@dougmerritt saw an interesting question on Physics Stack Exchange.

[johncarlosbaez@mathstodon.xyz]

@dougmerritt saw an interesting question on Physics Stack Exchange. Paraphrasing:

When neutron stars merge, a large amount of nuclear material is ejected. Some of it turns into elements with plenty of protons. So a lot of neutrons must turn into protons. Some say this happens much faster than neutrons naturally decay into protons. The mean lifetime of a free neutron is 15 minutes - a very long time. What makes them turn into protons faster than this?

The top rated answer sounds plausible to me. It says there are lots of positrons running around - plausible at high enough temperatures - and the reaction

𝑛 + 𝑒⁺ → 𝑝 + ν̅ₑ

converts neutrons to protons. Nice!

Of course high-energy electrons can turn protons back into neutrons:

𝑝 + 𝑒⁻ → 𝑛 + νₑ

But these electrons will need to have *higher* energy than the positrons to do this, so we'll see more neutrons turning to protons than vice versa... unless the temperature is so high that this difference becomes negligible. In equilibrium at temperature 𝑇, the chance of a nucleon being a neutron equal to

exp(−(𝑚ₙ−𝑚ₚ)𝑐²/𝑘T)

times the chance it's a proton. This approaches 1 as 𝑇→ ∞. But at really high temperatures, the nucleons will bust apart into quarks. (I doubt colliding neutron stars get *that* hot.)

These are just some instant thoughts. This sort of question really requires pretty serious calculations, or else better intuition for nuclear physics than I have! But it's fun:

https://physics.stackexchange.com/questions/861363/how-can-decompressing-neutronium-accelerate-beta-decay-to-form-heavy-elements

[dougmerritt@mathstodon.xyz]

@johncarlosbaez
...and some of the ejecta form various nuclei which eventually end up going into newly forming star systems, so it's always interesting as to *what* nuclei are created, and how, and in what ratios, and all sorts of complications like that.

There's some kind of threshold for "part of a neutron star" ... "intermediate stage?" ... "outside the neutron star", where phase 1 is largely neutrons (I thought it was neutronium, but at least one person said no, so where does that go), and phase 3 contains lots of heavy nuclei...

Maybe people have simulated that.

[johncarlosbaez@mathstodon.xyz]

@dougmerritt - serious astrophysicists simulate neutron star collisions and how they create various isotopes. But that's too complicated for me to do anything but read about.

The interior of a neutron star is popularly called "neutronium", but people with a certain higher level of scientific knowledge like to loudly avoid that word, since this stuff is actually a mix of neutrons, protons, electrons, pions, and even some heavier hadrons.

For example, the Wikipedia article on neutronium:

https://en.wikipedia.org/wiki/Neutronium

starts by calling it a "hypothetical substance made entirely of neutrons" before admitting that this term is also used to mean the stuff in neutron stars.

A more modern term is "nuclear pasta", and I recommend this article highly:

https://en.wikipedia.org/wiki/Nuclear_pasta

Heck, I'll quote the fun part!

(1/2)

[johncarlosbaez@mathstodon.xyz]

@dougmerritt -

"While nuclear pasta has not been observed in a neutron star, its phases are theorized to exist in the inner crust of neutron stars, forming a transition region between the conventional matter at the surface and the ultra-dense matter at the core.

Towards the top of this transition region, the pressure is great enough that conventional nuclei will be condensed into much more massive semi-spherical collections. These formations would be unstable outside the star, due to their high neutron content and size, which can vary between tens and hundreds of nucleons. This semispherical phase is known as the gnocchi phase.

When the gnocchi phase is compressed, as would be expected in deeper layers of the crust, the electric repulsion of the protons in the gnocchi is not fully sufficient to support the existence of the individual spheres, and they are crushed into long rods, which, depending on their length, can contain many thousands of nucleons. These rods are known as the spaghetti phase. Further compression causes the spaghetti phase rods to fuse and form sheets of nuclear matter called the lasagna phase. Further compression of the lasagna phase yields the uniform nuclear matter of the outer core. Progressing deeper into the inner crust, the holes in the nuclear pasta change from being cylindrical, called by some the bucatini phase or antispaghetti phase, into scattered spherical holes, which can be called the Swiss cheese phase.

The nuclei disappear at the crust–core interface, transitioning into the liquid neutron core of the star.

The pasta phases also have interesting topological properties characterized by homology groups."

https://en.wikipedia.org/wiki/Nuclear_pasta

[angusm@mastodon.social]

@johncarlosbaez @dougmerritt @cstross “... the electric repulsion of the protons in the gnocchi is not fully sufficient to support the existence of the individual spheres, and they are crushed into long rods ... known as the spaghetti phase.”

Reading this, I seriously expect to hear that String Theory has been replaced by Vermicelli Theory, and that physicists now believe that dark matter is just squid ink pasta.

Also, the Flying Spaghetti Monster cultists are now all going "We told you so!”