The gravitational field, as we saw in the first lesson, is space itself, in effect space-time. Therefore, when heat is diffused to the gravitational field, time and space themselves must vibrate . . . But we still don’t know how to describe this well. We don’t have the equations to describe the thermal vibrations of a hot space-time. What is a vibrating time? Such issues lead us to the heart of the problem of time: what exactly is the flow of time?
