The “how to build a black hole” video from PBS space time says that’s how you do it - you start with a neutron star and keep adding more mass to it, which paradoxically makes it smaller until it becomes smaller than its swarzchild radius, becoming a black hole. Blew my mind and anyone else’s when I share the video with them!

https://youtu.be/xx4562gesw0

That depends on what size of black hole you want to make. It works for large black holes (you need sufficient mass for the gravitational forces to be sufficient) but those black holes aren’t that useful.

Smaller black holes are harder to construct but way more useful.

I'm curious what useful applications we have for black holes of any size

There are actually a lot of theoretical applications.

Black holes are likely the most efficient form of power generation that we currently know of.

Black hole propulsion may well be the best form of interstellar starship propulsion.

Black holes may also be the ultimate computer.

If the universe continues based on it current understanding then eventually all the stars will be dead and the universe will be a dark place. The Black hole era will last trillions of time as long but may well be the golden age of civilization.

Each of these topics is a rabbit hole.

The theoretical limit of how efficiently you can turn energy into useful work depends on how cold your energy sink is. The coldest energy sink in an ever-expanding universe gets colder over time as everything spreads out and the fixed amount of energy occupies more and more space. So the hypothetical optimal end-state of an extremely old civilization is hibernating a bunch of very stable mass until the extreme far future, at which point they can drip-feed it into a black hole to do some extremely efficient computation.

It's assumed that little ones are the most efficient mass to energy converters, via Hawking radiation.

It's simple, just deflate spacetime!

I've seen a couple astrophysics videos over the last few years that put forward the theory that many black holes are the result of a neutron star forming in a binary or trinary star system. The initial explosion and the ejecta creating a situation where the neutron star begins to siphon material off of its partner.

Really don't want to be dismissive, but this guy appears to me as having one gesture (both hands towards the viewer, to underscore the graveness of his words), a good haircut and that's about it.

If find it hard to take a presentation like this serious. Most of the mentioned facts are right as far as i can judge it without having a major in physics, but to me this is so very much worse than a link to backreaction (Sabine Hossenfelder), startswithabang (Ethan Siegel) or almost any other serious physics blog out there as anything else.

I suppose this will get downvoted, and I'm fine with that, but please explain to me why a self-promoting video guy with probably 5% understanding of the matter compared to a serious pop-sci blog is so relevant to you.

Heck, even wikipedia has more information than this clip. And it doesn't cost 13 minutes + ads, just 5 minutes to read.

It's not a paradox a neutron star gets smaller the more mass you add to it. This is also true for Jupiter-sized objects; it is a function of density, matter and gravity, and not related (in this case) to special relativity.

That self-promoting video guy is an astrophysicist, with a significant amount of papers published about black holes.

You are free to not like his mass-market material, but from a technical level, he is quite qualified to talk about the subject.

https://en.wikipedia.org/wiki/Matt_O%27Dowd_(astrophysicist)

https://www.mattodowd.space/

https://ui.adsabs.harvard.edu/search/p_=0&q=author%3A%22O'Do...

I have no idea what you’re talking about? It’s a pbs YouTube channel, the host is a professor in astrophysics and has generally made sure he doesn’t oversimplify anything. I like Sabine’s videos sometimes when she’s not bonkers kookoo but this guys not trying to be controversial.

Matt O'Dowd is a well-known physicist. I'm not sure why you feel that you can judge him so quickly.

This could be one of the greatest own-goal posts on HN I can recall reading.

Lots of assumptions in your comment. They happen to be very wrong.

Maybe as a default position, you should assume less about someone’s credentials based on hand gestures and haircuts.

I was under the possibly incorrect impression that black holes result when the gravitational force is so great that it even overcomes Pauli's exclusion principle, collapsing the neutrons into an overlapping mass of staggering density. Now that I write that out, I'm a little more doubtful.

Close.

Giant blue stars burn all of the way up to the most stable element of all, which is iron. And then collects an iron core. As the core grows, the star shrinks and the pressure in the middle grows. Once the pressure in the middle is high enough, it overcomes the Pauli exclusion principle and electrons merge with protons to become neutrons with a release of energy, which mostly comes out as neutrinos.

Once a bit of that core disappears, the stuff around falls into the new gap, hits it, goes under way more pressure and the same thing happens. This starts a chain reaction that causes a supernova. The core collapses into a neutron star while the release of energy blows the outer shell out to eventually become a nebulae.

If the initial star was big enough, the newly created neutron star is big enough to transform into a black hole. Otherwise it is left as a neutron star.

If it forms a neutron star, the spin speeds up thanks to the collapse. Typical time to rotate can be anywhere from milliseconds to seconds. We can tell that because its magnetic field also gets trapped and forms a strong beam along the north and south magnetic poles. Since magnetic poles do not generally line up with the rotational poles, that beam becomes like a searchlight. If we're in the path of that beam, we see a pulsar, and the frequency of the pulsar is the rotational speed of the neutron star.

This is probably way more than you wanted to know. :-)

I probably also am going to get corrected on some details in 3, 2, 1...

Your comment did inspire me to go looking for the distribution of elements within the iron core: "carbon, neon, oxygen, and silicon burning leave a core composed of iron, cobalt, nickel, and neighboring species, referred to as the iron-peak nuclei".

For anyone else interested in a readable article on a slightly more detailed look at supernova than wikipedia, and a discussion on the difficulties of modelling supernova, I liked this article: https://aip.scitation.org/doi/10.1063/1.4870009

I was shocked to learn how long it takes a photon produced at the center of the sun by a fusion event to reach earth. The layers upon layers of subatomic particles that are all interacting in these tight quarters is simply astounding.

I've heard this same theory on supernovas from a number of people in or around physics, but I always wonder how handwavy it is, whether we actually understand how that works yet, and how different (confusing) the real process is from the given one.

We understand this one surprisingly well.

The seminal paper from 1957 is https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.29.... that figured out all of the reactions involved. Then used their predictions of the process to predict the distribution of observed isotopes of all of the elements. Those predictions were surprisingly accurate and have been refined more. See https://www.sciencealert.com/this-awesome-periodic-table-sho... for the outcome of that.

Incidentally the photon takes so long to escape because it gets absorbed and reemitted over and over again in a drunken walk. A supernova is fast because most neutrinos just escape, but with enough being stopped to rip the star to shreds.

...0: A nebula. The -e is a plural marker.

Hey, you asked for it.

Not exactly.

A black hole doesn't have to be very dense. The larger it is, the less dense it needs to be to prevent light from escaping. The supermassive black hole at the center of our galaxy may be less dense than water[1].

For black holes closer to a single solar mass, then yes, they will be super dense, and the degeneracy pressure helps keep it from becoming a black hole. You have to add enough mass to overcome the degeneracy pressure for it to develop the event horizon that makes it a black hole.

But not all black holes do that. Others can simply keep acquiring mass without that violent sudden event. They just gradually cross the point where no light can escape, but you might not even notice looking at it. The light just keeps gradually shifting redder and redder until the wavelength becomes infinite.

[1] https://en.wikipedia.org/wiki/Sagittarius_A*

Last update I heard on black holes was that the space inside of them increases at the speed of light. That's a hell of a lot of space inside of an event horizon.

I'm still fond of the theory that our universe is in fact inside of a black hole, and that the big bang is 'just' the moment we went supercritical.

One thing I don't understand about naked singularities - if you were inside of one, the lack of an accretion disk would allow you to see out of it, right? Could you detect that you are looking out across an event horizon?

AFAIK the moment you cross an event horizon is not supposed to be significantly different from being just outside of one (at first). Light is one-way within an event horizon though so I imagine that should cause a lot of strange phenomena.

The interesting thing about a naked singularity is you don't have to be "inside" it. You can be next to it but there is no even horizon.

seems like i was under the _likely_ incorrect impression black holes have mass but no volume (singularity and all..) and so infinite density. But, then again, what happens beyond the event horizon is meaningless anyway.

The description I have heard is that a black hole contains no matter at all, it is all energy expressed as infinitely(?) warped space and time.

I remember the infinities of black holes being a challenge to explain to my artsy friends after reading A Brief History of Time.

Correct me if I'm wrong, but isn't the curvature of space around - and within - a neutron star substantial enough that euclidean geometry doesn't really hold anymore? The volume of a basketball is the surface area x R/3, but is that true of a neutron star? I was under the impression that the difference between Euclid and actual was statistically significant, to the point that you get the wrong behavior if you don't account for it.

Imagine if black holes were just neutron stars made of second or third generation quark "neutrons."