**Title:** Displacement convexity of Boltzmann's entropy characterizes positive energy in general relativity

**Speaker:** Robert McCann

**Speaker Info:**

**Brief Description:**

**Special Note**:

**Abstract:**

Einstein's theory of gravity is based on assuming that the fluxes of a energy and momentum in a physical system are proportional to a certain variant of the Ricci curvature tensor on a smooth 3+1 dimensional spacetime. The fact that gravity is attractive rather than repulsive is encoded in the positivity properties which this tensor is assumed to satisfy. Hawking and Penrose (1971) used this strong positivity of energy to give conditions under which smooth spacetimes must develop singularities.By lifting fractional powers of the Lorentz distance between events in a globally hyperbolic spacetime to probability measures on these events, we show that the strong energy condition of Hawking and Penrose is equivalent to convexity of the Boltzmann-Shannon entropy along the resulting geodesics of probability measures. This new characterization of the strong energy condition on globally hyperbolic manifolds also makes sense in (non-smooth) metric measure settings, where it has the potential to provide a framework for developing a theory of gravity which admits certain singularities and can be continued beyond them. It provides a Lorentzian analog of Lott, Villani and Sturm's metric-measure theory of lower Ricci bounds, and hints at new connections linking gravity to the second law of thermodynamics.

Copyright © 1997-2024 Department of Mathematics, Northwestern University.