Parikh, MaulikMaulikParikhSarkar, SudiptaSudiptaSarkarSvesko, AndrewAndrewSvesko2025-08-222025-08-222018-01-012331-8422http://arxiv.org/abs/1801.07306https://d8.irins.org/handle/IITG2025/18272We find a local first law of thermodynamics, ΔE=TΔS−W, connecting gravitational entropy, S, with matter energy and work. For Einstein gravity S is the Bekenstein-Hawking entropy, while for more general theories of gravity S is the Wald entropy, evaluated on the stretched future light cone of any point in an arbitrary spacetime, not necessarily containing a black hole. The equation can also be written as ρΔV=TΔS−pΔV by regarding the energy-momentum tensor as that of a fluid.en-USe-Printe-Print123456789/615