“Time is not a line with two equal directions: It is an arrow with different extremities. And it is this, rather than the speed of its passing, that matters most to us about time. This is the fundamental thing about time. The secret of time lies in this slippage that we feel on our pulse, viscerally, in the enigma of memory, in anxiety about the future. This is what it means to think about time. What exactly is this flowing? Where is it nestled in the grammar of the world?” Here Rovelli is raising yet another of time’s mysteries. Why does it have such a clear directionality, with the future so easily distinguished from the past?

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Let me explain. If you see a film of a teapot first sitting on the edge of a table, then falling off the table and shattering into a thousand shards, the film appears perfectly normal. Evidently, someone jostled the table. But, if you were to watch a film in which shards of pottery scattered about on the floor suddenly rushed together and formed a teapot, which then jumped up to land on a table, you would interpret that film as being played backward in time. Because you’ve never seen such a sequence of events in the real world. Why not?

The answer is a matter of probability. A broken teapot could indeed re-form itself from vibrations and heat in the floor, but it is highly unlikely. Physics shows that moving from order to disorder is more probable than the reverse — much more probable when it comes to large collections of atoms such as constitute a teapot. In scientific terms, this movement determines the direction of time. And the fact that this direction is so definite in all that we see, both on earth and beyond, means that the cosmos must have begun in a state of relatively high order — with plenty of room to make a mess in the future. It is that room for increasing chaos that drives the evolution of the universe, that drives change. Without it, stars and planets would never form, humans and other life-forms would never exist, and teapots would never be made in the first place, broken or otherwise.

Physicists still do not understand why our universe appears to have been created with such a high degree of order. Rovelli suggests that it’s a matter of our human “perspective,” and depends on our “interactions” with the physical world. I respectfully disagree. It is theoretically possible for the universe to have been created in a state of nearly maximum disorder, in which case no evolution or change would occur. That would not be a matter of perspective. One possibility, entertained by a number of leading physicists, is that there are lots of universes, the so-called multiverse, with very different properties and initial conditions. Some of those universes may have started in conditions of maximum disorder, with nothing driving change, no distinction between future and past, where atom-size pottery shards gather themselves up to form atom-size teapots as often as the reverse. But some of these universes would have been created, by accident, with relatively high order. We live in such a universe because otherwise we wouldn’t be here to discuss the matter. The theory of “quantum gravity,” which is still not fully formulated, describes such a continuous creation of universes with random properties and initial conditions.

Some elements of Rovelli’s narrative, like the material on light cones and loop quantum gravity and spin networks, many readers will find incomprehensible. But the many other excellent explanations of science, the heart and humanity of the book, its poetry and its gentle tone raise it to the level and style of such great scientist-writers as Lewis Thomas and Rachel Carson. Listening to Rovelli’s book, as read by Cumberbatch, we hear the warm voice of a modest man searching to understand not only the physical world but also how he, and we, perceive it. “Time, then, is the form in which we beings whose brains are made up essentially of memory and foresight interact with the world,” Rovelli writes near the end. “It is the source of our identity.”