Does Anything Ever Come Out of a Black Hole?
Netta Engelhardt
Yesterday Evening (25-June-2025) Netta Engelhardt gave a lecture (which you can watch on YouTube) at the Perimeter Institute entitled Does Anything Ever Come Out of a Black Hole?” and I was there. So also was Leonard Susskind, sitting two rows in front of me. This was a good sign. I go to most of the PI public lectures. They vary a great deal in quality. They are intended for the general public. Sometimes they’re not even about physics. For instance I recently attended one in which a journalist interviewed Patchen Barrs about his biography of Roger Penrose. Even when the speaker is a physicist or mathematician, they are often dumbed down to the point of worthlessness. I was happy to see Susskind because his presence indicated that he, at least, though her lecture would not waste the time of an actual physicist.
It did not. Engelhardt nicely squared the circle of simplifying the physics of General Relativity and Quantum Mechanics down to a very basic level that almost anyone could follow without losing the essence of the problem.
What is the problem, the “Black Hole Information Paradox”? General Relativity says that when mass is sufficiently compressed it can collapse under its own gravity to form an object whose gravity is so strong that not even light can escape. If light can’t escape, nothing can escape, according to classical physics. (“Classical” in this context means “non-quantum.”) This,object, of course, is called a black hole. There’s a theorem in gravitation that says that when you do that, all information about the initial state disappears invisibly inside the black hole—it’s called the no-hair theorem (yes, really). Hawking, Bekenstein, and others showed that when you add quantum mechanics, stuff will come out of a black hole. In fact, stuff will come out until the black hole evaporates.
This raises the question, “What happens to any information that fell into the black hole?” Is it lost forever? The answer appeared to be “Yes.” This was a big problem, because quantum mechanics says that information can never be lost. (Formally, the evolution of any quantum system is a unitary process.) In fact, even classical physics says that, though less emphatically.
Engelhardt and co-workers showed that when a black hole forms and evaporates, all the information that went into the black hole comes back out. It comes out in an extremely mangled form, such that it is not practically possible to reconstruct the original information from the evaporate. But the information is there.
It was a good talk. 2025 is the 25th anniversary of the founding of the Perimeter Institute. They’ve been having a lot of conferences and symposia to celebrate. Engelhardt (probably, Susskind, as well) was in town for the “Quantum Information in Quantum Gravity” conference. These conferences bring a lot of really good physicists to town, and I’m glad Perimeter convinces some of them to give public lecture.