A bug's life: Millimeter-tall mountains on neutron stars | Hacker News Reader

A bug's life: Millimeter-tall mountains on neutron stars

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52 pointsdnetesn32 comments

A good science fiction book about an intelligent alien race that evolved on the surface of a neutron star, written by a physicist:

https://www.goodreads.com/book/show/263466.Dragon_s_Egg

That was just for sale a few days back on the Kindle store (at least in the US). Not sure if it still is.

Came here to post this. Love re re re reading this book.

The Xeelee Sequence also features post-post-posthumans designed to live on a neutron star in one of the books about midway through.

My favorite “people” from the series, second only to the Roman Catholic eusocial human hives of course.

Truly the largest series I’ve ever read in scope, if you can make it past the terribad writing & characterization in the first books (and you will, the ideas are worth it) it quickly gets better.

https://en.m.wikipedia.org/wiki/Xeelee_Sequence

Writing is pretty dry, but the ideas are grand. It's worth a read.

I’m currently reading Flux. Brilliantly bonkers.

Life on a neutron star, as understood by a local, in their own terms. It's brilliant.

A supreme triumph of the imagination. I'm going to reread it now that it's been brought to my attention here.

I came across a synopsis of this book a week or so ago, read about the time differential and it got me wondering: what's the explanation for the time differential?

We know that time passes slower in stronger gravitational fields. A neutron star has an enormously strong gravitational field near its surface, so time should move much slower there, not much faster... but in the book it's the other way around. (According to the synopsis.)

Am I missing something?

The explanation in the book is that neutron-star life is based on nuclear chemistry, whose reactions move much more quickly than our (electron-shell-based) chemistry, so everything happens much more quickly as a result.

Revelation Space by Alastair Reynolds also features an apparent neutron star in a cool way[0]

Warning: Link is [probably] a spoiler if you're thinking of reading the book

[0] https://revelationspace.fandom.com/wiki/Hades

One “wow” moment that blew me away during my grad physics studies was the realisation that a neutron star is effectively one gigantic atomic nucleus.

Fascinating, do you know if protons can exist within neutron stars? That would make it each one an element, right? Or if not, they would all be isotopes of Neutronium?

There are no isotopes with neutron stars because they are not actually nuclei.

Yes, it's thought that neutron stars are about 5-10% composed of protons and electrons. In fact, that's why a neutron star can have a magnetic field – charged particles are required to carry an electric current.

However, in principle there should be a (perhaps very small?) component due to the inner magnetic momentum of neutrons [1], right? After all, they are made of quarks, which have nonzero charge.

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

There's a gradient of material.

At the core, a neutron star is likely very nearly pure neutronium, as other particles simply won't exist long under the conditions present.

As depth decreases, other particles can exist, and at the surface there are likely free nuclei and atoms of elements, though my probably with low nuclear potential. Fission and fusion would be accelerated by the energies involved.

That's a lay reading / bits of stray collected memories, though quick checking seems to confirm most of this. See:

https://heasarc.gsfc.nasa.gov/docs/objects/binaries/neutron_...

That's stretching quite a bit the definition of a nucleus.

Neutron stars are held together by gravity, not nuclear forces. Also, the mix of particles is very different from an actual nucleus. The physics are pretty different.

It's a bit like saying the SpaceX Starship, and my mechanical pencil are the same thing, because they have the overall same shape, and are made of stainless steel.

> Neutron stars are held together by gravity, not nuclear forces.

Neutron stars are governed by both gravitational and the strong force. [edited]

Quite different from ealier phases of stars which are held together by gravity.

> Also, the mix of particles is very different from an actual nucleus.

Which particles? The key difference vs an atomic nucleus other than the immense size is that the neutron star is not charged like an atomic nucleus, as most of the precursor star’s protons and electrons have fused into neutrons.

https://en.wikipedia.org/wiki/Neutron_star#Density_and_press...

> In popular scientific writing, neutron stars are therefore sometimes described as "giant nuclei". However, in other respects, neutron stars and atomic nuclei are quite different. A nucleus is held together by the strong interaction, whereas a neutron star is held together by gravity. The density of a nucleus is uniform, while neutron stars are predicted to consist of multiple layers with varying compositions and densities.

It's just a pop-sci meme.

As for the structure, see here:

https://en.wikipedia.org/wiki/Neutron_star#Structure

I have a feeling you’re just trolling for trolling sake.

I’ll only comment on one part of the Wikipedia quote regarding constant densities of atomic nuclei.

The neutron star will have a constant density on any length scale as large as an atomic nucleus. The layered structure manifests over length scales of kilometers, which is 10^18 times larger than an atomic nucleus.

It could also be that your understanding of neutron stars may be superficial - you may feel, however strongly, that you understand this topic, while reality may be different. Have you considered that hypothesis?

Nobody who actually works in fields related to nuclear physics or astrophysics considers a neutron star as being "just a giant nucleus", no matter how often that image may be used in pop-sci articles and youtube videos.

More linky stuff, again related to your statement that neutron stars are held together by the nuclear force (which is false):

https://www.nuclear-power.com/nuclear-power/reactor-physics/...

> Since they have some similar properties as atomic nuclei, neutron stars are sometimes described as giant nuclei. But be careful, neutron stars and atomic nuclei are held together by different forces. A nucleus is held together by the strong force, while a neutron star is held together by gravitational force.

This is an enjoyable and worthwhile lecture by Jocelyn Bell, who discovered pulsars:

https://www.youtube.com/watch?v=-335gUOvdhA

Just watched it, very worthwhile!

Amazing. Thank you!

MISSION OF GRAVITY by Hal Clement.

Addresses this very scenario.

A fine fine bit of scifi.

A wonderful, wonderful book. Captain Barlennan is a fantastic character. Time to reread this magnificent story.

> Neutron stars are some of the densest objects in the Universe: they weigh about as much as the Sun, yet measure only around 10km across, similar in size to a large city.

I'm going to be the Hacker News pedant here and say that this sentence bugged me. Weight is a measurement of force. Mass is the word they were looking for.

Just wait until you see the next sentence.

Why do neutron stars have "crusts", as opposed to behaving more like a liquid?

Because the pressure on the surface is only from one side.

It's exactly like surface tension in water.

I don't think that's what causes surface tension in water. However, it turns out that surface tension is more mysterious than I thought.

I thought surface tension was caused by inter-molecular attraction due to water being a polar molecule, which seemingly wouldn't apply to a neutron star.

But this thread https://physics.stackexchange.com/questions/2615/are-there-a... contains a reply saying that surface tension is an inevitable result of the interface between two materials, and there are no liquids (even superfluids, apparently) that don't have surface tension.

In any case I don't see why we'd expect condensed matter physics to be generalizable to neutronium...

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