Finding the Time Andrew Jaffe Takes the Measure of Two Books on the Tangled Concept of the Temporal

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PHYSICS Finding the time Andrew Jaffe takes the measure of two books on the tangled concept of the temporal.

hat is time? If you’re a practising I wonder whether this is sufficient. physicist, it’s a quantity in your Physicist Richard Muller also seems equations, t. This is the variable exercised by this conundrum. His Now Wthat you use for one of the four dimensions of attempts to lay out a solution. He starts with

the manifold of space-time, the term coined a pop-science introduction to the required MARK GARLICK/SPL by mathematician Hermann Minkowski physics: the broad theories of relativity and after Albert Einstein’s theories of relativ quantum mechanics, and the specific roles ity began to show that time and space are of cosmology and particle physics in our fungible. And yet we can move freely back Universe, such as those of the Higgs boson and forth in space but not in time. Why? and its mass-giving field. His introductions In Time Travel, science writer James to modern physics are probably too technical Gleick reviews the science of time by focus for most lay readers, despite relegating most ing (mostly) on the science fiction of time of the harder maths to a series of appendices. travel. He starts from, and often returns to, A ‘wormhole’ — a favourite time-travel device. Unfortunately, after dispensing with H. G. Wells’s The Time Machine, which pre physics, Muller delves into philosophy, a dis dates Einstein’s 1905 special theory of relativ Arthur Eddington opined that this alone is cussion that hardly rises above the university- ity by a decade. It’s a pleasurable romp over responsible for the ‘arrow of time’. The prob bar level. For example, he takes for granted Wells’s fourth dimension and polished Vic lem is that the second law is not really about that free will is not compatible with determin torian machinery; ‘golden age’ science-fiction physics, but probability — and hence knowl ism. This has been debunked in philosophy, authors such as Isaac Asimov, who provided edge. We know less about the details of a high- for instance by Daniel Dennett in the 1991 the templates for modern treatments of entropy system than a low-entropy one, so it’s Consciousness Explained (Little, Brown), or time travel; and the Doctor Who franchise harder to extract useful work. this year by Sean Carroll in The Big Picture (A. Jaffe Nature 502, 620–622; 2013). Gleick The symmetry of time is also broken in (R. P. Crease Nature 533, 34; 2016). Instead, also explores more highbrow offerings from quantum mechanics, which describes a phys Muller opts for the manifestly non-scientific writers such as David Foster Wallace and ical system by its wavefunction, but gives us idea of a non-physical soul with causal powers Jorge Luis Borges (who envisaged time as a probabilities, not definite results. When we over the quantum-mechanical wavefunction. “Garden of Forking Paths”), and filmmaker make a quantum measurement, we some This is pretty far-out, but is just a side note. Chris Marker, whose 1962 sci-fi short La Jetée times say that the wavefunction collapses, a Muller’s main thesis is that the expansion inspired 1995 time-travel noir 12 Monkeys. process that has only one direction. But this is of the Universe “is continually creating not Gleick doesn’t exactly wear his knowledge about knowledge, too, in contemporary ways only new space but new time”. That is a good lightly, but he does cram a lot in, especially of understanding quantum mechanics such soundbite, but cosmologists debate whether in discussions of the physics. Einstein’s 1915 as the many-worlds interpretation — the idea the starting point — the idea of creating new general theory of relativity seems to allow that every possible outcome exists out there space — is itself meaningful. Since writing for “closed timelike curves”, paths that start in the multiverse. When we make a measure the book, Muller has expanded on his ideas at one place and time, and end at exactly the ment, we gain information about the system. more mathematically, and applied them same place and time. Unfortunately, actually Gleick spends some pages on the ‘problem to this year’s observations of gravitational creating space-time with such a curve — that of now’, the question of how the equations of waves (R. A. Muller and S. Maguire. Preprint is, a time machine — may be impossible, an physics seem to give us a Universe in which at http://arxiv.org/abs/1606.07975; 2016). idea formulated in Stephen Hawking’s “chro time isn’t just one of four space-time dimen Kudos to him for proposing an idea that may nology protection conjecture”. In this, the sions. Instead, it is special: why do we always be testable. Very few popular or professional Universe conspires to make time machines live at a specific moment, only remember physics books bother to make an argument, impossible to build: they require physically ing the past and waiting for the future? The summarizing the state of the art instead. impossible states of matter, or their creation issue nags at many physicists, including Unfortunately, I don’t buy Muller’s argument: may also generate a black hole around the me. Sometimes, I’m convinced that ‘now’ whether or not ‘now’ is a non-problem, Mul machine, making it impossible to access. is a non-problem. Once quantum mechan ler’s idea is a non-solution in my view. But even the normal perceived flow of ics and thermodynamics have given time a Both Gleick and Muller want us to time in one direction is mysterious. Most of direction, ‘now’ isn’t physics, but a combi realize that time is central to our experience the microscopic equations of physics have nation of time’s arrow with psychology and — that having a now is what constitutes hav a fundamental symmetry: they can’t tell physiology. The past is what is encoded in ing an experience at all. Even if travelling whether time is moving forwards or back our memories. To a rock, an electron or a into the past is a fantasy, the physics of time wards (mathematically, they look identical if galaxy, there is no now. But occasionally encompasses almost everything that physi we replace t with −t). But this is not how we cists study. Perhaps understanding its flow experience time. We move inexorably from Time Travel: A History will give us a more complete picture of our past to future; we remember the past and have JAMES GLEICK changing Universe. ■ no direct knowledge of the future. One excep Pantheon: 2016. tion to time-reversal symmetry is thermo Now: The Physics of Time Andrew Jaffe is a cosmologist at Imperial dynamics, whose second law says that entropy RICHARD A. MULLER College London. always increases with time. Astronomer W. W. Norton: 2016. e-mail: [email protected]