Adam Brown: observation

Transcript context

…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. gth, 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. In fact, what I showed in my paper is that you need a tensile strength to weight ratio that is as strong as is consistent with the laws of nature. So, in fact, the laws of nature bound this quantity. The finiteness of the speed of light means you cannot have an arbitrarily strong rope with a given mass per unit length. There is a bound set by the C squared in some units that bounds the maximum possible tensile strength that any rope can have. Any rope, in fact, that has that, or an example of a rope that has that, is a string. So a string is, I mean, a fundamental string from string theory is an example of a hypothetical rope that is just strong enough to saturate that bound, that strength bound. And then the problem is the following. The problem is that if you have a rope that saturates the bound as strong as any rope can be, it is just strong enough to support all of its own weight exactly on the edge there, with exactly no strength left over to support any payload it might wish to carry. And that's ultimately what dooms these mining black holes, you know, these rapid mining black hole proposals. And what happens if you try to make the rope stronger?…

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