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So, layman's question about black holes, almost 100% sci-fi derived. Starting from the time mechanics shown in, e.g., Interstellar. If when you're near a massi

by phn 5y ago

So, layman's question about black holes, almost 100% sci-fi derived. Starting from the time mechanics shown in, e.g., Interstellar. If when you're near a massive black hole time passes differently (more time passes away form the hole, so to speak), couldn't it be said that the regions near and far the black hole are drifting apart in the "time dimension"? If we take the black hole to be an extreme case of that, isn't the black hole a region that is drifting away so "fast" that light isn't fast enough to reach "us" on the outside? In that case, there would be no paradox, right? Whatever is inside the black hole is still there, but with no way to communicate.

philipov 5y ago
> isn't the black hole a region that is drifting away so "fast" that light isn't fast enough to reach "us" on the outside? I've seen some models of black holes that are similar to this. Specifically, what is happening in those models is that the space inside the event horizon is growing faster than the speed of light, so more space is created than light can traverse. This is the inverse of how cosmological horizons work. The reason we can only observe a limited portion of the universe is because objects are uniformly moving away from all other non-gravitationally bound objects. Space is being created between them. The farther you look, the faster galaxies are moving away from us because space is being created at every point in between. If you try to look far enough, the speed that objects are moving away from us becomes faster than the speed of light: space is being created faster than light can traverse it. This sort of faster-than-light travel doesn't break the relativistic speed limit because these objects aren't inertially accelerating inside their frame of reference, the frame of reference itself is expanding.
- hinkley 5y ago
  > space is being created faster than light can traverse it. Which is precisely the sort of behavior one might expect if they lived inside of a black hole.
  - alasdair_ 5y ago
    Isn’t this what is happening in our universe as well? Space is expanding faster than light can traverse it?
    - hinkley 5y ago
      I am at this point waiting to see someone prove that we aren't. Though ultimately I don't know if it matters. What we have is so vast that we can't really fathom it ending except in the abstract. No matter how big a ripple I make in the pond it's not going to matter in a couple billion years. I content myself by thinking in centuries.
defrex 5y ago
This is a reasonable conceptualization, IMO. However, the problem isn't that we can't access the information in a black hole (there are other places in the universe where information becomes inaccessible). The problem is that black holes evaporate. If the particles released via evaporation don't contain the information about the particles that entered, information is lost when the black hole is completely gone. The proposed solution is that the information is encoded onto the surface of the black hole and thus into the hawking radiation being released from that surface.
- eloff 5y ago
  This idea in physics that information is conserved, neither created nor destroyed, just transformed seems awfully similar to a computer to me. A classical computer is not the right metaphor really when you think of the universe as a possible computational process, but the parallels are striking to me.
  - macrolocal 5y ago
    Nb. wave function collapse messes with this, and a computer would use something like lazy evaluation to avoid generating the Everettian multiverse.
  - causasui 5y ago
    Physics layman, but I agree as a computer scientist. It also sometimes feels like there are "optimizations", e.g., delayed-choice quantum erasure (https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser) I'm open to the idea that it's just me projecting what I understand onto what I don't.
  - simonh 5y ago
    I suspect it may be just necessarily true that information is preserved in a consistent universe. I don't know though, maybe someone could come up with a model for a consistent universe with information loss, but it seems to me that would lead to physically possible states that are not derivable from consistent laws of physics.
macrolocal 5y ago
Well, objects falling into a black hole can reach the singularity in a finite amount of time. So you're going to have to enrich these singular spacetime points with a lot of extra structure if you want whatever passes through them to still exist. ¯\_(ツ)_/¯ To wit, you can imagine classical black holes as pulling whatever's near them into the future. The effect is so severe when you pass the event horizon that escaping the black hole amounts to traveling backwards in time. The effect is so severe when you reach the singularity that the entire timeline of the universe is in your past. So the singularity itself is more like an infinitely distant future than a point in space, with the caveat that the black hole slings you toward it with enough acceleration that you either actually reach it or something about this classical picture breaks down.
- saalweachter 5y ago
  I feel like there's something so scary about falling into a black hole, literally unable to escape, that we just really, really want it to be "survivable", somehow. Which is kind of funny when you consider that no one would expect to survive falling into a star, but we don't grasp at straws the same way to say, "Oh, you wouldn't actually be immolated, the solar wind would blow you back into space first."
  - macrolocal 5y ago
    To be fair, virtually any place in the universe, except where you happen to be right now, is probably not survivable.
marcyb5st 5y ago
The problem is that black hole evaporates due to Hawking radiation, which is a special case of Unruh radiation. This radiation is independent of what falls into the black hole, but just its size (and hence mass). This is the paradox. Two black holes of the same mass can be created with completely different matter and they would radiate exactly the same way and in so doing destroying the quantum information of matter they are made of. Your point of view/theory would hold if black holes were eternal, but they probably are not if our understanding of physics is correct. In fact, if black holes "die" then the quantum information has to be released back into the universe somehow. This paper proposes a mechanism for that to happen.
- kstrauser 5y ago
  Why does that information have to be released? To my naive layman's thinking, if you told me that a black hole permanently destroys that information, I'd think "sure, it destroys lots of other things, so that makes sense". What problem does it cause if we believe that the information is gone forever?
  - naasking 5y ago
    All information is physical. The information you're reading right now could conceivably be stored in electron charge, or spin, or polarization on optical storage, etc. Information simply must have some physical form. Accepting the destruction of any information means that none of the physical symmetries we observe actually hold up, like conservation of momentum and energy. That would change literally everything.
  - hollerith 5y ago
    The problem is that black-hole evaporation is a high-level description of many "fundamental" events, and at the level of fundamental physics, there is no known process that destroys information. Or so popular-science articles tell me.
  - amluto 5y ago
    Because quantum mechanics _really_ does not like destroying information. Mangling information beyond recognition is just fine, but the laws of quantum mechanics are very insistent that, if you have a complete description of the state of the universe, you can solve the equations backwards and figure out what happened in the last. When you throw in a black hole following Hawking’s rules, or any other device that irretrievably chews things up and spits them out in a way that can’t, even in theory, be undone, quantum mechanics breaks.

by phn 5y ago

philipov 5y ago

> isn't the black hole a region that is drifting away so "fast" that light isn't fast enough to reach "us" on the outside? I've seen some models of black holes that are similar to this. Specifically, what is happening in those models is that the space inside the event horizon is growing faster than the speed of light, so more space is created than light can traverse. This is the inverse of how cosmological horizons work. The reason we can only observe a limited portion of the universe is because objects are uniformly moving away from all other non-gravitationally bound objects. Space is being created between them. The farther you look, the faster galaxies are moving away from us because space is being created at every point in between. If you try to look far enough, the speed that objects are moving away from us becomes faster than the speed of light: space is being created faster than light can traverse it. This sort of faster-than-light travel doesn't break the relativistic speed limit because these objects aren't inertially accelerating inside their frame of reference, the frame of reference itself is expanding.

hinkley 5y ago
> space is being created faster than light can traverse it. Which is precisely the sort of behavior one might expect if they lived inside of a black hole.
- alasdair_ 5y ago
  Isn’t this what is happening in our universe as well? Space is expanding faster than light can traverse it?
  - hinkley 5y ago
    I am at this point waiting to see someone prove that we aren't. Though ultimately I don't know if it matters. What we have is so vast that we can't really fathom it ending except in the abstract. No matter how big a ripple I make in the pond it's not going to matter in a couple billion years. I content myself by thinking in centuries.

defrex 5y ago

This is a reasonable conceptualization, IMO. However, the problem isn't that we can't access the information in a black hole (there are other places in the universe where information becomes inaccessible). The problem is that black holes evaporate. If the particles released via evaporation don't contain the information about the particles that entered, information is lost when the black hole is completely gone. The proposed solution is that the information is encoded onto the surface of the black hole and thus into the hawking radiation being released from that surface.

eloff 5y ago
This idea in physics that information is conserved, neither created nor destroyed, just transformed seems awfully similar to a computer to me. A classical computer is not the right metaphor really when you think of the universe as a possible computational process, but the parallels are striking to me.
- macrolocal 5y ago
  Nb. wave function collapse messes with this, and a computer would use something like lazy evaluation to avoid generating the Everettian multiverse.
- causasui 5y ago
  Physics layman, but I agree as a computer scientist. It also sometimes feels like there are "optimizations", e.g., delayed-choice quantum erasure (https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser) I'm open to the idea that it's just me projecting what I understand onto what I don't.
- simonh 5y ago
  I suspect it may be just necessarily true that information is preserved in a consistent universe. I don't know though, maybe someone could come up with a model for a consistent universe with information loss, but it seems to me that would lead to physically possible states that are not derivable from consistent laws of physics.

macrolocal 5y ago

Well, objects falling into a black hole can reach the singularity in a finite amount of time. So you're going to have to enrich these singular spacetime points with a lot of extra structure if you want whatever passes through them to still exist. ¯\_(ツ)_/¯ To wit, you can imagine classical black holes as pulling whatever's near them into the future. The effect is so severe when you pass the event horizon that escaping the black hole amounts to traveling backwards in time. The effect is so severe when you reach the singularity that the entire timeline of the universe is in your past. So the singularity itself is more like an infinitely distant future than a point in space, with the caveat that the black hole slings you toward it with enough acceleration that you either actually reach it or something about this classical picture breaks down.

saalweachter 5y ago
I feel like there's something so scary about falling into a black hole, literally unable to escape, that we just really, really want it to be "survivable", somehow. Which is kind of funny when you consider that no one would expect to survive falling into a star, but we don't grasp at straws the same way to say, "Oh, you wouldn't actually be immolated, the solar wind would blow you back into space first."
- macrolocal 5y ago
  To be fair, virtually any place in the universe, except where you happen to be right now, is probably not survivable.

marcyb5st 5y ago

The problem is that black hole evaporates due to Hawking radiation, which is a special case of Unruh radiation. This radiation is independent of what falls into the black hole, but just its size (and hence mass). This is the paradox. Two black holes of the same mass can be created with completely different matter and they would radiate exactly the same way and in so doing destroying the quantum information of matter they are made of. Your point of view/theory would hold if black holes were eternal, but they probably are not if our understanding of physics is correct. In fact, if black holes "die" then the quantum information has to be released back into the universe somehow. This paper proposes a mechanism for that to happen.

kstrauser 5y ago
Why does that information have to be released? To my naive layman's thinking, if you told me that a black hole permanently destroys that information, I'd think "sure, it destroys lots of other things, so that makes sense". What problem does it cause if we believe that the information is gone forever?
- naasking 5y ago
  All information is physical. The information you're reading right now could conceivably be stored in electron charge, or spin, or polarization on optical storage, etc. Information simply must have some physical form. Accepting the destruction of any information means that none of the physical symmetries we observe actually hold up, like conservation of momentum and energy. That would change literally everything.
- hollerith 5y ago
  The problem is that black-hole evaporation is a high-level description of many "fundamental" events, and at the level of fundamental physics, there is no known process that destroys information. Or so popular-science articles tell me.
- amluto 5y ago
  Because quantum mechanics _really_ does not like destroying information. Mangling information beyond recognition is just fine, but the laws of quantum mechanics are very insistent that, if you have a complete description of the state of the universe, you can solve the equations backwards and figure out what happened in the last. When you throw in a black hole following Hawking’s rules, or any other device that irretrievably chews things up and spits them out in a way that can’t, even in theory, be undone, quantum mechanics breaks.