In one minute

Ted Jacobson derives the Einstein equation from two ingredients: entropy proportional to horizon area, and the thermodynamic relation δQ = T dS linking heat, entropy and temperature. The relation is required to hold for every local Rindler horizon through each point of spacetime, with heat read as energy flux and temperature as the Unruh temperature seen by an accelerated observer. For that to work, matter must bend the causal structure of spacetime in exactly the way the Einstein equation describes. Seen this way, the Einstein equation is an equation of state. Jacobson suggests it may be no more appropriate to quantize it canonically than to quantize the wave equation for sound in air.

Thermodynamics of Spacetime: The Einstein Equation of State