Book Review: The Order of Time by Carlo Rovelli

Time and space are central not only to our common sense, but also to physics and philosophy.  The title phrase is excerpted from the very short surviving fragment that is our oldest writing of Greek philosophy, by Anaximander of Miletus.  Rovelli's examination of time is based on modern physics, but it leans heavily on ancient philosophy as well, with Aristotle, Augustine, Husserl, Kant, Hegel, Reichenbach, and many others making appearances.  Paul McCartney, the Grateful Dead, and the Mahabharata also get quoted.  Most chapters start with a quote from one of Horace's Odes.

So what is the nature of time?  Rovelli's first exhibit is due to Aristotle, who saw time as the ordering of events.
Aristotle is the first we are aware of to have asked himself the question “What is time?,” and he came to the following conclusion: time is the measurement of change. Things change continually. We call “time” the measurement, the counting of this change. 
Rovelli, Carlo. The Order of Time (p. 63). Penguin Publishing Group. Kindle Edition. 
Aristotle's time is relative to the change in things around us.

Newton's view contrasts strongly.  His time was absolute and uniformly flowing, a river in which the changes in material things were embedded.

Rovelli says that Einstein unified the ideas of Aristotle and Newton - well maybe - but Special Relativity, as Minkowski showed, is a theory of absolute spacetime.  As such, it annihilated the idea of uniformly flowing time - now becomes a local construct, and while identifying now here with now in New York is not a bad approximation, we can't say the same for now here and now on Mars, much less for now in the Virgo cluster.  Rovelli points out, very interestingly, that Einstein's job at the patent office involved synchronizing railroad clocks.

An oddity well known to physicists, but strange to lay persons, is the fact that the equations of mechanics, electrodynamics, Special and General Relativity, and quantum mechanics have no place for the concept of now, or of any special direction of time.  Only one equation of physics does - and it's the only equation in the book - the second law of thermodynamics, which states that entropy never decreases.

A lot of talk about entropy follows, and Rovelli identifies it with blurring of the past.  More technically, I would say that computing entropy always involves coarse graining, or averaging over some microstates.

Having introduced entropy as the source of the direction of time, Rovelli develops some ideas about a quantum theory of time.  Unfortunately, I didn't really understand this and consequently found it unconvincing.

His third section of the book (of three) is more philosophical, poetic and even mystical.  I'm not sure I got a lot out of it either.

Clearly Rovelli has wrestled deeply with time.  Has he gotten further than others?  I'm not sure.  Maybe I will reread some of the confusing stuff sometime.  Or maybe somebody else.


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