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Are Black Holes Hollow?
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If we drop a lamp into a black hole, the lamp might reach the event horizon without being crushed. We would see the lamp becoming redder, but if we could observe longer and longer radio waves, it would not disappear. When the black hole started to evaporate, we might observe particles, with a total mass equal to that of the original lamp, emerging.
In fact, everything that approached the black hole would apparently stay at the event horizon before evaporating. The black hole would be hollow. In this view, information is not lost, but it is at odds with the viewpoint of an observer who might fall passed the event horizon.
In fact, everything that approached the black hole would apparently stay at the event horizon before evaporating. The black hole would be hollow. In this view, information is not lost, but it is at odds with the viewpoint of an observer who might fall passed the event horizon.
Answers
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Hardly makes it hollow, just because it appears things falling in stop at the event horizon. Besides most things will get swept up in the accretion disk and take a ridiculous length of time to even get near it.
17:13 Sun 14th May 2023
err no, the event horizon is merely the point at which light speed is balanced thus the what you see would be still. Black holes have zero space and infinite density. The "size" is given by the amount of mass iut has swallowed and this the gravity is exerts in the vicinity. Black holes are not hollow.
I'm struggling to parse the question, perhaps because you're trying to comprehend too much at once. It's better to break this down and take things in turn. I don't mean to discourage asking questions at all! But it can be useful sometimes to take a step back, and figure out what's important to ask *first*, and what turns out to be a distraction when trying to get to grips with a topic for the first time.
I'll begin my answer with what isn't relevant:
a) black holes evaporating;
b) "information", which, in any case, is subtle to define;
c) the specific object we drop -- a lamp is as good an "observer" as any human;
d) (from OG's answer) accretion disks, and the practicalities of objects that have to pass through them;
e) (from TTT's answer) what happens at the exact centre of a black hole. This is such a massively complicated, and ongoing, area of research that I don't want to touch on, but the basic point is that it's more complicated than TTT says, but in any case is not relevant.
TTT's and OG's intuitions that black holes aren't hollow is correct, and likewise what they're saying about how things only "appear" to stop at the event horizon is accurate, so I don't mean here to dismiss their answers out-of-hand. I just think that there's a lot of baggage here that is distracting.
The two key points are that:
1. A black hole can be thought of as an object whose total mass fits inside its event horizon. Most form from dying stars, which are not hollow; neither, too, can be black holes.
2. An event horizon is not an impenetrable barrier through which nothing can pass. It only appears that way to anybody outside the event horizon. Objects that approach a horizon close enough inevitably cross it, and then continue on towards the centre of the black hole, in a finite proper time.
Proper time here can be understood as "time according to the local observer". From the lamp's point of view, it wouldn't even necessarily notice that it *had* crossed the event horizon. This is particularly true for supermassive black holes.
There's lots of complexity here, and part of that is also because there's lots still to be understood. Black holes remain at the cutting edge of research. That said, they are not hollow; or, at least, not in the way implied by your question.
I'll begin my answer with what isn't relevant:
a) black holes evaporating;
b) "information", which, in any case, is subtle to define;
c) the specific object we drop -- a lamp is as good an "observer" as any human;
d) (from OG's answer) accretion disks, and the practicalities of objects that have to pass through them;
e) (from TTT's answer) what happens at the exact centre of a black hole. This is such a massively complicated, and ongoing, area of research that I don't want to touch on, but the basic point is that it's more complicated than TTT says, but in any case is not relevant.
TTT's and OG's intuitions that black holes aren't hollow is correct, and likewise what they're saying about how things only "appear" to stop at the event horizon is accurate, so I don't mean here to dismiss their answers out-of-hand. I just think that there's a lot of baggage here that is distracting.
The two key points are that:
1. A black hole can be thought of as an object whose total mass fits inside its event horizon. Most form from dying stars, which are not hollow; neither, too, can be black holes.
2. An event horizon is not an impenetrable barrier through which nothing can pass. It only appears that way to anybody outside the event horizon. Objects that approach a horizon close enough inevitably cross it, and then continue on towards the centre of the black hole, in a finite proper time.
Proper time here can be understood as "time according to the local observer". From the lamp's point of view, it wouldn't even necessarily notice that it *had* crossed the event horizon. This is particularly true for supermassive black holes.
There's lots of complexity here, and part of that is also because there's lots still to be understood. Black holes remain at the cutting edge of research. That said, they are not hollow; or, at least, not in the way implied by your question.
douglas9401 please consider Hawking radiation.
Old_Geezer I agree completely.
ToraToraTora there is no point in a singularity (forgive the pun): even if enough material could be confined within a Planck length, if no light could escape then no gravity could either.
ClareTGOld: I agree that "hollow" was a provocative word on which I placed too much weight. A better title is "From the point of view of a distant observer, can a black hole be regarded as a volume containing a relatively small amount of material, surrounded by an accretion disc or sphere where most of the material which produces the black hole's gravitational force resides."
Your second key point more clearly expands my remark beginning "at odds with the viewpoint" .
I agree that "c)" is irrelevant .
Old_Geezer I agree completely.
ToraToraTora there is no point in a singularity (forgive the pun): even if enough material could be confined within a Planck length, if no light could escape then no gravity could either.
ClareTGOld: I agree that "hollow" was a provocative word on which I placed too much weight. A better title is "From the point of view of a distant observer, can a black hole be regarded as a volume containing a relatively small amount of material, surrounded by an accretion disc or sphere where most of the material which produces the black hole's gravitational force resides."
Your second key point more clearly expands my remark beginning "at odds with the viewpoint" .
I agree that "c)" is irrelevant .
// can a black hole be regarded as a volume containing a relatively small amount of material, surrounded by an accretion disc or sphere where most of the material which produces the black hole's gravitational force resides? //
In that case, no. The accretion disk holds relatively little mass. It's a total distraction, both to your original question and apparently to the modified one.
In that case, no. The accretion disk holds relatively little mass. It's a total distraction, both to your original question and apparently to the modified one.
// What happens when a black hole is can't swallow anything more or is full. //
It gets bigger. Although, in practice at least, probably not by much. Accretion discs contain relatively little mass compared to the overall black hole mass, so the gain in mass is quite slow -- possibly, for sufficiently small black holes, much slower than the mass lost from Hawking radiation and jet emission, etc.
// I agree that "no" would be the answer of an observer who had fallen in, but would you accept "yes" in answer to the question if it were preceded by "From the point of view of a distant observer"? //
Yes to which question? No to both of the ones you've asked, as far as I can tell. It's true that external observers can't "see into" the event horizon, but that doesn't mean that they'd see the black hole as hollow. Indeed, that would kind of mean being able to see into it to distinguish any internal structure or lack of it.
The fundamental point is that the accretion disc is not really part of the black hole itself, so much as a part of things "outside" it. It certainly isn't most of the mass, any more than, say, Saturn's Rings. It's a distraction. Granted, black holes likely don't exist in isolation, and most or all of them will have an accretion disc to some extent, but they evidently *could*, if for example given enough time to absorb all the mass in their immediate neighbourhood.
I'm probably coming across as more of a black hole expert than I actually am, so to be clear I'd encourage you to read around this more in order to sanity-check my answers. But I'm confident at least in my assertion that you're getting too bogged down in what happens in or around the accretion disc, rather than the black hole itself, when trying to understand this.
It gets bigger. Although, in practice at least, probably not by much. Accretion discs contain relatively little mass compared to the overall black hole mass, so the gain in mass is quite slow -- possibly, for sufficiently small black holes, much slower than the mass lost from Hawking radiation and jet emission, etc.
// I agree that "no" would be the answer of an observer who had fallen in, but would you accept "yes" in answer to the question if it were preceded by "From the point of view of a distant observer"? //
Yes to which question? No to both of the ones you've asked, as far as I can tell. It's true that external observers can't "see into" the event horizon, but that doesn't mean that they'd see the black hole as hollow. Indeed, that would kind of mean being able to see into it to distinguish any internal structure or lack of it.
The fundamental point is that the accretion disc is not really part of the black hole itself, so much as a part of things "outside" it. It certainly isn't most of the mass, any more than, say, Saturn's Rings. It's a distraction. Granted, black holes likely don't exist in isolation, and most or all of them will have an accretion disc to some extent, but they evidently *could*, if for example given enough time to absorb all the mass in their immediate neighbourhood.
I'm probably coming across as more of a black hole expert than I actually am, so to be clear I'd encourage you to read around this more in order to sanity-check my answers. But I'm confident at least in my assertion that you're getting too bogged down in what happens in or around the accretion disc, rather than the black hole itself, when trying to understand this.
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