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Carlo Rovelli: Straddles the divide between science and the arts (Fronteiras do Pensamento CC BY SA 2.0)

In his famous essay on the unfortunate distrust between science and the arts in British intellectual life, C. P. Snow wrote that he had often been at gatherings of “highly educated” people “who have with considerable gusto been expressing their incredulity at the illiteracy of scientists. Once or twice I have been provoked and have asked the company how many of them could describe the Second Law of Thermodynamics. The response was cold: it was also negative. Yet I was asking something which is the scientific equivalent of: Have you read a work of Shakespeare’s?”

Indeed, the second law of thermodynamics, that heat passes from warm things to cold things rather than the other way round, or more precisely that available energy always tends to become unavailable, is fundamental to our notion of time’s arrow. By contrast, Newton’s physical laws work equally forwards or backwards in time, so something has to distinguish one direction from the other and thermodynamics does this, giving what Carlo Rovelli in his new book (The Order of Time, Allen Lane, £12.99) calls “thermal time”. Yet he starts with subtleties that most of us never think about: time passes more quickly up a mountain than it does down in the plains. This is not a matter of perception but the simple fact that an atomic clock up a mountain will count more nanoseconds than one on the plain. The stronger the gravitational field the weaker the passage of time, until on the surface of a black hole there is none at all.

This feature of Einstein’s General Theory of Relativity is easier to grasp than the usual “space-twins paradox” of special relativity where the twin who travels very fast out into space and back ages less than the one left at home. Nothing travels faster than a ray of light, for which time stands still, and Rovelli uses this to point out there is no such thing as a present time throughout the universe. There is a here and now, but the speed of light is not infinite and the concept of “now” can only be a local one: “Our present does not extend throughout the universe. It is like a bubble around us.” If we measure time in nanoseconds the bubble extends only a few metres, if by milliseconds it extends over thousands of kilometres, and if we blur things to the nearest tenth of a second it extends over the whole planet. Blurring things avoids quantum phenomena, but time seems to vanish at the very tiniest scales and Rovelli phrases things in picturesque terms: “The events of the world do not form an orderly queue, like the English. They crowd around chaotically, like the Italians . . . [things] are happening; it is not stasis . . .  The fundamental equations do not include a time variable, but they do include variables that change in relation to each other.”
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