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Comment by cletus | Hacker News Reader

Let me add some perspective here. This only became apparent to me fairly recently and it blows my mind. It's something I hadn't thought of before.

Black holes are relatively "simple". I mean that you can completely define a black hole with 3 properties, one of which isn't really relevant.

The two most relevant properties are spin and mass. The last is electric charge. The reason this is somewhat irrelevant is because electric repulsion is about 60 orders of magnitude more than gravitational attraction so it's not expected black holes have a significant charge.

Neutron stars OTOH are arguably the most complex objects in the Universe. Why? Because you're dealing with gravity, electric charges, nuclear forces and QCD such that there's no equation of state for describing dense nuclear matter.

Probably my favourite variant of neutron stars is the extremely rare magnetar [1], the most powerful we've found has a magnetic field 100 trillion times that of Earth's [2]. This field is so strong it would flatten atoms and rip electrons from your body.

So as much difficulty as we have of describing the nucleus of an atom (as referenced in this post), imagine a whole star of that stuff.

[1]: https://en.wikipedia.org/wiki/Magnetar#:~:text=A%20magnetar%....

[2]: https://www.space.com/magnetar-eruptions-sculptor-galaxy#:~:....

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Black holes are relatively "simple". I mean that you can completely define a black hole with 3 properties, one of which isn't really relevant.

Classically, yes. But more recently we've discovered not.

One of the major conundrums about black holes is how information gets lost in their creation. There is a lot more information in the stuff that creates a black hole than in the black hole itself. Which violates the third law of thermodynamics. This is called the Black Hole Information Paradox.

But as https://blogs.scientificamerican.com/observations/have-we-so... explains, the real state of a black hole includes all of the stuff that you can see in the process of falling in. (We never actually see anything hit the event horizon. And in theory something on its way there can still be retrieved a million years after it started falling.) When you track things carefully, a real black hole is a very complex thing indeed. With no information loss.

This brings up a question I have had for a while: what is the life story of a photon shot at a black hole?

From an outside observer, presumably you shoot a laser at a black hole and if your photons don't hit anything on the way to the horizon, they never hit it either but just approach asymptotically as time goes to infinity.

As, a photon though, you don't "notice" crossing the horizon and in a measurable amount of time you go from being emitted to hitting the singularity.

Black holes don't last until infinity though, they evaporate in a large but measurable time. (let's say 10^100 years, it depends on time and how the universe dies and how big the black hole is, but whatever, presuming it happens it is some extraordinarily large number of years)

So... as something falling into a black hole an outside observer will watch you infinitely slowly approaching a growing horizon until at some point the growth goes negative and you watch the horizon falling away from your friend the photon until at last the horizon disappears entirely and releases the photon to go about its merry way to hit something sometime around the heat death of the universe.

In other words, 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? Do you experience going through the event horizon like it's nothing and hit the singularity in a few hours by your own watch...

Or 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 and you come out at the end having never crossed the horizon in what was for you a very short ride.

Or is there some third option where from the outside it takes an infinite amount of time for you to fall in, but that infinity "for math reasons" actually takes place and passes in a Zeno's paradox kind of way at some distinct point in your timeline and you meanwhile pass the horizon and hit the singularity in short order and are no more?

I don't really know anything but these are questions and vague thoughts I have had. The central question is reconciling the outside apparent infinite time to watch something cross an event horizon with the finite lifespan of evaporating black holes.

The wrinkle in your question which sibling comments miss is this: it's difficult to reason about how a photon 'experiences' things under any circumstances.

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 I was a bit imprecise and wandered about with my various examples... a photon doesn't experience time but an observers in a reference frame inertial-ish to the microwave background and generally far away from a gravity well will have a consistent agreement of when a photon is emitted, when it crosses an event horizon, and perhaps what happens to that photon as the black hole evolves. From the photon's experience it is emitted and absorbed at the same instance, my questions about duration are about how an external observer would assign times and locations to events for that photon.

As a particle, you continuously accelerate even after the event horizon (which you don't realize). Immediately after you passed the horizon, any photon you can send won't ever leave the black hole.

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...

But... let’s watch that image or at least keep a model of our friends falling into the black hole for 10^100 years. Forever is a long time.

If we keep watching that image the black hole eventually stops growing, the “image” never crosses the event horizon and when the universe cools down enough the event horizon starts getting smaller and hotter until we watch our friends getting roasted outside the evaporating black hole which eventually is gone and just normal matter.

The timeline of the “image” would seem to reconnect with the real article having never crossed the event horizon.

In other words it would seem if we waited long enough the image of our friends outside the event horizon would outlive the black hole and we could go say hello after it evaporated.

In other, other words, how do we see the universe outside aging as we fall into a black hole? Do we not get to watch the heat death of the universe as we approach and consequently the black hole very quickly evaporating in front of us as we fall towards it?

This has been my line of thinking also. Many models of black holes in the past have been way oversimplified, to the point of absurdity. Their temporal existence is bounded in both directions, and neither is an instant cut-off!

1) When a star collapses, the black hole it becomes isn't formed instantly, that would violate the very theory of relativity that predicts their formation.

2) Black holes evaporate, as you've mentioned.

But this begs the question: Forget the hypothetical "probe" particle. What about the particles that made up the original star? They're like many probe particles!

Imagine the black hole starting at the centre of the collapsing star, expanding outwards, slowing down time for the infalling matter. Does any of the rest of the star fall in to the black hole? Or, like your test particle, does the rest of the star just hang there, frozen in time, a thin layer above the event horizon?

Regressing that back to the formation: Does anything ever truly fall into the black hole?

I suspect not. My thinking is that a black hole truly is a hole in spacetime, and contains nothing, not even spacetime. It's like poking your finger through a loose-knit wool jumper and making a hole. From the perspective of the threads, you've made a distortion that can be approached, but not entered.

Then the evaporation of the black hole is the hole in spacetime closing up, releasing the compressed matter near the horizon as Hawking radiation...

You’re correct: for an outside observer watching the formation of a black hole, the matter going into it will never really fall in. It’ll just continuously get dimmer and dimmer.

Outside observers are more diverse than perhaps you suspect.

For instance, what does an observer hovering just above the horizon of a different black hole see?

There is more than one astrophysical black hole in our universe, and one should not forget that when reasoning based on the Schwarzschild exact solution. Worse, most of the astrophysical black hole candidates have non-negligible spin. Certainly it would be weird for a stellar black hole to have low spin, as they will have previously have been rotating stars, and so with any sort of trust in the exterior Kerr solution[1], it seems unwise to ignore the observables generated by the ergosphere, in particular the non-stationarity of objects within it.

It doesn't really change the thrust of my question two paragraphs up: can an accelerated observer see a blueshifted infall more similar (in terms of counting time by the ticks of the observer's wristwatch) to the left than to the right in https://en.wikipedia.org/wiki/File:Gravitational_time_dilati... ? Or, equivalently, are there families of observers who are not (in this case Boyer-Lindquist) coordinate observers ? (The second answer should be, (a) yes, we can find arbitrarily accelerated observers, and (b) we may not be able to find a mapping among the local inertial frames of the infaller, the coordinate observers, and arbitrarily accelerated observers, but we should not ascribe any physical meaning to our failure to do so. cf [2]).

Perhaps a more interesting way of thinking about it is that there is essentially no practical difference between a black hole with the infaller collided with the gravitational singularity and a black hole with the infaller at 2GM+epsilon (in units such that c=1 etc) above the gravitational singularity, and if there was a practical difference and it persisted longer than a light-crossing time, we would have disproven the no-hair conjecture. However, LIGO/VIRGO evidence shows a very clear ring-down for BH/BH and BH/NS mergers, which fails to support such a challenge to no-hair.

For us weakly accelerated Earthbound observers, neutron stars and black holes at a wide variety of distances from us completely fall into each other in finite (indeed, short in human terms) time.

Non-compact infallers don't raise much of a bump on the horizon of the black hole in comparison, but the result is the same: M changes (as does the spin parameter a), and we now "find" the horizon at a new set of points. (Here it's tempting to talk about slicing up spacetime into space and time so we can talk how it is easy to forget what we mean when we talk about e.g. the "before" horizon and the "after" horizon, or alternatively discuss whether, if we had a sensitive gravitational wave detector, we could use that to decide whether a large-mass infalling object was inside or outside the black hole).

- --

[1] There are objects like https://carinaemajoris.wordpress.com/2012/07/07/a-very-errat... whose observations are much closer matches to Kerr (exterior) solutions with spin parameter ~ 0.9 than to Kerr (exterior) solutions with spin parameter ~ 0 (in particular they're pretty clearly not generating the sorts of timelike geodesics that one would find in a Schwarzschild spacetime).

[2] https://physics.stackexchange.com/a/458855

> it would seem if we waited long enough the image of our friends outside the event horizon would outlive the black hole and we could go say hello after it evaporated

You would see your friend's image, but your friend's image is not your friend. If you tried to go towards where the image appeared to be coming from, you would just find empty space, where the black hole that evaporated away (with your friend inside, having long before hit the singularity) used to be. Your friend would not be there any more.

Note, btw, that once you see the final image of your friend (the image of him just crossing the horizon, which you see in a big flash of light when you see the hole finally evaporate), you see nothing more; your friend, and the hole, and all of the other things that fell into the hole (and whose images as they crossed the horizon you also saw in the big flash of light) all vanish after that (no more images are coming).

> Black holes don't last until infinity though, they evaporate in a large but measurable time.

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.

> the objects actually fell in and hit the singularity inside the hole long before you see the final image of them crossing the horizon.

It's not an illusion at all, and neither observers reference frame is more privileged than the other.

There is no global reference frame that allows you to say that the objects actually fell in and hit the singularity inside the hole long before you see the final image

> neither observers reference frame is more privileged than the other.

I am not talking about any particular observers. I am talking about invariant properties of the spacetime geometry and invariant curves within that geometry. Nothing I have said depends on any particular choice of observers. I only phrased it in terms of what a particular observer sees because the post I was responding to was asking about what can be seen from outside the horizon. I could just as easily have phrased it in terms of what null curves intersect with a particular timelike curve; the underlying geometric facts, and the fact that they are invariant, are the same either way.

> There is no global reference frame that allows you to say that the objects actually fell in and hit the singularity inside the hole long before you see the final image

Yes, there is. There are a number of frames that cover both the exterior and interior regions; for example, Painleve coordinates, Eddington-Finkelstein coordinates, and Kruskal coordinates.

Thank you for your reply, going back to my original question that sparked all of this uninformed speculation... how much extra time and distance is covered/experienced traveling to an event horizon? (It's not exactly easy to construct the question correctly)

> how much extra time and distance is covered/experienced traveling to an event horizon?

I'm not sure what "extra" means, but a general rule of thumb is that the time to fall goes like the 3/2 power of the radial coordinate you are falling from. (Strictly speaking, this is the time to fall to the singularity, not the horizon, but for falling in from large radial coordinates, which will be the case for virtually all cases of interest, the difference is small.) The only technical complication is that this rule is true if the time and radial coordinate are in units of the Schwarzschild radius of the hole; you then have to convert to conventional units. A couple of specific examples:

Black hole of one solar mass, observer falling into the hole from a distance of 3 million kilometers (which is about 4.3 times the radius of the actual Sun--the distance is chosen for reasons that will appear in a moment). This makes the radial coordinate 1 million, 10^6, in units of the Schwarzschild radius of the hole (which is 3 km), so the time to fall will be 10^9, 1 billion, times the light-travel time across that distance, which is 10 microseconds (10^-5 seconds). So the time to fall will be 10^4 seconds, or about 3 hours.

Black hole of one million solar masses, observer falling into the hole from a distance of 30 billion kilometers (or about 5 times the semi-major axis of the orbit of Pluto around the Sun). Here the radial coordinate is only 10^4 in units of the Schwarzschild radius, so the time to fall will be 10^6 times the light travel time across that radius, which is 10 seconds (a million times 10 microseconds). So the time to fall will be about 10^7 seconds, or about 4 months.

The second option is accurate. As you fall from the event horizon to the singularity, you see the entire future of the universe unfold since all arrows of time now point toward the singularity so as you move along it, you are moving in time.

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.

>As you fall from the event horizon to the singularity, you see the entire future of the universe unfold since all arrows of time now point toward the singularity so as you move along it, you are moving in time.

This would suggest that as I see the entire future of the universe as I fall towards the event horizon I also see the entire future of the horizon itself... which means that I will never cross the horizon because it will at some point far in the future no longer exist.

i.e. there is no spacetime path which ever crosses the event horizon

I'm near the point where I'm going to have to learn some black hole math to convince myself otherwise. (I think I'm wrong but can't see why)

Relevant discussion: https://physics.stackexchange.com/a/79086

> i.e. there is no spacetime path which ever crosses the event horizon

As I understand it, there isn't, from the perspective of an observer external to the event horizon.

(warning: I am only an avid watcher of PBS Spacetime)

> As I understand it, there isn't, from the perspective of an observer external to the event horizon.

This is not correct. An observer outside the horizon cannot see anything at or inside the horizon; but that doesn't mean he can't calculate that there is a region of spacetime inside the horizon, from observations he can make from the outside. He can.

As I understand it at this point... there is no round trip spacetime path to the horizon and back and best-case light which hits (or is arbitrarily close to hitting the event horizon) can get back to you the moment the black hole finally evaporates. However the first half of that trip is possible and happens in finite time.

> As you fall from the event horizon to the singularity, you see the entire future of the universe unfold

No, you don't. Most of the universe will never be in your past light cone as you fall to the singularity. The fact that the singularity is unavoidably in your future when you are inside the hole, does not mean it is also to the future of everything outside the hole.

It's basically all the future events that happen inside the black hole isn't it? At least that's what I meant.

> It's basically all the future events that happen inside the black hole isn't it?

No, you won't even see all of those if you fall in; the most you will see are the events that end up in your past light cone before you hit the singularity. That is only a portion of the entire spacetime region inside the horizon.

This is unnecessarily pedantic. What parent meant is more akin to "if Earth would exist forever, and you would live forever on it, you'd see the entire future of the universe unfold". Obviously you won't see what happens on the moon when it is behind the horizon, but that was not the point.

> What parent meant is more akin to "if Earth would exist forever, and you would live forever on it, you'd see the entire future of the universe unfold".

No, that's not what the post I responded to meant. That post was specifically talking about what an observer falling into a black hole and hitting the singularity would see. That is not the same as existing forever outside the hole's horizon.

Why do you think the post did not refer to its author's understanding, that your experience would be the same as hanging forever just outside the horizon except in finite time from your perspective? This would be my default interpretation of their comment, and since you are successfully attacking a different stronger interpretation you should reconsider that this strong interpretation is the right way to interpret the comment. That's scenario is actually covered in HN discussion rules.

To be fair until your comment I wrongly believed the moment you'd be crossing horizon you'd be hit by all light ever emitted toward that point of the horizon in the entirety of the future (assuming no hawking radiation of course).

That's officially above my pay grade. :-)

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?

Well, the other wrinkle here is that the "singularity" is a mathematical approximation. There's no such thing as hitting it -- from the outside, you just move asymptotically slower as you approach it.

Not so fast. Black holes have a singularity in the middle, and an event horizon around them. There is no singularity at the event horizon, and that is all that we can see..

There might be naked singularities in case of a rapidly rotating black hole. The singularity would have a donut shape and might be outside of the event horizon. https://en.wikipedia.org/wiki/Naked_singularity

Of course, this is very theoretical and we don't actually expect that to exist.

Huh, I thought that naked singularities were thought to not exist. But apparently in 2018 someone found a case where they could.

Thanks!

You move asymptotically slower from the point of view of the particle falling in the black hole. From outside the black hole, the falling particle should fall into the black hole at a speed close to the speed of light. (but we can't observe that from the outside)

The image an outsider would see "printed" on the black hole has nothing to do with where the falling particle is.

Wasn't the issue of black hole information loss the subject of the Hawking-Thorne bet that Hawking eventually conceded (that black holes didn't destroy information)? [1]

(This may be a separate issue; I'm genuinely curious).

[1]: https://physicsworld.com/a/hawking-loses-black-hole-bet/#:~:....

It doesn't violate any law of thermodynamics (in as much as it does violate the arrow of time in some sense). The problem is hawking radiation is pure white noise which means if a laplace demon existed (and for our purposes, when quantum mechanics meets real world, we assume an infinite number of laplace demons exist and are created every moment), it cannot reverse a black hole back into its constituents that fell in. This is a HUGE problem.

However, there was a solution to this published in 2019. Just look up Black Hole Information Paradox and it should appear as the last section on the wiki page.

All the stuff around a black hole and in a black hole is under a lot of tidal forces and will be crushed and crumbled.

Imagine a blender. According to quantum mechanics, using a blender for an hour is an unitary transformation, so it will not destroy the information, and it's an invertible operation, you can reconstruct the original items. [And even a classic blender is invertible at the molecular level. It's only not invertible if you consider the friction and other average macroscopically properties.] Anyway, after an hour of blending, you will get a horrible homogeneous mix.

But (if the current models are correct) a Neutron star is very different in the crust that is a few km near the surface. It has many levels with different properties that are called "nuclear pasta" https://en.wikipedia.org/wiki/Nuclear_pasta and one with graphics that explain the names https://astrobites.org/2017/10/05/nuclear-pasta-in-neutron-s...

Going again to the blender example, it's like a bad blended mix, that has foam at the surface, a liquid in the middle and solids at the bottom.

It depends on the size of the black hole. One with the mass of the Sun would rip your head from our feet under a tidal force of thousands of gravities. One the size of the monster at the heart of our galaxy has tidal forces so gentle that you wouldn't even notice them until after you were inside the event horizon.

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.

You are correct.

But the "no-hair theorem" that says that a black hole can be characterized using only 3 properties does not apply instantly, but only after some time of isolation.

For a small black hole the time is very short, but for a huge black hole that will not tear you apart it takes a long time. So even a classic huge black hole has more internal state that should be measurable from outside.

> using a blender for an hour is an unitary transformation, so it will not destroy the information

> 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.

> Which violates the third law of thermodynamics

Wouldn't this be a violation of the second law rather than the third?

I just looked it up, and it is a violation of the fact that the laws of physics should be reversible. So from the wave function at any point of time you should be able to reconstruct the wave at any other.

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

It's not quite Maxwell's demon because the event horizon is a one-way causal ticket. The information might not be destroyed but it is totally inaccessible on this side of the horizon.

> This field is so strong it would flatten atoms and rip electrons from your body.

...which is underselling it a bit. :)

(As quoted in Wikipedia) the field is so strong that pure vacuum itself has ~10,000 times the density of lead, due to the energy contained in the field. Imagine that.

Neutron stars seem like what you would see in a black hole if the interesting parts weren't shrouded by an event horizon. Although I suppose the event horizon does give you jets.

But yeah, city-sized objects with the density of an atomic nucleus. Pretty mind-bending! (literally, I guess?)

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."

I really like this take, but on a weird philosophical way I gotta disagree. The surface of the earth has by a long shot the most difficult physics in the universe we know about.

Try to imagine a model that predicts the things going on there.

At least the neutron star probably has some scale at which it starts to looks homogeneous.

This, btw, is a pretty good marketing strategy for condensed matter physics (and indeed, biology and chemistry [1]):

"The most interesting physics in the universe takes place on the surface of earth."

[1] Biology and Chemistry are literally subdisciplines of physics so large we call them different names. And they're subdisciplines which are only so large because the surface the earth exists. Without the surface of the earth as part of the universe, as far as we know biology wouldn't exist, and chemistry would probably still be nothing more than a medium-sized subdiscipline of physics.

if free will is to be believed, then it can't be modeled, right?

I mean, free will isn't to be believed. But if you wanna, be my guest. If quantum mechanics is to be believed, then the model is certainly probabilistic at best, so you'll have enough wiggle room for it.

Even granting the weirdness of quantum mechanics there may not be any wiggle room, as random will is not free will.

Oh don't you know that acts of free will only take place in the subset of chains of cause and effect that can be traced to the (decoherence-induced) collapse of a quantum state, and also just happen to turn out to be perfectly randomly distributed samples of the particular distribution quantum physics predicts for that situation?

Free will of the gaps lives on [1].

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

>In 2020, a fast radio burst (FRB) was detected from a magnetar.[⁷][⁸][⁹][¹⁰][¹¹][¹²][ᵉˣᶜᵉˢˢᶦᵛᵉ ᶜᶦᵗᵃᵗᶦᵒⁿˢ]

Wikipedian humor at its finest. Because of course "excessive citations" looks like a citation.

> [ᵉˣᶜᵉˢˢᶦᵛᵉ ᶜᶦᵗᵃᵗᶦᵒⁿˢ]

FWIW, superscript i (contra ɪ) is at U+2071:

[ᵉˣᶜᵉˢˢⁱᵛᵉ ᶜⁱᵗᵃᵗⁱᵒⁿˢ]

I'm not seeing a superscript square bracket anywhere, unfortunately.

ah, good, there is a `i`. that struck me as weird too. I just grabbed a random "write in superscript" site tbh :)

>The reason this is somewhat irrelevant is because electric repulsion is about 60 orders of magnitude more than gravitational attraction so it's not expected black holes have a significant charge.

I hadn't thought about it but I guess that means protons or electrons have a maximum resting density. A group of those will never collapse from gravity?

Protons and electrons are both Fermions, which means they can not have identical quantum numbers (have to obey the Pauli exclusion principle as mentioned elsewhere in the thread). In the case of a very dense system, like the sun, this can lead to an effect known as degeneracy pressure (which acts against gravity). Essentially you have filled all the lower quantum numbers and then adding an extra proton/electron to the system requires a certain amount of energy. It's quite handwavy but the degeneracy pressure of electrons is mostly what keeps a white dwarf from contracting, whereas in the case of a neutron star it is the degeneracy pressure of neutrons (plus repulsive strong force and other effects as indicated in the OP). This kind of high level discussion is often covered in first year astronomy courses auditing a MOOC like the following may be of interest (https://www.edx.org/course/astrophysics-the-violent-universe)

If that were true, you would not get a neutron star where the electrons and protons are collapsed into neutrons. Not sure how that stands to the overall strength question, though.

> Neutron stars OTOH are arguably the most complex objects in the Universe.

In OUR universe, at least.

If a black hole ends in a white hole, Gosh only knows how complex things will end up at the other end. Witness, for example, our entire reality.

A black hole is just a very heavy object

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