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Comment by zw123456 | Hacker News Reader

I know this is probably dumb on my part but am I the only one that had "dark matter" pop into my mind?

Dark matter isn't made of protons, neutrons, or anything as pedestrian as that. It's a sort of "placeholder name" for an effect seen in situations like the collision of galaxies, where there seems to be more gravitational attraction between the two of them than there should be given the matter observed in them. It's as if they had a whole bunch more mass to them, but for some reason that mass is interacting gravitationally, and not in any other way or through any other forces. Scientists can model this by assuming that this type of gravitational-only matter is real, calling it "dark matter", because we don't have a better sense of what it really is.

The stuff made in this experiment, on the other hand, has more to do with what you find in something like a neutron star—clumps of neutrons (which are regular, non-dark matter) packed closely together—albeit in an extremely small scale compared to the star.

Neutrons are electrically neutral but they still interact electromagnetically (they're composed of charged quarks). They have a magnetic moment, for instance [1]. So they wouldn't be dark the way "dark matter" appears (heh) to be.

[1] - https://en.wikipedia.org/wiki/Neutron_magnetic_moment

If dark matter particles had a magnetic moment, how would we know?

They would interact with photons, allowing us to see some effects of concentrations of dark matter against background light sources.

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.

>Solo neutrons don't exist for longer than 10-20 minutes

Don't equate the bound states with the unbound states.

"If a four-neutron nucleus did occur it lasted about a billionth of a trillionth of a second before decaying into other particles."

Sure, but his reasoning of "single neutron is unstable implies a bound state containing neutrons is also unstable" is not necessarily sound.

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?

The number of centuries for which we've even known of the neutron's existence is 1, and that's rounding up!

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.

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

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).

> 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.

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.

Neutron stars are just mostly neutrons. Like atomic nuclei, they also contain protons.

(In their core, they could potentially even contain quarks or hyperons, although recent neutron star measurements make this a bit more unlikely)

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? :)

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

> 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.

Dark matter is probably popping into and out of all our minds, all the time, and we just don't notice it.

Only because we can't feel the gravity of the situation.

One idea is it is a very thin gas permeating the entire galaxy. Because it does exchange photons it doesnt chemically clump together like regular matter and stays a diffuse gas concentrated only by gravity.

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