# Why are string theorists interested in entanglement entropy?

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I have been reading some papers on Ryu-Takyanagi but I am not seeing a good explanation as to why entanglement entropy of the boundary CFT a good observable to probe the possible bulk quantum gravity.

Any kind of deformation invariance? (like Witten index)
Is it even defined for CFTs at finite temperature or at arbitrary coupling?

Could this Maldacena talk be helpful?

Yes, I remember that Lumo has mentioned entanglement entropy in one of his ER/EPR TRF posts too. Might be related to the strenght or the ER bridge?

I read through this paper (PDF), and I don't see anything exceptionally surprising about the entanglement entropy being a suitable observable to probe the dual quantum gravity theory. The entanglement entropy $S$ is equal to $\frac{A}{4G_N}$, which is of course related to the possible bulk quantum gravity. You can read more on pg 2 of the linked paper (pg 2 of the paper, not of the PDF file).
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