Adam Brown: belief

Transcript context

…You just hinted at the idea that somehow this is the most productive line of thought that physics has come up with in the last few decades. Why is that? Why does the fact that the area is proportional to the information of a black hole tell us so much about the universe? It's been extremely important for our understanding of quantum gravity. It's perhaps the central fact that we know about quantum gravity: the information scales with the area. That fact, which was known since the 70s, was a big hint that became very influential later on. As understood by Bekenstein and Hawking, it was just a weird fact about black holes, perhaps. But we now understand it as a strong indication of what we call the holographic principle. The holographic principle has been a powerful idea in quantum gravity, and it's the following. If you took a non-gravitational system in which you ignored gravity, like the pile of hard drives, the information storage would scale like the volume, as we discussed. Whereas in fact, it scales like the area. Another way to say that is if you take a three-dimensional, three-plus-one-dimensional theory in which you have both quantum mechanics and gravity, the information storage scales like R squared rather than R cubed. That is, it scales as though you had a non-gravitational system in one fewer dimension. So if you had a two-dimensional theory in which there was no gravity, the information stored in a given region would also scale like R squared because the information would be just the two-dimensional volume, as in the area. So in other words, at least as far as information density, the information capacity is concerned, a gravitational theory in three dimensions is like a non-gravitational theory in two dimensions. Or more generally, a gravitational theory in n dimensions is like a non-gravitational theory in n minus 1 dimensions. So that is a big hint that forms the basis of the holographic principle. It's like gravity eats information. There's less information than you thought there was, than you naively thought there was if you didn't include information. And so the holographic principle says that maybe that's not just a neat observation. Maybe it is, in fact, the case that for every, or for some quantum gravitational theories, there is another theory that is exactly equivalent to it in one fewer dimension. And so this led to Maldacena's ADS/CFT correspondence, the gauge gravity duality, which was the most cited paper in high-energy theoretical physics ever, I think, maybe at this stage. In the late 90s, he wrote down an exact, we believe, an exact duality between a particular theory of quantum gravity, some particular flavor of string theory, and a non-gravitational theory that lives on the boundary of that space. And what problem does it solve if you can model the world in fewer dimensions that doesn't involve gravity?…

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