Dark matter uneducated question

[Grumble]

Maybe this should be in the less serious forum, but it’s definitely a weighty matter.

Could dark matter be neutrons? Do we know that it isn’t?

[dyqik]

Maybe this should be in the less serious forum, but it’s definitely a weighty matter.

Could dark matter be neutrons? Do we know that it isn’t?

Free neutrons decay into protons and electrons with a half life of 10.3 minutes, so we know that diffuse cold dark matter can't be them, as it wouldn't last any time.

Neutrons would also scatter photons, meaning that they wouldn't be dark.

[Bewildered]

Maybe this should be in the less serious forum, but it’s definitely a weighty matter.

Could dark matter be neutrons? Do we know that it isn’t?

Edit: Haha I can’t read in the morning, didn’t notice you wrote neutrons not neutrinos until I saw dyqik’s post and initially thought what the f.ck is he talking about. No neutrons can’t be dark matter. roughly speaking it should be non-baryon matter that interacts weakly (or just gravitationally). I’ll leave my original answer below since you might also want to know about neutrinos…

Neutrinos do make up a tiny bit of the dark matter relic abundance but from the the neutrinos we know about it’s far too small. Also those neutrinos have small masses so they give rise to hot dark matter which doesn’t work for galaxy structure formation. cold dark matter means dark matter that moves mores slowly which means the particles have more mass and this is ~needed for galaxy formation (though there is also now some evidence for warm matter which could work). So it can’t be neutrinos in the standard model of particle physics, but it can be heavy sterile (means they have no gauge interactions) neutrinos in physics beyond the standard model. This has been considered and discussed a lot, see e.g. this review: https://arxiv.org/abs/1807.07938

[Grumble]

Thanks gents

[bjn]

What is the mechanism that stops bare neutrons in a neutron star decaying? It’s something to do with pressure from the weight of the star. Do they decay then recombine immediately due to pressure or similar?

[shpalman]

What is the mechanism that stops bare neutrons in a neutron star decaying? It’s something to do with pressure from the weight of the star. Do they decay then recombine immediately due to pressure or similar?

In a neutron star they aren't really bare, it's like a big atomic nucleus. Of course that doesn't explain why neutrons in a nucleus don't always decay.

[bjn]

Ah. Quantum magic. My layman’s brain will have to accept that.

I did try to do learn quantum mechanics a while ago to figure out quantum computation, but the text book I was using had obvious mistakes in the tensor maths and I got sad and gave up.

[dyqik]

What is the mechanism that stops bare neutrons in a neutron star decaying? It’s something to do with pressure from the weight of the star. Do they decay then recombine immediately due to pressure or similar?

In a neutron star they aren't really bare, it's like a big atomic nucleus. Of course that doesn't explain why neutrons in a nucleus don't always decay.

And that's fundamentally because the strong nuclear force that binds protons and neutrons into a nucleus is stronger than the weak nuclear force that mediates neutron decay. Beta decay, OTOH, is neutron decay, or the reverse, in a nucleus.

Which is really only a restatement of the fact that neutrons don't decay when bound into stable nuclei, but the standard model does have the details.

[dyqik]

Ah. Quantum magic. My layman’s brain will have to accept that.

I did try to do learn quantum mechanics a while ago to figure out quantum computation, but the text book I was using had obvious mistakes in the tensor maths and I got sad and gave up.

You really don't need tensors to understand quantum computation.

[IvanV]

Ah. Quantum magic. My layman’s brain will have to accept that.

There's a fairly simple explanation of the stability of neutrons in stable nuclei at our usual favourite unreliable source.

Tldr: Converting a neutron to a proton in a stable bound state, such as within a stable nucleus, is typically impeded by one or both of the Pauli exclusion principle applied to nucleons, and energetic constraints taking into account the energy of bound nucleons. The energetic constrains arise because the energy of a bound nucleon is different from a free nucleon. So whilst (free) neutron -> (free) proton + electron + antineutrino is energetically favourable, that is not necessarily the case when the nucleons are instead bound in a stable configuration.

The Pauli exclusion principle is quantum magic to me, though.

There's a lovely story about Enrico Fermi, who first proposed the theory of beta decay. Fermi wasn't very good at the complex maths generally used in quantum mechanics these days. Fermi was of course famous for his order of magnitude arguments, which are often called Fermi estimates, or Fermi problems if set for you to assess. He once is reputed to have said, later in life, to Emilio Segré after a seminar, 'I cannot follow the highbrow theory developed by Oppenheimer's pupils any more. I went to their seminar and was depressed by my inability to follow them. Only the last sentence cheered me up. It was "And that was Fermi's theory of beta ray decay."' And indeed one of the original reviewers of Fermi's original paper which proposed the theory of beta decay said it was unfit for publication for a lack of rigor - going from memory, I think that might have been Leo Szilard, who found himself on the wrong side of history on that occasion.

[dyqik]

The Pauli exclusion principle is quantum magic to me, though.

That's because it is, as it doesn't have any classical physics analogue, and it doesn't work if you're picturing quantum particles as either waves or billiard balls, or even sometimes as waves and sometimes as billiard balls.

[Bewildered]

What is the mechanism that stops bare neutrons in a neutron star decaying? It’s something to do with pressure from the weight of the star. Do they decay then recombine immediately due to pressure or similar?

Which textbook? Dyqik is right you don’t need tensors to understand QM.

Actually speaking of maths you don’t need to understand, maybe Leonard Susskind has a QM lectures in his weird YouTube lecture series where he explains things with the minimal maths necessary to explain the theories, aimed at people who don’t need to use it to do calculations etc but just want to understand. Maybe that would work for you?

[bjn]

I’m a computation guy and wanted to understand Q computation well enough to at least understand and possibly devise algorithms for it. I picked up a QC textbook about 5 years ago to teach myself and it started off with tenors maths, which had blatant errors in the exercises. Can’t recall the name of the book and I don’t have it anymore. Happy to be pointed in other directions.

[bjn]

I worked for years with a guy that has PhD in QC, he said it’s basically complex linear algebra at heart.

[dyqik]

I worked for years with a guy that has PhD in QC, he said it’s basically complex linear algebra at heart.

That's correct. The quantum information theory course I took has zero tensor maths in it.

[shpalman]

I’m a computation guy and wanted to understand Q computation well enough to at least understand and possibly devise algorithms for it. I picked up a QC textbook about 5 years ago to teach myself and it started off with tenors maths, which had blatant errors in the exercises. Can’t recall the name of the book and I don’t have it anymore. Happy to be pointed in other directions.

https://qiskit.org/

[bjn]

Tx! Will have a play.

[shpalman]

To get, for example, the state vector of a two-qubit system you do the outer product of the two single qubit state vectors, and the outer product is also called the tensor product if you do it between tensors, but the state vector is just a 1-d vector with 2^N elements (for N qubits) and any circuit is a unitary 2-d 2^N by 2^N matrix. (A gate acting on a single qubit is a 2x2 matrix so if you want the matrix for that gate acting on one qubit in a two-qubit system you do the outer product of that gate with the identity matrix; the order depends on which qubit is which.)