# Thermodynamics of Spacetime: The Einstein Equation of State?

Hi,

I have recently been delving into Quantum gravity related material... and I came accross a paper by Ted Jacobson "Thermodynamics of Spacetime: The Einstein Equation of State" http://arxiv.org/abs/gr-qc/9504004 and as far as I understand the argument it is very impressive who would have thought that three seemingly unrelated theories could be so closely tied (GR, thermo and QFT). But I have two questions:

1) Why starting from the 2nd law which assumes an arrow of time can we extract a theory which is time reversal invariant GR? (this is probably closely related to the work of Hawking which I am unfamiliar with)

2) His interpretation of heat puzzles me:

"In thermodynamics, heat is energy that ﬂows between degrees of freedom
that are not macroscopically observable. In spacetime dynamics, we shall
deﬁne heat as energy that ﬂows across a causal horizon."
specifically he identifies $$Q$$ (the heat) with $$T^{\alpha\beta}$$ (the energy momentum tensor.)

cheers

M