Adam Brown: prediction

Transcript context

…Tell me about it. Okay, so what do we mean by "mine a black hole?" Mining a black hole means taking energy out of a black hole that used to be in a black hole. Obviously, if our distant descendants have used up all of the energy in stars and everything else, the black hole might be the last thing they turn their eye to. Can you get energy out of black holes at all? The old story, pre-1970s, is no. A black hole is one way: matter falls in, it never comes out, it's stuck. The thing that Hawking and Bekenstein discovered in the 70s is that once quantum mechanics is involved, that's not true anymore. Once quantum mechanics is involved, in fact, energy, even without you doing anything, starts to leave black holes. The problem, as far as our distant descendants will be concerned, is that it leaves black holes extremely slowly. So if you took a solar mass black hole, same mass as the sun, just collapsed to form a black hole, there'll be this little quantum, what's called Hawking radiation nowadays, little quantum Hawking radiation in which the energy will leach out again very, very slowly. The temperature of a solar mass black hole is measured in nanokelvins, a very low temperature. So the energy leeches out when something that cold, so cold you couldn't even see it in the cosmic microwave background, it leeches out incredibly slowly back into the universe. And that's bad news because it means the energy comes out super duper slowly. So the mining question is, can you speed that up? A solar mass black hole, if you don't help it, will take about 10 to the 55 times the current age of the universe to have given out all its energy back into the universe. Can you make that faster? There were these proposals stretching back a few decades that you could do what's called mining black holes, where we see the Hawking radiation that escapes when we are a very long way away from the black hole. But actually, mathematically, it's known that much of the Hawking radiation doesn't escape. It just sort of makes it a little bit out of the black hole and then falls back in again. And there was this proposal that you could kind of reach in with a mechanical claw, obviously not crossing the horizon, because otherwise, you've lost the claw and you're somewhat counterproductive, but just outside the horizon, just grab some of that Hawking radiation and just drag it a long way away from the black hole and then feast on it or do whatever it is you want to do with it. In that way, you could mine a black hole. You could speed up the evaporation of a black hole by a huge factor. So in fact, the lifetime would no longer go like the mass cubed, like it does with just unaided Hawking radiation, but would scale like just the mass, so considerably faster for a large black hole. So this was these proposals and what I had a somewhat pessimistic contribution to the story, which is that the existing proposals did not work. They didn't work to speed it up. And in fact, you can't speed it up. these proposals and what I had a somewhat pessimistic contribution to the story, which is that the existing proposals did not work. They didn't work to speed it up. And in fact, you can't speed it up. You can't get down that M cubed down to M. You can't, in fact, get it anything less than M cubed. It still scales like the mass cubed. The length of time you need to wait to get all the energy out of a black hole still scales like the mass cubed. And what goes wrong is ultimately a material science problem. So this scoop that comes down really close to the horizon, now, from one point of view, that's just like a space elevator, albeit a very high-performance space elevator. Space elevators, you'll remember, are these ideas for how we might get things off the surface of the Earth without using rockets. The idea is that you have some massive orbiting object sort of very long way away, beyond geostationary orbit, and then you dangle off that a rope down to the surface of the Earth, and then you can essentially just climb up the rope to get out. That's the space elevator idea. And already around Earth, it's hitting pretty hard material science constraints. So if you want to make a space elevator, the trouble with making a space elevator isn't so much supporting the payload that you're trying to have climb up. It is merely just the rope supporting its own weight because each bit of the rope needs to support not only its own weight but also the weight of all of the rope beneath it. So the tension that you require keeps getting more and more and more as you go up. At the bottom, there is no tension effect. It doesn't even touch the Earth. It's not like a compression structure that's like a skyscraper that's pushed up from below. It's a tension structure that's held up from above. But as you go up, because you need more and more tension, you also need to make the rope thicker and thicker and thicker. And if you try and on Earth or around Earth, build a space elevator out of steel, say, it just doesn't work. Steel is not strong enough. You need to keep doubling the thickness until, by the time you get to geostationary orbit, the thickness of the steel rope is more than the size of the Earth. Like, the whole thing just doesn't work at all. But carbon nanotubes are this material that we discovered that are much stronger than steel. So, in fact, around Earth, carbon nanotubes will just about work. If we can make them long enough and pure enough, then they will be strong enough that we will be able to build a space elevator around Earth in, you know, maybe sometime in the next century, that you only need a couple of doublings of the thickness of the carbon nanotubes along its entire length. So carbon nanotubes work great around Earth, but they are totally inadequate for black holes. For black holes, the critical material science property you need for this rope is the tensile strength to mass per unit length ratio. It needs to be strong, high tensile strength, but low weight, light, low mass per unit length. And that's the critical ratio. And carbon nanotubes is 10 to the minus 12 or something on that scale. And that is simply not strong enough at all.…

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