Adam Brown: belief

Transcript context

…to the sun’s surface, this would be larger. It’d be a million times larger, because the sun is a few million times the mass of the Earth, but then it’s also bigger, so that takes it away a little bit. You end up with 2x10-6, the famous redshift from the sun’s surface. You can escalate from there. You can imagine cramming a sun-like mass into an Earth-like radius to make this formula even bigger. Sun mass, Earth radius. That’s pretty much exactly what happens in a white dwarf like Sirius B. And this would get even bigger again. A larger fraction of the mass of the object you’d be extracting by lowering it down to the surface. But it really feels like something has to give before we make an object that is too massive and too compact. In particular, if you look at this formula, what happens for r less than or equal to GM over c2? If this object were so compact and so heavy that it had a radius less than the mass of the object divided by c2, it sure looks like you could get more than a hundred percent. The fraction would be bigger than one. You could get more than a hundred percent of the mass of your brick back by lowering it down to the surface of this object. And that feels wrong. That feels, in fact, more wrong than what’s going on here, because now you’ve got all this energy a long way away. You could perhaps use it to make a whole new brick. You’ve got all this more than mc2 out there. Lower that one down, and it feels like we’ve figured out a way to make a huge amount of energy where there was no energy before. This argument is pretty suggestive that something has to go wrong by the time you get down to that radius. Indeed, when you do the calculation—this is a Newtonian calculation, so it’s only suggestive —in full general relativity, indeed something does go wrong. The thing that goes wrong is that you form a black hole. You can imagine two ways that you could avoid this conclusion. One would be somehow that gravity becomes very weak when you get close to a massive object, weaker than the Newtonian law would predict. That’s sort of what saves you if you try and repeat this same trick in electromagnetism, lowering a charge down towards another charge and trying to extract the electrostatic energy between them. What happens is, essentially due to quantum effects, when one gets too close to the other, they start to fuzz out. The energy going like inverse r gets softened, and you can’t extract more energy because they stop attracting each other so hard. So that’s one possibility, the force gets weaker than Newtonian law would predict as you approach the other object. That’s actually the opposite of how general relativity resolves this. General relativity resolves this paradox by the force getting stronger than Newtonian law would predict. In particular, the force gets so strong when you try to get within this radius, that in fact you cannot slowly lower the brick down towards the surface because you’ve formed a black hole. The gravitational force becomes infinite at a finite distance away—not at r=0, but at some finite value of r—and the brick simply gets ripped out of your hand and you’re unable to extract any more energy out of it. nal force becomes infinite at a finite distance away—not at r=0, but at some finite value of r—and the brick simply gets ripped out of your hand and you’re unable to extract any more energy out of it. That’s the resolution that general relativity provides to this paradox. In particular, you will find that you’ve formed a black hole. So far, everything we’ve written down on the board is Newtonian. It’s just Newtonian, and you start plugging in the speed of light, and you start getting confused. To actually answer some of these questions that we’re asking, you need to go to general relativity, the theory that correctly unifies the speed of light with gravity. This was first done in the context of black holes by Schwarzschild, who wrote down the Schwarzschild metric that describes the gravitational field around a central mass, including potentially around a black hole. Let me write down some of the formulas that emerge. In fact, I think I’m going to write down three formulas, three direct consequences of the Schwarzschild metric. They’re going to give us intuition for what it’s like outside and indeed inside a black hole. The first formula I’m going to write down is the formula for the gravitational field that you would experience if you were trying to remain static outside a central mass. So let’s just talk about static observers. I can discuss how these will get upgraded for observers who are moving around. But for now, I’m just going to imagine that you’re trying to sit here at some fixed radius r away from the black hole. The reason you don’t fall in, maybe I’m lowering you down on a pulley. You’re just sitting here holding the pulley. The question is, how strong a force do you need to stop you falling down? You’re abseiling down very slowly. You’re static. What is the local force of gravity that you experience? Or you can imagine that you’re sitting here, and the reason you’re static is that you’re firing a rocket very hard. The question is, how much acceleration do you locally feel? So by whatever mechanism, you’re remaining static. What is the local force of gravity that you feel? In Newtonian physics, you know what the answer to that question would be. The force of gravity is GM/r2, which is Newton’s famous inverse-square law. But this gets a correction from general relativity. The correction is 1/√(1-2GM/(c2r)), this same 2GM/c2 that we find all over the place. What this tells you: first of all, if you’re a very long way away from the black hole, this here is essentially one. r is very big, and you get Newton’s force law back again. For the Earth, this is very small. As we discussed, it’s down by a factor of 10-10, and then you take the square root. So you don’t really notice it, but you can Taylor-expand this at large r, and you find that you get corrections. You get an inverse-square law plus an inverse-cube law correction plus an inverse-fourth law correction. You find that gravity at short distances is stronger than it would have been in Newtonian physics. This is the general relativity correction and it’s making the gravitational field stronger. You have to accelerate harder to not fall into the black hole. than it would have been in Newtonian physics. This is the general relativity correction and it’s making the gravitational field stronger. You have to accelerate harder to not fall into the black hole. In particular, once r is equal to 2GM/c2, what’s called the Schwarzschild radius, you have to accelerate infinitely. The proper acceleration required to not move in r goes to infinity. In fact, if we now convert this Earth to a black hole, this is a very significant radius over here, 2GM/c2. It’s called the event horizon. It’s called the event horizon because if you want to remain static outside the event horizon, further away from the event horizon, you just need to accelerate with some finite velocity in order to remain static. You need to have a finite gravitational field. But the gravitational field, as you approach the event horizon, becomes infinite. So once you’re at or beyond the event horizon, it is impossible to remain static. You will inevitably get sucked into the black hole no matter how hard you fire your rocket. Now, this is just a static formula. You might imagine, “Okay, it’s impossible to remain static closer than that, but maybe I could avoid falling into the black hole by orbiting really, really fast. If I orbit really fast, I have a huge centrifugal force that pushes me away from the black hole, and I can stay out of the black hole that way.” That actually doesn’t work. The reason it doesn’t work is somewhat instructive for the way gravitational attraction happens in general relativity. Of course, if you think about the International Space Station, why doesn’t it fall towards the Earth? It is precisely the fact that it’s orbiting. The fact that it’s orbiting gives it a centrifugal force that shoots the astronauts away from the Earth and precisely balances the gravitational field on the astronauts, which is why they feel weightless there. So orbital angular momentum, if you’re a long way away from the black hole, helps you stay away from the black hole, stops you falling in. There is this kind of sci-fi notion that black holes just suck in everything around them. Not true. You are perfectly able to orbit around a black hole if you’re a long way away from it, just like you would orbit around any central mass. You are not inevitably falling into the black hole. You can orbit just fine. But orbiting stops helping when you get too close to the black hole. We said that the event horizon is 2GM/c2. In fact, once you’re already within 3GM/c2, orbiting is counterproductive if you’re trying to stay away from the black hole. That’s because there are two effects of orbiting. One effect helps you stay away from the black hole. That’s the centrifugal effect. Orbital angular momentum pushes you away from the black hole due to the centrifugal effect. It’s not too hard to write down the version of this formula that applies when you have angular momentum, and you would see that pushing you away from the black hole. But there’s another effect which drags you towards the black hole, and that is the fact that in general relativity, all energy gravitates, not just rest mass energy. Kinetic energy also gravitates.…

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