Adam Brown: belief

Transcript context

…What is the correct conceptual way to think about this? One version is, the boundary is one way to simplify the processes that are actually four-dimensional. Another is—I don't know how we think about this in the context of black holes—but maybe in the context of black holes, no, the information actually is on the horizon. The analogous thing here would be, no, somehow we are on the boundary of the universe somehow. Is there a sense in which one of these interpretations is correct? This duality idea, where you have two different descriptions of the same thing, is not new. The AdS/CFT correspondence was not the first such example in physics. It's a common trope in physics that you can have two different descriptions of the same thing, some of which are more useful in one scenario, some of which are more useful in the other scenario, but both are exactly correct. There are non-gravitational examples in physics that go back a long way. You may then ask, "Which one is right, and which one is not right? Is it actually a CFT that's pretending to have this weird alternative description as a gravitational theory? Or is the gravitational theory correct, and the other one is not correct?" I think this is more of a philosophical question. My answer would be that if the isomorphism was just an approximation -- if it was really one thing and you were just pretending it was the other thing, and that approximation worked in some region of validity and not others -- then I would say that one was right and the other one was just an alternative, fanciful description. That is not our understanding of AdS/CFT as we understand it today. Our understanding is that this is a precise isomorphism. It's not an analogy, it's not a metaphor, it is not an approximation that is valid in some domain and not another. It really is the case that these two theories are exactly equivalent to each other. And if that's correct, then as a matter of philosophy, I would say those are both equally real. So it's not the case that one is more real than the other; they're perfect simulations of each other. Are you an AdS dreaming you're a CFT, or a CFT dreaming you're an AdS? I think these are just two completely different, inequivalent descriptions of the same identical physics. Tell me if this is just a question that doesn't make sense. Because when I was, if you try to ask somebody about the quantum many worlds, "Where are the other worlds?" right? And they're just like, "They're in Hilbert space." "Where is Hilbert space?" "No, dude, it's just a conceptual thing. Stop asking questions." Intuitively, it feels like there should be a sense in which there is some physical existence. Either that existence is in this four-dimensional space, or it's in some space that exists on the boundary. Is this just going to lead us into philosophical loops, or is there something that can be said more about it? And also, in a de Sitter space, in a world like ours, what exactly would the boundary mean?…

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