear force for merging two neutrons, which is extremely difficult to do.

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    $\begingroup$ Yes, but how difficult ? :-) . We only get stellar fusion (and later on, neutron stars) because gravity overcomes various repulsive fields. $\endgroup$
    – Carl Witthoft
    Commented Sep 22, 2021 at 13:16
  • $\begingroup$ @CarlWitthoft It’s not gravity that overcomes repulsion to allow fusion, but pressure. The pressure may be caused by gravity, but its force is exerted on the fusing atoms via electromagnetism. $\endgroup$
    – Mike Scott
    Commented May 11, 2023 at 5:17
  • $\begingroup$ @MikeScott Interesting -- certainly gravity is what causes the initial pressure and density. Can you provide some references for E-M forces taking over (before the 2 Nuclear forces come into play $\endgroup$
    – Carl Witthoft
    Commented May 11, 2023 at 13:25

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