Spacetime thermodynamics without hidden degrees of freedom

Understand

A celebrated result by Jacobson is the derivation of Einstein's equations from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius relation.

• This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric.
• We show that a different interpretation of Jacobson result is possible, which does not imply the existence of additional degrees of freedom, and follows only from the quantum properties of gravity.
• We introduce the notion of quantum gravitational Hadamard states, which give rise to the full local thermodynamics of gravity.