杏吧原创

ARE you better off getting your popular science from someone who (like me) writes for a living, or from practising scientists who are experts in their field? The experts are likely to make fewer mistakes (as long as they stick to their speciality), but they might find it harder to ramp down the jargon and technicality. Besides, most experts are as ready as any hack to reach for the clich茅s commonly used to popularise their subject.

But then there鈥檚 Carlo Rovelli.

The Italian theoretical physicist, who works mostly on quantum gravity, found himself unexpectedly awarded guru status when his short book Seven Brief Lessons on Physics, based on a series of articles published in an Italian newspaper, became an international bestseller.

It wasn鈥檛 hard to see why. The book was very short and easily digestible. It was beautifully packaged by Penguin, which recognised the value of making an object of almost fetishistic desire that fitted neatly into your pocket.

But most of all, Rovelli鈥檚 prose style was a breath of fresh air. It seemed to belong to a distinctly Italianate tradition, reminiscent of both Primo Levi鈥檚 writings on chemistry and Italo Calvino鈥檚 visionary fantasies: poetic without being twee, learned without ostentation, authoritative yet conversational. I will accept being addressed as 鈥渄ear reader鈥� in this style, when elsewhere I鈥檇 find it an irritating affectation.

Rovelli distinguishes himself among scientist-communicators by including ideas and images from philosophy, history, art and literature (produced by the likes of St Augustine, Kant, Proust, Rilke) not as a bit of window dressing, but because they have genuinely valuable things to tell us. The reader feels the presence of a cultured and humane, but in no way self-important, thinker. When Rovelli eulogises Newton and Einstein, he acknowledges this is no way to do real history (if only some other physicists would follow his example).

鈥淲e experience time鈥檚 arrow because the past seems to us to have lower entropy鈥�

Rovelli鈥檚 sequel to Seven Brief Lessons, calledReality Is Not What It Seems, was far less brief and, for all its eloquence, a little more unwieldy. For his latest, The Order of Time, he has returned to the original formula in both length and format, and the result deserves to achieve comparable classic status.

Here, Rovelli shows what a real expert 鈥� when possessed of rare expressive and imaginative skills 鈥� can bring to the table. In his brisk survey of how physicists have thought, and think today, about the nature of time, much of what he covers is rather familiar territory: the Newtonian notion of absolute time, its disintegration in the face of Einstein鈥檚 special and general relativity, the quantum granularity of space-time at the absurdly tiny Planck scale, the entropic arrow of time, and so forth.

But there is no sense of retreading old ground because Rovelli offers all manner of newly minted metaphors and perspectives. Better still, he reaches into the heart of the matter in ways that make you think afresh about ideas you thought you had understood.

Take his discussion of the arrow of time. The usual story here is that it comes from the peculiar circumstance of the universe having had a very low entropy state in the past, from which entropy is now perpetually and inevitably increasing because of the sheer probability of it.

But as Rovelli points out, entropy is all a matter of perspective. The entropy of a system depends on an inability to distinguish particular configurations of its components, and thus on a degree of blurriness (more about that later). So it is subjective, contingent on what information is accessible to us. We experience time鈥檚 arrow, Rovelli says, because the past seems to us to have lower entropy, and not because that is true in any absolute sense. 鈥淚nexorably, then, the study of time does nothing but return us to ourselves,鈥� he writes.

Best of all is the chapter about why the past differs from the future (trust me, by that stage the answer won鈥檛 seem as obvious as you think). The point about the past is that it leaves traces. And such traces exist only because of dissipation: the degrading of energy into heat. Otherwise all interactions would be reversible. Falling objects wouldn鈥檛 stay on the ground 鈥� they would rebound, elastically, forever. Rovelli offers the most striking image of ink from the quill of a medieval scribe bouncing off parchment, defying all attempts to record any important thoughts for posterity.

All of this is rather wonderful, but it truly takes wing because of Rovelli鈥檚 facility with the elegant aphorism (and let鈥檚 not forget this elegance must owe something to Rovelli鈥檚 regular translators, Erica Segre and Simon Carnell). Take this example. 鈥淭he world is not a collection of things, it is a collection of events 鈥� networks of kisses, not of stones.鈥�

He adds, with a merry embrace of cultural stereotypes: 鈥淭he events of the world do not form an orderly queue, like the English. They crowd around chaotically, like Italians.鈥� And he concludes with a wisdom that echoes both physicist Niels Bohr and philosopher Ludwig Wittgenstein: 鈥淲hen we have found all the aspects of time that can be spoken of, then we have found time.鈥�

There are two small but telling glitches in what is an almost flawless little book. They are related because they illustrate the pitfall of a specialist perspective.

One of the criticisms of Reality Is Not What It Seems was its partisanship. Rovelli鈥檚 approach to quantum gravity eschews string theory in favour of the model he helped to shape, loop quantum gravity.

鈥�The Order of Time will establish Rovelli among the pantheon of great scientist-communicators鈥�

There is a similar bias towards his preferred theories in The Order of Time. But he is explicit and gracious about that, acknowledging that alternatives exist and that he is giving the view that makes most sense to him: a personal position, not an established consensus.

Yet it is precisely when Rovelli describes his own theories that his masterful lucidity falters. For example, he explains how the roots of a local definition of time might lie with the phenomenon behind the quantum uncertainty principle, namely the 鈥渘oncommutativity of conjugate variables鈥�, the fact that it matters in which order quantum operations are applied.

This quantum grammar, he explains, can create a kind of 鈥渇low鈥�, out of which the physicist鈥檚 conventional t for time can be spun from equations that don鈥檛 have it already inserted. It is fascinating stuff: because of the blurring through quantum indeterminacy, Rovelli says, 鈥渢he time of physics is, ultimately, the expression of our ignorance of the world鈥�.

But the description contains lapses into physics-speak, even to the extent of Rovelli casually qualifying a definition with the phrase 鈥渦p to certain internal symmetries鈥� 鈥� a nod to his colleagues straight out of the seminar room rather than a cautionary aside to general readers. When you are in this deep, it is hard to keep sight of the horizon.

In fairness, readers should know, too, that some of the material in this book overlaps with discussion of related themes in his two previous books 鈥� although to my mind, the reprise adds richness.

The Order of Time will surely establish Rovelli among the pantheon of great scientist-communicators, while at the same time showing that it is possible to reach a wide audience with a different voice from the slightly bland, culturally narrow plain-spokenness of many of his contemporaries and predecessors. More of this please.

• See Carlo Rovelli speak on the nature of time at聽聽on 22聽September in London

This article appeared in print under the headline 鈥淲hy we need to rethink time鈥�