# "Bulk" in the context of Black Holes and Quantum Mechanics

In this video [1], what Preskill means by "Bulk physics?" I mean, what suppose to mean a "Bulk" in black hole physics context?

$$* * *$$

[1] (7:14 min) https://www.youtube.com/watch?v=PblxOXJt9pY&t=466s I

## 2 Answers

Many speculative theories today involve additional dimensions. In some of these theories, the extra dimensions are “compactified” and thus microscopic. In others, they are cosmological in size but invisible to us because we live on a 3+1-dimensional “brane” within the larger-dimensional “bulk”, and the physics we experience is constrained to that brane.

This idea arose in string theory once physicists realized that in addition to there being vibrating one-dimensional strings, there could be vibrating k-dimensional branes. The word “brane” comes from “membrane”, generalizing the 2-dimensional case.

In this instance “bulk physics” refers to gravity (AdS) side of AdS/CFT correspondence. Its counterpart on the CFT would then be “boundary physics”.

For more info on AdS/CFT have a look at this question and this section of nlab wiki.