Following up on this post: Anti-Particle of Neutron, one very important part of it is unanswered. If a neutron collides with an anti-neutron, will it violently explode in a flash of energy? The Wikipedia article on it also doesn't shed light on this. We know a proton will be attracted to its anti-particle and create energy, but I suppose there is nothing (apart from a very weak gravity) that will attract a neutron to its anti-neutron. So if I took a gas made up of neutrons and another made of anti-neutrons and mixed them up, would nothing happen? Would it depend on the density?

Also, what about a neutron star and an anti-neutron star? I suppose they would revolve around each other due to gravity, but there would come a time when they collide. Would the collision be the same as or different from the collision of two neutron stars?

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    $\begingroup$ Yes, an antineutron will annihilate with a neutron, or any other piece of matter (Its antiquarks will), producing huge amounts of energy as gamma rays. That is exactly how WP says it was discovered in 1956. What do you gather its statement "Instead, the products of its annihilation with ordinary matter are observed" means? $\endgroup$ Nov 20, 2021 at 22:54
  • $\begingroup$ Yes, I meant photons. $\endgroup$ Nov 21, 2021 at 4:23
  • $\begingroup$ I think the concept of 'violently exploding' is very much a macro-scale thing. I don't know that it's a good way to imagine particle-antiparticle annihilation, there's no analogy to the build of pressure as far as I'm aware. $\endgroup$
    – Jojo
    Nov 21, 2021 at 11:13
  • $\begingroup$ A gas of neutrons (or antineutrons) is not a static thing. It's extremely energetic. Free neutrons & antineutrons have a half-life of ~610 seconds. See physics.stackexchange.com/q/437245/123208 $\endgroup$
    – PM 2Ring
    Nov 21, 2021 at 15:44

2 Answers 2


For education, the first observation of an antineutron in an antiproton beam at Berkley in 1958.

The beam of antiprotons is coming from the top. One antiproton does a charge exchange reaction with a proton at rest in the chamber, leading to a pair of neutron antineutron, the antineutron taking most of the momentum of the antiproton.


The star shown in the drawing on the right is the antineutron annihilating on a proton into pions (the charges of the pions do not add up to zero, so it is a target proton).


original letter in Physical Review.

  • $\begingroup$ Nice images! Also I like 'For education:' $\endgroup$
    – Jojo
    Nov 21, 2021 at 11:12

Yes, they will annihillate. It will happen slower like a proton-antiproton gas mix, because they have no charge, thus nothing attracts the neutrons to the antineutrons.

Annihillation does not convert matter to energy, it converts particle-antiparticle pairs to photons. Energy is not matter, it is a number what we assign to particles.

A mix of neutron- and antineutron gas will create photons, neutrinos and antineutrinos.

Neutrinos and antineutrinos will appear because beside the annihillation, other processes will also happen. Neutrons and antineutrons are not elemental particles, they are from 3 quarks or antiquarks, and these annihillate. The other two builds pions, some of them decays to muons and antimuons before they annihillate to neutrinos and antineutrinos. The muons decay to electrons, positrons and (anti)neutrinos. The electrons and the positrons annihillate to photons.

If the gases are really big or you have a much time to watch them, then also the neutrinos + antineutrinos will annihillate, and the result will be only photons. But this would require sizes and time comparable to the visible Universe.

Currently there is no experimental technology to create stable antineutron gas. Even to create stable neutron gas is hard, because the neutrons have no charge, so there is no easy way to trap them (they have a little magnetic moment, so very slow neutrons can be trapped by very strong magnetic fields).

The collision of a neutron and antineutron star would initiate a terrible strong annihillation. The result would be similar to a supernova with an extreme gamma photon flash. It is hard to say, what would be the result. In the Universe, no significant amount of antimatter exists.

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    $\begingroup$ Where you wrote "Annihillation does not convert matter to energy", did you mean something like "Annihillation does not convert matter to energy because matter is already included in the system's total energy"? $\endgroup$ Nov 20, 2021 at 23:09
  • $\begingroup$ @ChiralAnomaly I tried to say: $\rm{x + \overline x \rightarrow E}$ is not correct, but $\rm{x + \overline x \rightarrow \gamma + \gamma + ...}$ is correct. $\rm{E}$ is a property of all the terms in the equation, but not a term itself. $\endgroup$
    – peterh
    Nov 21, 2021 at 14:05

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