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There aren't any known arrangements of neutrons that are stable without protons. Solo neutrons don't exist for longer than 10-20 minutes before decaying into pr

There aren't any known arrangements of neutrons that are stable without protons. Solo neutrons don't exist for longer than 10-20 minutes before decaying into protons and I doubt these tetraneutrons exist for longer than a few seconds unless they're moving at relativistic speeds (and even then, for not much longer). Unless there is something drastically different going on in the interstellar medium, neutrons as dark matter is extremely unlikely.

noobermin 11y ago
>Solo neutrons don't exist for longer than 10-20 minutes Don't equate the bound states with the unbound states.
- ars 11y ago
  "If a four-neutron nucleus did occur it lasted about a billionth of a trillionth of a second before decaying into other particles."
  - noobermin 11y ago
    Sure, but his reasoning of "single neutron is unstable implies a bound state containing neutrons is also unstable" is not necessarily sound.
    - akiselev 11y ago
      Why is this reasoning unsound? The fact that, after centuries of theoretical and experimental physics, we have yet to see stables neutrons in any configuration other than with protons in an atomic nucleus? We know pretty accurately the decay rates of single neutrons and now we know that all the way up to a tetraneutron that the particles are unstable on the order of minutes and seconds. What evidence is there that we are wrong?
      - caf 11y ago
        The number of centuries for which we've even known of the neutron's existence is 1, and that's rounding up!
        mikeash 11y ago
        Fun fact: the time from the discovery of the neutron to detonating an atomic bomb was a mere 13 years. From the discovery of nuclear fission it was only 7 years.
pklausler 11y ago
What about the neutrons in a neutron star -- do they also decay into protons? Aren't neutron stars long-lived?
- ars 11y ago
  One way to think about it: They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.) Another way to think about it: They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into. You could ask why don't the neutrons in nucleus's decay into protons? It's because it's not energetically favorable for them to do so. That's basically all decay is: Is it energetically favorable to be something else. The more energy is involved the "faster" it decays (sort of, it's more like the more energy relative to the barrier).
  - swombat 11y ago
    > They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Just being pedantic here but in a neutron star, it's neutron degeneracy pressure that's at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there's enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable. The principles for both types of degeneracy are similar (it's the Pauli exclusion principle at work basically) but the particles are different.
    - piannucci 11y ago
      GP is correct-- the neutron degeneracy pressure prevents the stellar remnant from getting any smaller, but it is the electron degeneracy pressure which makes beta decay energetically unfavorable. In order to be produced by neutron beta decay, an electron must enter a valid state. Pauli exclusion (electrons are fermions) requires the electron to enter a state at least as energetic as the electron Fermi level, which due to the neutron star's gravity is comparable to the rest mass of the electron. Neutron decay is energetically favorable, but not that favorable. The reverse process accelerates and equilibrium is reached.
pc86 11y ago
Is there an ELI5 about how a neutron decays into a proton? Aren't they separate particles? Or maybe an ELI-only-took-undergrad-physics-101? :)
- dragonwriter 11y ago
  A neutron decays into a proton by emitting an electron and an electron antineutrino. A proton decays into a neutron by emitting a positron and an electron neutrino. (These are the antiparticles of the ones emitted in neutron to proton beta decay, above.) https://en.m.wikipedia.org/wiki/Beta_decay https://en.m.wikipedia.org/wiki/Beta_decay
- deleted 11y ago
  [deleted]
- Manishearth 11y ago
  > Aren't they separate particles? They are. Subatomic decay doesn't happen with particles "contained within" other particles; a neutron does not contain a proton but can decay into a proton, electron, and electron antineutrino via the W boson.

noobermin 11y ago

>Solo neutrons don't exist for longer than 10-20 minutes Don't equate the bound states with the unbound states.

ars 11y ago
"If a four-neutron nucleus did occur it lasted about a billionth of a trillionth of a second before decaying into other particles."
- noobermin 11y ago
  Sure, but his reasoning of "single neutron is unstable implies a bound state containing neutrons is also unstable" is not necessarily sound.
  - akiselev 11y ago
    Why is this reasoning unsound? The fact that, after centuries of theoretical and experimental physics, we have yet to see stables neutrons in any configuration other than with protons in an atomic nucleus? We know pretty accurately the decay rates of single neutrons and now we know that all the way up to a tetraneutron that the particles are unstable on the order of minutes and seconds. What evidence is there that we are wrong?
    - caf 11y ago
      The number of centuries for which we've even known of the neutron's existence is 1, and that's rounding up!
      - mikeash 11y ago
        Fun fact: the time from the discovery of the neutron to detonating an atomic bomb was a mere 13 years. From the discovery of nuclear fission it was only 7 years.

pklausler 11y ago

What about the neutrons in a neutron star -- do they also decay into protons? Aren't neutron stars long-lived?

ars 11y ago
One way to think about it: They do decay into protons and electrons, and then are immediately squished back into neutrons by the immense pressure. (There is a missing neutrino to wonder about though.) Another way to think about it: They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Basically all particles decay, unless there is something stopping them - for example there is nothing lighter for them to decay into. You could ask why don't the neutrons in nucleus's decay into protons? It's because it's not energetically favorable for them to do so. That's basically all decay is: Is it energetically favorable to be something else. The more energy is involved the "faster" it decays (sort of, it's more like the more energy relative to the barrier).
- swombat 11y ago
  > They are unable to decay into protons plus electrons because there isn't enough "room" for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure. Just being pedantic here but in a neutron star, it's neutron degeneracy pressure that's at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there's enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable. The principles for both types of degeneracy are similar (it's the Pauli exclusion principle at work basically) but the particles are different.
  - piannucci 11y ago
    GP is correct-- the neutron degeneracy pressure prevents the stellar remnant from getting any smaller, but it is the electron degeneracy pressure which makes beta decay energetically unfavorable. In order to be produced by neutron beta decay, an electron must enter a valid state. Pauli exclusion (electrons are fermions) requires the electron to enter a state at least as energetic as the electron Fermi level, which due to the neutron star's gravity is comparable to the rest mass of the electron. Neutron decay is energetically favorable, but not that favorable. The reverse process accelerates and equilibrium is reached.

pc86 11y ago

Is there an ELI5 about how a neutron decays into a proton? Aren't they separate particles? Or maybe an ELI-only-took-undergrad-physics-101? :)

dragonwriter 11y ago
A neutron decays into a proton by emitting an electron and an electron antineutrino. A proton decays into a neutron by emitting a positron and an electron neutrino. (These are the antiparticles of the ones emitted in neutron to proton beta decay, above.) https://en.m.wikipedia.org/wiki/Beta_decay https://en.m.wikipedia.org/wiki/Beta_decay
deleted 11y ago
[deleted]
Manishearth 11y ago
> Aren't they separate particles? They are. Subatomic decay doesn't happen with particles "contained within" other particles; a neutron does not contain a proton but can decay into a proton, electron, and electron antineutrino via the W boson.