The Order of Time

by Carlo Rovelli

Many parts of this story are solid, others plausible, others still are guesses hazarded in an attempt at understanding the whole. Practically all the things recounted in the first part of the book have been ascertained from innumerable experiments: the slowing down of time according to altitude and speed; the nonexistence of the present; the relation between time and the gravitational field; the fact that the relations between different times are dynamic, that elementary equations do not recognize the direction of time; the relation between entropy and blurring. All this has been well ascertained.127 That the gravitational field has quantum properties is a shared conviction, albeit one currently supported only by theoretical arguments rather than by experimental evidence. The absence of the time variable from the fundamental equations, as discussed in Part Two, is plausible—but on the form of these equations debate still rages. The origin of time pertaining to quantum noncommutativity, of thermal time, and the fact that the increase in entropy which we observe depends on our interaction with the universe are ideas that I find fascinating but are far from being confirmed or widely accepted.