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Black Hole Puzzle

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Re: Black Hole Puzzle

#21

Post by jiggawatts »

while_true_ wrote: post #14
jiggawatts wrote: post #13

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

It irks me so many physicists/cosmologists jump from the mathematical GR singularity at the center of a BH to "matter there has infinite density." That's highly unlikely, it's probably quark plasma.

My simple model of it is to just think about spatial surfaces and do "accounting" of the total flux through them.

If you draw a sphere around a star collapsing into a black hole, you can treat it as a closed system. If the black hole evaporates, then all of the mass-energy of its progenitor original star needs to leave through these concentric surfaces. This is on the same order of magnitude as a supernova, as it is equal to the collapsed core of the star being converted into pure radiated energy!

An infalling observer accounting of the mass-energy flows must match this external view point. As they cross smaller and smaller bounding spheres, they must see this energy flowing out through those boundaries, adding up to the same total. (There is nowhere else for the energy to go; it has to be blasting you in the face as you fall in!)

Oversimplified models of black holes concentrate the mass-energy to a point, leaving spacetime around it an empty vacuum. So an infalling observer will see zero, zero, zero, zero... infinite energy density for an infinitesimal time. This is non-physical nonsense, and isn't even mathematically sound!

From Hawking we know that infalling (and distant) observers see some finite energy flux, and from Einstein's GR we know that infalling observers will see this blue-shifted and time-accelerated on the way in.

The logical conclusion is that the flux is observed to increase smoothly by infalling observers until the entire amount is accounted for in a finite time. This is a staggering total amount of radiated energy, equivalent to an matter/anti-matter explosion of matter the density of a neutron star core! There is no way anything could "fall through" this while jotting down their observations. It's not a survivable journey.

There are no wormholes, reachable parallel universes, and there are no separate white holes "elsewhere" in the universe. A black hole is the white hole, smeared out trillions of years into the future so that the enormous total energy is radiated out so slowly from an outside perspective that they just look black. Infalling observers see the "true" white nature of these explosions frozen in time.

Re: Black Hole Puzzle

#22

Post by cyberax »

mtdewcmu wrote: post #20
cyberax wrote: post #11

Earlier quoted context omitted.

> Isn't this true of all matter that enters? Not quite. He will see the light emitted by _all_ of the matter that has fallen in before him, but only in an infinitely small area.

A single photon can't be seen multiple times, right? So, if photon A goes into Alica's retina, then Bob can't see photon A. If a big, opaque object passes through the event horizon right in front of you, it would absorb or scatter the photons in its path, and you would not see them.

Yep, I explained it a bit more here: https://news.ycombinator.com/item?id=42299891

Re: Black Hole Puzzle

#23

Post by daxfohl »

cvoss wrote: post #3

It is argued that Bob sees light from Alice's crossing of the horizon at the same instant Bob himself crosses. Isn't this true of all matter that enters? When Bob enters, he sees everything that ever fell into the black hole "before" him, at all once? Is it blinding? Does it fry and scramble Bob? Or is it so redshifted that Bob survives?

The article is missing on a couple points. The analysis assumes all Alice's light is emitted radially exactly outward from the center of the black hole. In reality, light is emitted in all directions, and anything emitted at even slightly different angles would get sucked into the black hole. But, Bob might still see it, because he can catch up with it. So when the article says Bob sees Alice cross the horizon when Bob crosses the horizon, it really means that Bob won't see any photons they emitted from inside the BH until Bob crosses into the BH. But similarly, one second prior Bob will be encountering photons Alice emitted roughly 0.99 seconds prior to crossing the horizon, and so on, because those are all getting redshifted too.

It's similar to if you and the car in front of you are accelerating at the same rate, they have a small head start, and they've got someone throwing fastballs at you at 100mph. When you get to the point where you are going 100mph relative to the ground, you'll hit the ball that was thrown from exactly that spot relative to the ground. So, sure, crossing the 100mph barrier implies something interesting mathematically, but it's not something that the observer would particularly notice. The math for GR isn't exactly the same (baseballs won't redshift), and in particular there isn't even a piece of dirt to compare the photon's motion to, as the horizon is just a mathematically-defined "place", but to a first-order approximation, it's the same thing going on with photons and spacetime distortion. It's a continuous function.

There's a bit more to it than can be explained in a comment, but the main thing to know is that (as far as we know) nothing special happens at the horizon if you're falling in.

Re: Black Hole Puzzle

#24

Post by mtdewcmu »

cyberax wrote: post #22
mtdewcmu wrote: post #20

Earlier quoted context omitted.

A single photon can't be seen multiple times, right? So, if photon A goes into Alica's retina, then Bob can't see photon A. If a big, opaque object passes through the event horizon right in front of you, it would absorb or scatter the photons in its path, and you would not see them.

Yep, I explained it a bit more here: https://news.ycombinator.com/item?id=42299891

So you don't see _everything_ that went in before you, mostly just the _last_ thing.

Re: Black Hole Puzzle

#25

Post by af3d »

while_true_ wrote: post #14
jiggawatts wrote: post #13

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

It irks me so many physicists/cosmologists jump from the mathematical GR singularity at the center of a BH to "matter there has infinite density." That's highly unlikely, it's probably quark plasma.

From what I understand of it the singularity is analogous to what happens in the 2D Cartesian plane with functions such as f(x) = 1/x. When x equals 0, the function itself "breaks down" because the y-coordinate extends to infinity (ie f(0) is "undefined"). In the context of a black hole that means that neither time nor space (hence neither does matter) "exist" at this singularity. In other words upon arrival the falling object has essentially "reached the end of time".

Re: Black Hole Puzzle

#26

Post by pierrec »

readyplayernull wrote: post #18

If I got BH theory right, each particle of the ship entering the event horizon will almost fully stop, while the next particles entering will still slightly move, compressing the whole ship into a thin shell or crust that, due to atomic mechanics, won't function as normal matter, so the crew won't know what happened to them. The entering ship will redshift until it dissapears. Their particles will "spacetime-travel"…

Ignoring the "atomic mechanics" and "spacetime travel" parts, what you're describing is roughly what might be seen by an external observer, far from the black hole. From the ship's point of view, passing the event horizon happens (and in the case of a large-enough black hole, could be quite uneventful). This puzzle is all about reconciling those two points of view, and exploring intermediate points of view. If you're trying to describe things from a single authoritative point of view, of course, there is no such thing (although here's the disclaimer: I am also not an astrophysicist)

Re: Black Hole Puzzle

#27

Post by af3d »

daxfohl wrote: post #17
jiggawatts wrote: post #13

Any such blog / article / video that features a Penrose diagram is just wrong, because it's using mathematics that doesn't apply to the physical universe. Penrose diagrams draw black holes as if they have existed forever, and will last forever -- that's what the "future infinity" line means . Obviously black holes form at some finite time, and Stephen Hawking showed that they evaporate in a finite time. This matters.…

This is inaccurate. The object can fall in. Where the argument fails is that it relies on the idea that an observer will see the object redshift forever. But that applies in the classical GR realm only. However, when combined with QFT (which is required for BH evaporation) it no longer holds. In the classical approach, the light that the object emits a second before crossing the horizon will take years to reach the o…

Somewhat tangentially, what really perplexes me about Hawking radiation (HR) is this: What happens to the individual particles within the black hole as it evaporates? Like say we start with X particles. The black hole emits HR and shrinks a bit. But how exactly does it "give up" the energy from the black hole without destroying something in return? Does one particle just disappear and now we are left with X-1 particles (or what)? I haven't found any good explanations for this.

Re: Black Hole Puzzle

#28

Post by daxfohl »

af3d wrote: post #27
daxfohl wrote: post #17

Earlier quoted context omitted.

This is inaccurate. The object can fall in. Where the argument fails is that it relies on the idea that an observer will see the object redshift forever. But that applies in the classical GR realm only. However, when combined with QFT (which is required for BH evaporation) it no longer holds. In the classical approach, the light that the object emits a second before crossing the horizon will take years to reach the o…

Somewhat tangentially, what really perplexes me about Hawking radiation (HR) is this: What happens to the individual particles within the black hole as it evaporates? Like say we start with X particles. The black hole emits HR and shrinks a bit. But how exactly does it "give up" the energy from the black hole without destroying something in return? Does one particle just disappear and now we are left with X-1 particl…

At steady state, a classical black hole is fully described just mass, charge, and angular momentum. So there are no individual particles. Which itself was disconcerting to physicists because in the quantum world, information is supposed to be conserved. But they were okay-ish with it being "trapped in there somewhere". Hawking radiation is what blew that up because now the black hole evaporates. So now, nobody really knows. Lots of ideas, the most prominent being holograms on the boundary, but the math is still far from complete, and obviously experiments are even further.

Re: Black Hole Puzzle

#29

Post by readyplayernull »

pierrec wrote: post #26
readyplayernull wrote: post #18

If I got BH theory right, each particle of the ship entering the event horizon will almost fully stop, while the next particles entering will still slightly move, compressing the whole ship into a thin shell or crust that, due to atomic mechanics, won't function as normal matter, so the crew won't know what happened to them. The entering ship will redshift until it dissapears. Their particles will "spacetime-travel"…

Ignoring the "atomic mechanics" and "spacetime travel" parts, what you're describing is roughly what might be seen by an external observer, far from the black hole. From the ship's point of view, passing the event horizon happens (and in the case of a large-enough black hole, could be quite uneventful). This puzzle is all about reconciling those two points of view, and exploring intermediate points of view. If you're…

Granted. Thinking twice, BHs move in space really fast, for example when they orbit other BHs. Meaning their mass is not frozen in time, otherwise these will elongate or rip apart. So the time dilation wouldn't be have such big effect, and atoms might not be compacted too much?

Re: Black Hole Puzzle

#30

Post by mtdewcmu »

readyplayernull wrote: post #18

If I got BH theory right, each particle of the ship entering the event horizon will almost fully stop, while the next particles entering will still slightly move, compressing the whole ship into a thin shell or crust that, due to atomic mechanics, won't function as normal matter, so the crew won't know what happened to them. The entering ship will redshift until it dissapears. Their particles will "spacetime-travel"…

Let's see if I can state this properly. The atoms of the ship will pass right through the event horizon like nothing. To see the ship, though, photons have to travel from the ship to your eyes. As the ship goes deeper into the black hole's gravity, the photons will "appear to" be getting slowed down by the black hole's gravity well, each photon more than the last. So, an outside observer would have to wait longer and longer to get the next photon. In fact, he'd have to wait an infinitely long time to get all of them. It's like an optical illusion, except that a real physicist would say it's not an optical illusion; it's time dilation, etc.
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