The Origin of the Null Energy Condition

Abstract: The null energy condition plays a critical role in general relativity, in particular for guaranteeing the existence of the Big Bang. However, there has so far been no first-principles derivation of this ad hoc condition. In this talk, I will argue that the null energy condition has been misunderstood as a property of matter, when in fact it is really a property of spacetime geometry. With this insight, I will then derive the geometric formulation of the null energy condition in two different ways: starting from string theory or starting from thermodynamics. Thus we will find that, rather than being an incidental property of matter, the validity of the null energy condition hints at the deepest origins of gravity.