Events in the quantum world do not form an orderly queue but crowd around in a chaotic fashion
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.”
As well as dealing in a light-hearted way with some very serious physics, Rovelli (who works on quantum gravity), mixes in literary references showing that he at least straddles the cultural divisions that annoyed Snow. He quotes Horace, Shakespeare and others, but the longest quote is from Hugo von Hofmannsthal’s wonderful libretto to Richard Strauss’s Rosenkavalier. Yet the quotation is put together from three separate musings by the Marschallin: the first alone after Ochs has left and she recalls her younger life, the second in conversation with Octavian as she fears losing him, and the third from her soliloquy on the passing of time, Die Zeit die ist ein sonderbar’ Ding (Time is an extraordinary thing). But having reached this far into the book I already had my doubts about the presentation.
The author starts very well with differences in the passage of time under different gravitational field strengths, or different states of motion, along with the direction of time’s arrow that emerges from thermodynamics, but the quantum phenomena in chapters eight and nine are not well explained. “The theory [loop quantum gravity] describes how things change one in respect to the others, how things happen in the world one in relation to each other. That’s all there is to it.” Surely there must be more one can say, but in the end-note for this excerpt the author gives mathematical formulations without adequate explanation, basically saying: I know what I’m talking about, but can’t explain it to you — which is a case of talking down to the reader.
Earlier when he is dealing with the light-cones of relativity theory, he alludes to the passing of a gravitational wave creating conditions where “advancing always towards the future, one can return to the same point in spacetime”. This is time travel, as usually understood, but we need more context to the statement that “there is no logical contradiction entailed by the existence of closed temporal lines or journeys into the past; we are the ones who complicate things with our confused fantasies about the supposed freedom of the future”. Rather than explain what he’s getting at here he goes straight on to black holes: “It is the curious local structure of the present that produces black holes.” Really? I thought it was gravity that produces black holes, and that as you approach them the “local structure” (of space-time) breaks down.
As a popular guide to the author’s specialisation of quantum loop gravity this does not cut the mustard. It is more a poetic meditation on the passing of time, which ends with allusions to Augustine, the Vedas, and the Benedictus from Beethoven’s Missa Solemnis, rounding off with a quotation from Ecclesiastes: “The silver thread is broken, the gold lantern is shattered, the amphora at the fountain breaks, the bucket falls into the well, the earth returns to dust.” He omits the next line, “and the spirit shall return unto God who gave it”, betraying a distrust of “eternalism” and disbelief in anything outside spacetime. He stays therefore very much in the realm of physics, but I would have loved more clarity on how quantum gravity creates the time from which the author finds his inspiration. Certainly Rovelli straddles the divide between science and the arts, but after a strong start the arguments dissipate about halfway through, and the book leaves scientific matters too vague.
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