# The Off-shell String Effective Action and Black Hole Entropy

### APA

Ahmadain, A. (2023). The Off-shell String Effective Action and Black Hole Entropy . Perimeter Institute. https://pirsa.org/23040092

### MLA

Ahmadain, Amir. The Off-shell String Effective Action and Black Hole Entropy . Perimeter Institute, Apr. 25, 2023, https://pirsa.org/23040092

### BibTex

@misc{ pirsa_PIRSA:23040092, doi = {10.48660/23040092}, url = {https://pirsa.org/23040092}, author = {Ahmadain, Amir}, keywords = {Quantum Fields and Strings}, language = {en}, title = {The Off-shell String Effective Action and Black Hole Entropy }, publisher = {Perimeter Institute}, year = {2023}, month = {apr}, note = {PIRSA:23040092 see, \url{https://pirsa.org}} }

**Collection**

**Subject**

Taking string theory off shell requires breaking Weyl invariance on the worldsheet, i.e. the worldsheet theory is now a QFT instead of a CFT. I will explain Tseytlin’s first-quantized approach to taking the worldsheet theory off-shell in a consistent manner, with a particular emphasis on the subtleties involved in calculating the sphere amplitude. This approach allows for the derivation of a classical string action which gives rise to the correct equations of motion and S-matrix, to all orders in perturbation theory.

I'll also explain the underlying conceptual structure of the Susskind and Uglum black hole entropy argument. There I will show explicitly how the classical (tree-level) effective action and entropy S = A/4G_N may be calculated from the sphere diagrams.

Time permitting, I will discuss ongoing efforts to derive the Ryu-Takayanagi formula in string theory.

Based on arXiv:2211.08607 and arXiv:2211.16448.

Zoom link: https://pitp.zoom.us/j/93668017324?pwd=K2pxZEhjalRTWkVVbVRESCtRVDFmUT09