Janna Levin: disagreement

Transcript context

…So this is what catapulted Hawking’s fame. When he was a young researcher, he was thinking about black holes and wanted to just add a little smidge of quantum mechanics, just a little smidge. Wasn’t going for full-blown quantum gravity, but just asking, “Well, what if I allowed this nothing, this vacuum, this empty space around the event horizon, the stars gone, there’s nothing there, what if I allowed it to possess ordinary quantum properties, just a little tiny bit, nothing dramatic? Don’t go crazy.” And one of the properties of the vacuum that is intriguing is this idea that you can never see the vacuums actually completely empty. We talked about Heisenberg, the Heisenberg uncertainty principle really kicked off a lot of quantum mechanical thinking, it says that you can never exactly know a particle’s position simultaneously with its motion, with its momentum. You can know one or the other pretty precisely, but not both precisely. And the uncertainty isn’t a lack of ability that we’ll technologically overcome. It’s foundational. In some sense, when it’s in a precise location, it is fundamentally no longer in a precise motion. And that uncertainty principle means I can’t precisely say a particle is exactly here, but it also means I can’t say it’s not. And so it led to this idea that what do I mean by a vacuum? Because I can’t 100% precisely know. In fact, there’s not really meaningful to say that there’s zero particles here. And so what you can say however, is you can say, “Well, maybe particles froth around in this seething quantum sea of the vacuum. Maybe two particles come into existence and they’re entangled in such a way that they cancel out each other’s properties so they have the properties of the vacuum. They don’t destroy the properties of the vacuum. They cancel out each other, spin maybe, each other’s charge maybe, things like that, but they froth around. They come, they go, they come, they go.” And that’s what we really think is the best that empty space can do in a quantum mechanical universe. Now, if you add an event horizon, which, as we said, is really fundamentally what a black hole is, that’s the most important feature of a black hole. The event horizon, if the particles are created slightly on either side of that event horizon, now you have a real problem. Now the pair has been separated by this event horizon. Now, they can both fall in, that’s okay, but if one falls in and the other doesn’t, it’s stuck. It can’t go back into the vacuum because now it has a charge or it has a spin or it has something that’s no longer the property of that vacuum it came from, it needs its pair to disappear. Now it’s stuck. It exists. It’s like you’ve made it real. t has a charge or it has a spin or it has something that’s no longer the property of that vacuum it came from, it needs its pair to disappear. Now it’s stuck. It exists. It’s like you’ve made it real. So, in a sense, the black hole steals one of these virtual particles and forces the other to live. And if it’ll escape, radiate out to infinity and look like to an observer far away that the black hole is actually radiated a particle. Now, the particle did not emanate from inside. It came from the vacuum. It stole it from empty space, from the nothingness that is the black hole. Now, the reason why this is very tricky is because in the process, because of the separation on other side of the event horizon, the particle it absorbs, it has to do with the switching of space and time that we talked about, but the particle, it absorbs, well, from the outside you might say, “Oh, it had negative momentum. It was falling in from the inside.” You say, “Well, this is actually motion and time.” This is energy. If it has negative energy and it has absorbs negative energy, it’s mass goes down, black hole gets a little lighter. And as it continues to do this, the black hole really begins to evaporate. It does more than just radiate. It evaporates away. And it’s intriguing because Hawking said, “Look, this is going to look thermal, meaning featureless. It’s going to have no information in it. It’s going to be the most informationless possibility you could possibly come up with when you’re radiating particles. It’s just going to look like a thermal distribution of particles like a hot body. And the temperature is going to only tell you about the mass, which you could tell from outside the black hole anyway, you know the mass of the black hole from the outside. So it’s not telling you anything about the black hole. It’s got no information about the black hole. Now you have a real problem.” And when he first said it, not everyone understood how really naughty he was being. He did. But some people who love quantum mechanics were really annoyed, people like Lenny Susskind, Gerard’t Hooft, Nobel Prize winner, they were mad because it suggested something was fundamentally wrong with quantum mechanics if it was right. And the reason why it says there’s something fundamentally wrong with quantum mechanics is because quantum mechanics does not allow this. It does not allow quantum information to simply evaporate away and poof out of the universe and cease to exist. It’s a violation of something called unitarity, but really the idea is it’s the loss of quantum information that’s intolerable. Quantum mechanics was built to preserve information. It’s one of the sacred principles. As sacred as conservation of energy. really the idea is it’s the loss of quantum information that’s intolerable. Quantum mechanics was built to preserve information. It’s one of the sacred principles. As sacred as conservation of energy. In this example, more sacred, because you can violate conservation of energy with Heisenberg’s uncertainty principle a little tiny bit, but so sacred that it created what became coined as the black hole wars where people were saying, “Look, general relativity is wrong, something’s wrong with our thinking about the event horizon, or quantum mechanics isn’t what we think it is, but the two are not getting along anymore.” And just to tell you how dramatic it is, so the temperature goes down with the mass of the black hole, heavier black hole, the cooler it is. So we don’t see black holes evaporate, they’re way too big. But as they get smaller and smaller, they get hotter and hotter. So as a black hole nears the end of this cycle, of evaporating away, it takes a very long time, much longer than the age of the universe. It will be as though the current and the event horizon is yanked up, it’ll literally explode away, just boom. And the event horizon in principle would be yanked up, everything’s gone. All that information that went into the black hole, all that sacred quantum stuff gone. Poof. Because not in the radiation, because the radiation has no information. And so it was an incredibly productive debate because in it are the signs of what will make gravity and quantum mechanics play nice together, some quantum theory of gravity, whatever these clues are, and they’re hard to assemble, if you want a quantum gravity theory, it has to correctly predict the temperature of a black hole, the entropy of a black hole. It has to have all of these correct features. The black hole is the place on which we can test quantum gravity.…

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