Black holes and academic walls

Original author: Sabine Hossenfelder
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It used to be easy to fall into a black hole: you would not have noticed anything. But can a wall come your way?

According to Einstein, you would not have noticed the moment of the intersection of the black hole event horizon. But now researchers are proving that a fiery wall (or brick) may appear on your way. Did they all go crazy at once?



Tl; dr: Yeah.

Sometimes it is very difficult to understand why someone will waste time on solving such an academic task as the loss of information in a black hole. And I say this, having spent a substantial part of the last decade in thinking about black holes. Unless physicists have nothing more to do in the world, suffering from wars and diseases, or at least grammatical errors? What moves these researchers apart from hoping to get into the headlines thanks to the solution of a 40-year puzzle?



Many physicists in our time are working on topics such as the loss of information in black holes, which seem to be divorced from reality. Black holes will be able to destroy information after complete evaporation, and this will not happen in the next 100 billion years. These scientists are driven not by the practical application of their ideas, but by the realization that today someone must pave the way for science, which will become necessary in a hundred, thousand, or ten thousand years. And reading the news headlines describing the current mess, I think that the unearthly purity of this argument and the inevitable logic that leads to a paradox only adds weight to it.

And I can be carried away with wire puzzles.



If the loss of information in black holes is a cosmic detective, then quantum theory is a victim. Stephen Hawking showed in the early 1970s that the combination of quantum theory with gravity gives us the thermal radiation of black holes. This “Hawking radiation” consists of particles that contain no information other than temperature. And when the black hole completely evaporates, all the information about what has fallen into it is destroyed. But such a destruction of information is incompatible with the quantum theory itself, used to prove this conclusion. In quantum theory, all processes are reversible in time, but the evaporation of a black hole is, apparently, irreversible.

And this is a serious puzzle for physicists, since it shows that gravity and quantum theory do not want to be combined. To explain the problem, appealing to the unknown theory of quantum gravity, also failed. Hawking radiation is not a quantum gravitational process, and although quantum gravity at some point becomes important in the final stages of black hole evaporation, it is argued that by that time it was too late to extract all the information.



The situation changed dramatically by the end of the 1990s, when Maldacena suggested that certain theories of gravity are equivalent to gauge theories. This famous “correspondence between calibration and gravity” was discovered in string theory, and although it has not been mathematically proven, it eliminates the problem of information loss in a black hole, because whatever happens during the evaporation of a black hole, this is equivalently described in the gauge theory. And the gauge theory does not kill information, and therefore there is no problem either.

And although the conformity of calibration and gravity convinced many, including Stephen Hawking himself, that black holes do not destroy information, it did not shed light on how information escapes from a black hole. Research continued, but complacency spread among theoretical physicists. String theory seems to have resolved the paradox, and it remains only a matter of time when we understand its details.



But everything went wrong. Instead, in 2012, a group of physicists, Almgeiri, Marolf, Polchinski and Sally (Almheiri, Marolf, Polchinski, Sully - AMPS) showed that this “solution” was inconsistent. They showed that four assumptions, in which most string theorists believe, cannot be fulfilled simultaneously. These assumptions are:

  1. Black holes do not destroy information.
  2. The standard model of particle physics and GR work when approaching the event horizon.
  3. The amount of information stored in a black hole is proportional to its surface area.
  4. An observer crossing the event horizon will not notice this.

The second assumption rephrases the statement that Hawking radiation is not a quantum gravity effect. The third is the derivation from calculations of the black hole microstates in string theory. The fourth is the Einstein equivalence principle. In short, AMPS say that one of these assumptions is incorrect. One of the witnesses is lying, but who exactly?

In their work, the AMPS suggested, perhaps not very seriously, to reject the least challenged assumption: the fourth. If you abandon the number 4, the other three will lead to the fact that an observer falling into a black hole, stumbles upon the "wall of fire" and burns to hell. However, the equivalence principle is a dogma of GR, and it can only be discarded as a last resort.



To the uninitiated observer, it will seem obvious that the 3rd witness is lying. Unlike other assumptions that follow from already known and with great accuracy proven theories, number 3 follows from unverified. So if we throw away one of the assumptions, then perhaps it will be an assumption that the string theory is right about the information content of black holes. But this opportunity, of course, is not very popular among string theorists.

Less than a few months, as a section of theoretical high energy physics site arxiv was crammed with attempts to reconcile these assumptions. Absolutely all solutions were proposed, from adopting the theory of the fiery wall to the multiverse and complex mental experiments, demonstrating that an observer falling into a black hole would not notice that he was going to burn. Yes, that’s modern physics.

I also contributed to this discussion. I found all the witnesses convincing, none of them gave the impression of a liar. By accepting this, I realized that their apparent incompatibility came from the fifth, unvoiced assumption used in the proof. Just as, seemingly contradictory testimony may suddenly make sense when you realize that the victim was killed not where they found the body, so these four statements make sense when you do not need to save the information in a certain way (such that its final the condition will not be “typical”). Instead, the requirement of local energy conservation near the event horizon makes the fiery wall impossible, and at the same time shows how exactly the black hole evaporation remains compatible with quantum theory.



It seems to me that nobody liked my work. Perhaps due to the fact that it has an incomprehensible picture. Or because of the conclusion that somewhere near the event horizon, there is a boundary that changes quantum theory, moreover, so that it is invisible to any observer who is near a black hole. The effects produced by this boundary can be measured, but only at a great distance.

And although my guess was solving a puzzle with a fiery wall, it did not solve the problem of losing information in a black hole. I mentioned in a note that, in principle, it is possible to use this boundary to transmit information to outgoing radiation, but this would still not explain how the information will even reach this boundary.

After the publication of the work, I vowed again not to think about the evaporation of the black hole. But last week a preprint from 't Hooft appeared in arxiv. This is one of the first people to fiddle with the thermodynamics of the black hole, and in his new work, 't Hooft assumes that the black hole event horizon works like a border, reflecting information - a “brick wall”, as described by New Scientist. The idea was inspired by Stephen Hawking's recent suggestion that much of the information falling into a black hole remains on the horizon. If so, and you can give the horizon a chance to work, then the information will once again be able to leave the black hole.

Of course, I do not consider bricks a great improvement over fire, and I am sure that this idea will not take off. But after all this confusion, it can help us better understand how exactly the horizon interacts with Hawking radiation and how it manages to encode information in it.

Fast forward a thousand years. There we are waiting for the theory of quantum gravity, which allows us to understand the behavior of space and time at the shortest distances, and, as many hope, the source of the quantum theory itself, or even matter. Progress comes in short steps, and sometimes history leads us in circles, but physicists support the knowledge that a solution must exist, and that the killer of information will be caught. There is no trick in reading the last page of the book - it has not been written yet.

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