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I still don't understand what they mean by "loss". Why is this even a problem? The information isn't gone, its right -there- in the black hole, which until prov

by xevrem 6y ago

I still don't understand what they mean by "loss". Why is this even a problem? The information isn't gone, its right -there- in the black hole, which until proven otherwise, is part of the universe. Until someone can prove the universe cares whether the info is in a black hole or not, its not really a problem is it? If anything the universe usually shows it doesn't care what we humans think, its going to do its own thing, regardless: i.e., weak nuclear force and "symmetries"

whimsicalism 6y ago
But then the black hole evaporates...
gizmo686 6y ago
The black hole is not eternal. Once it is fully evaporated, you still need to account for the information that was contained within (or accept information loss).
- seppel 6y ago
  > Once it is fully evaporated, you still need to account for the information that was contained within (or accept information loss). One way to escape this is: * You accept that the Hawkin radiation contains the original information. * But it is scrambled in a reversible way, but so hard that you cannot reverse it with the energie available in the universe. There are nice talk by Scott Aaronson about this, e.g.: https://simons.berkeley.edu/events/theoretically-speaking-series-scott-aaronson https://simons.berkeley.edu/events/theoretically-speaking-se...
  - lokedhs 6y ago
    But Hawking radiation is black-body radiation. Isn't it just photons where its frequency is proportional to the black hole mass? Where would the information be encoded?
cli 6y ago
I believe this was addressed in the first few paragraphs of the article: this problem is not about 'information', which is a vague phrase. Rather, it is about how black hole evaporation is fundamentally time irreversible.
- xevrem 6y ago
  but wasn't the evaporation of black holes a suggested solution by hawking for information being "lost forever" in a black hole? i.e., hawking radiation is itself unconfirmed, so its a "solution" for something that remains unproven :|
  - ssivark 6y ago
    Right, but that is using a semi-classical calculation, whereas we know that ultimately any process (evolution of a closed system like the universe) compatible with quantum mechanics needs to be unitary/reversible. That mismatch is what sets up the BH information “paradox”.
pdonis 6y ago
> The information isn't gone, its right -there- in the black hole No, it isn't; it hits the singularity inside of the hole and gets destroyed. At least, that's what Hawking's original model, the one he used to predict that black holes evaporate, says. One way of seeing why Hawking's model had to say this is to combine the following facts about the evaporating black hole and the Hawking radiation in Hawking's model: (1) The hole itself cannot contain any information other than its mass, charge, and spin (because of the "black holes have no hair" theorem), which is far too little information to describe everything that fell into the hole. (2) The Hawking radiation cannot contain any information about what fell into the hole because it is thermal, black-body radiation, i.e., the only information it contains is its temperature, which is related to the mass of the hole. So the information can't be stored either inside the hole or outside the hole, which means it must be destroyed, and the only place it can be destroyed is by hitting the singularity inside the hole. The black hole information loss problem is that the above is inconsistent with quantum unitarity. So Hawking's original model can't be right; but nobody knows what model should replace it.
- effie 6y ago
  > The hole itself cannot contain any information other than its mass, charge, and spin... which is far too little information Maybe the information gets encoded in digits of value of mass expressed in some unit. There is enough digits to store any finite number of bits.
  - SmooL 6y ago
    Sure, and the point of this video is that while that may be _mathematically and theoretically_ sound, there's no way you can realistically make any measurements or any observations to confirm or deny your particular idea. What we have a lot of these ideas, with no way to discern between theories which accurately represent nature and theories which are merely mathematically correct.
    - pdonis 6y ago
      > while that may be _mathematically and theoretically_ sound The particular idea suggested in the GP actually isn't. See my post upthread.
pa7x1 6y ago
All of Quantum Mechanics respect unitary evolution, unitary evolution can be rewinded back. Black hole evaporation breaks unitary evolution, at least in the semi-classical approximation of Hawking. If you prepare a pure quantum state it will come out as mangled thermal radiation. You cannot return to the pure quantum state from the thermal radiation (i.e. we have lost information about it). This transition is impossible in Quantum Mechanics and it would suppose a killing blow to Quantum Mechanics if true. So a better way to rephrase our worries is that if Black Holes do not respect unitary evolution then our most precise physical theory is fundamentally wrong.

by xevrem 6y ago

whimsicalism 6y ago

But then the black hole evaporates...

gizmo686 6y ago

The black hole is not eternal. Once it is fully evaporated, you still need to account for the information that was contained within (or accept information loss).

seppel 6y ago
> Once it is fully evaporated, you still need to account for the information that was contained within (or accept information loss). One way to escape this is: * You accept that the Hawkin radiation contains the original information. * But it is scrambled in a reversible way, but so hard that you cannot reverse it with the energie available in the universe. There are nice talk by Scott Aaronson about this, e.g.: https://simons.berkeley.edu/events/theoretically-speaking-series-scott-aaronson https://simons.berkeley.edu/events/theoretically-speaking-se...
- lokedhs 6y ago
  But Hawking radiation is black-body radiation. Isn't it just photons where its frequency is proportional to the black hole mass? Where would the information be encoded?

cli 6y ago

I believe this was addressed in the first few paragraphs of the article: this problem is not about 'information', which is a vague phrase. Rather, it is about how black hole evaporation is fundamentally time irreversible.

xevrem 6y ago
but wasn't the evaporation of black holes a suggested solution by hawking for information being "lost forever" in a black hole? i.e., hawking radiation is itself unconfirmed, so its a "solution" for something that remains unproven :|
- ssivark 6y ago
  Right, but that is using a semi-classical calculation, whereas we know that ultimately any process (evolution of a closed system like the universe) compatible with quantum mechanics needs to be unitary/reversible. That mismatch is what sets up the BH information “paradox”.

pdonis 6y ago

> The information isn't gone, its right -there- in the black hole No, it isn't; it hits the singularity inside of the hole and gets destroyed. At least, that's what Hawking's original model, the one he used to predict that black holes evaporate, says. One way of seeing why Hawking's model had to say this is to combine the following facts about the evaporating black hole and the Hawking radiation in Hawking's model: (1) The hole itself cannot contain any information other than its mass, charge, and spin (because of the "black holes have no hair" theorem), which is far too little information to describe everything that fell into the hole. (2) The Hawking radiation cannot contain any information about what fell into the hole because it is thermal, black-body radiation, i.e., the only information it contains is its temperature, which is related to the mass of the hole. So the information can't be stored either inside the hole or outside the hole, which means it must be destroyed, and the only place it can be destroyed is by hitting the singularity inside the hole. The black hole information loss problem is that the above is inconsistent with quantum unitarity. So Hawking's original model can't be right; but nobody knows what model should replace it.

effie 6y ago
> The hole itself cannot contain any information other than its mass, charge, and spin... which is far too little information Maybe the information gets encoded in digits of value of mass expressed in some unit. There is enough digits to store any finite number of bits.
- SmooL 6y ago
  Sure, and the point of this video is that while that may be _mathematically and theoretically_ sound, there's no way you can realistically make any measurements or any observations to confirm or deny your particular idea. What we have a lot of these ideas, with no way to discern between theories which accurately represent nature and theories which are merely mathematically correct.
  - pdonis 6y ago
    > while that may be _mathematically and theoretically_ sound The particular idea suggested in the GP actually isn't. See my post upthread.

pa7x1 6y ago

All of Quantum Mechanics respect unitary evolution, unitary evolution can be rewinded back. Black hole evaporation breaks unitary evolution, at least in the semi-classical approximation of Hawking. If you prepare a pure quantum state it will come out as mangled thermal radiation. You cannot return to the pure quantum state from the thermal radiation (i.e. we have lost information about it). This transition is impossible in Quantum Mechanics and it would suppose a killing blow to Quantum Mechanics if true. So a better way to rephrase our worries is that if Black Holes do not respect unitary evolution then our most precise physical theory is fundamentally wrong.