Neutron stars may be bigger than expected, measurement of lead nucleus suggests

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

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.

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...

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*

My bad for having stuck thermodynamics in there at all. :-(

Seriously my course in general relativity was about 30 years ago. And work on the information paradox work is about exactly what you're talking about.

I honestly don't fully know what happens when you're approaching a growing black hole. Do you cross the horizon then?

What outside observers will see "frozen" is the instant just before you cross the horizon. A bit like if your image 1ms before you cross the horizon will be seen by the observers years after, your image 1µs before will be seen millions of years after, etc...

This is one of the things shown in interstellar before he hits the tesseract and they go into pure speculative scifi. He gets bombarded with extremely energetic blue shifted particles as he moves through time while falling.

Regarding what happens at the event horizon, your question is slightly inaccurate in that the photon is what you observe so it doesn't emit anything. If you assume, say, a torch falling in facing toward you, the reason that it feels delayed is ofc that it has to move out of the gravity well of the black hole. I haven't thought about the question myself, but basically think about the entire set of photons your torch emits and assume it is a magically resilient thing that never tears apart.

Then it emits a fair few trillion packets of energy before it crosses the event horizon. Most of this is caught in the photon sphere and never makes it out (depends on incident angle and some other calculations I forget).But we will assume photons are ejected in one line out on some radius vector of the black hole. Then all photos ejected before the event horizon will eventually escape and you will see them. The last one is supposed to escape asymptotically as you approach the horizon but it is only constrained by how much energy it loses traversing that piece of space time from the event horizon to infinity (i.e. where the observer is located).

So we can deduce that it gets emitted at some point in time. We assume infinity to make the math easier but real life is quantized so it'll likely be sooner than that. I can't do any better than that unfortunately.

No, not because they don't have sensory organs or the like, we can wave that aside, but because time doesn't pass for objects moving at the speed of light. It's so compressed that everything is instantaneous: a photon from another galaxy arrives in our eyeballs at the same moment it is emitted.

So you're asking a question about the duration experience around a black hole, of a particle which can't have duration experience.

Yes, but things can still fall in and hit the singularity inside during that time. You can't see the inside from outside, but you can still calculate that it is there based on observations you can make outside.

> if you are a conscious very resilient little particle doomed to fall into a black hole... do you really appear from the outside to be falling in until the black hole evaporates?

Sort of. If you are watching from outside, when the hole finally evaporates, you will see a big flash of light that includes images of everything that fell into the hole before it evaporated away, at the instant those things crossed the hole's horizon. Up until that point, yes, you will see infalling objects appear to gradually slow down as they approach the horizon. But that is only a sort of optical illusion; the objects actually fell in and hit the singularity inside the hole long before you see the final image of them crossing the horizon.

> is a black hole sort of a time machine to the end of the universe where in the short process of falling in you get to watch the whole history of existence pass you by

No, it's not. If you fall into a black hole, you will only see a short portion of the future of the rest of the universe from the time you started falling. The rest of the future of the universe will never become visible to you.

> is there some third option

No.

If this seems non-intuitive, remember that the Schwarzschild radius varies linearly with mass, and tidal forces scale like mass / radius^3. So tidal forces at the event horizon scale like 1/mass^2. A black hole that is 100 million times heavier will have tidal forces that are 10 quadrillion times smaller at its event horizon.

> Anyway, after an hour of blending, you will get a horrible homogeneous mix.

This (plus hawking radiation being that horrible homogeneous mix) sums up the solution to the black hole information paradox really well, thank you.

Inside could be shaped like a dolphin a potted plant or a whale. Outside of horizon we couldn’t tell from outside.

I’m saying that “neutron stars have lower density than previously thought” means “neutron stars can be more massive before reaching constraint of Schwarzschild radius for given mass”.