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Issue 215 February 2018 Beyond the Big BangReality is not what it seems: the journey to quantum gravityCarlo RovelliPenguin books, 2017, £9.99Reviewed by Pete MasonAfter 90 years of frustrating debate and countless blind alleys, best-selling Italian physicist Carlo Rovelli presents a convincing solution to the greatest challenge in physics. Summing up the conceptual breakthroughs of the current century, he discusses the search for an understanding of deep questions about the nature of space, time and gravity. Albert Einstein continually attacked quantum theory in the 1930s. Crucially, he showed that quantum mechanics, the theory that describes atoms and fundamental particles so well, implied some kind of instantaneous connection between particles across the universe, contradicting his relativity theory. Einstein’s relativity says that nothing can travel faster than the speed of light. Quantum mechanics seemed to suggest that fundamental particles in some sense exist ‘beneath’ space and time. It was 30 years before Irish physicist John Bell figured out how to test this, and another 30 before the first convincing results. The famous Schrödinger wavefunction told scientists that a pair of particles, such as photons of light or electrons, can be ‘entangled’ across the entire universe. Tickle one and the other laughs – metaphorically speaking, that is! And, astonishingly, decades of experimental results have proved it is true. Relativity describes the universe in terms of matter, space and time. By contrast, quantum theory deals only with the probabilities of discrete events and is happy to ignore both space and time in between. Rovelli comments: "An electron is nowhere when it is not interacting… things only exist by jumping from one interaction to another" in quantum theory. The two theories speak a different language, as if about a different universe. They don’t communicate. So the greatest challenge for physicists was how to unite these two outstanding theories. Matvei Bronstein, a Russian theorist of the 1930s, provided a critical insight into nature that is central to the book: that Einstein’s general theory of relativity, the pinnacle of classical physics, and quantum mechanics both tell us that space itself is made up of particles. Rovelli notes that, tragically, Bronstein died in a Stalinist purge, adding in a footnote that Bronstein happened also to be the real surname of Leon Trotsky, leader of the opposition to Stalin. Space is ‘granular’, just as Newton thought when, in passing, he referred to "every particle of space" in his famous Mathematical Principles of Natural Philosophy, which established the mathematical basis and precision of modern science. But Rovelli is not a crude ‘reductionist’ (one who reduces everything to ‘atoms and the void’). Here is a scientist who embraces art, who loves his fellow Italian poet Dante, and who, in an astonishing passage, convinces us that Dante saw the universe as Einstein did, artistically anticipating Einstein’s vision. The works of Dante and Einstein together "count among the most beautiful and significant flights the mind can conceive". Understanding gravity is critical to the task of unifying relativity and quantum mechanics. It lies at the heart of Rovelli’s book. Quantum scientists have searched for decades for a ‘graviton’ particle and a theory of gravity formulated in terms of quantum theory – without success. Why could quantum mechanics not replace Einstein’s theory with a better version of gravity? The standard Big Bang theory of the origin of the universe, which developed from Einstein’s relativity, starts with a hot dense state which expands in the way we can now observe, producing the atomic elements and large-scale structure of the universe. This model has stood the test of time. But Einstein’s theory dissolves into infinities when scientists try to look back further in time, and a ‘singularity’ emerges – something infinitely small. This cannot be right. Scientists know that any actual infinity, like the old Newtonian infinite universe, contains physical contradictions. Newton recognised this and looked to the hand of God to help him out. Similarly, quantum mechanics is plagued with infinities. Yet, if space is granular, there is a way out. Over the last three decades or so, two new theories have attempted to unify relativity and quantum theory. The competition between advocates of these two theories – string theory and loop quantum gravity – has been mercilessly satirised in the TV comedy, The Big Bang Theory. Let’s just say there is no love lost. Rovelli, a loop quantum gravity specialist, allows himself only one passing reference to string theory, which is based on the idea that everything is made up of tiny one-dimensional strings. It is a quiet celebration: the Large Hadron Collider at CERN has failed to find the precursor particles (supersymmetry particles) which string theory is based on. String theory is not done for yet, however. More powerful colliders are needed for that. String theory may never provide an experimental test to validate itself. Loop quantum gravity, the Cinderella of the physics community which proposes that space looks like a chainmail of loops, is also some way from any convincing test. Future empirical tests are outlined by Rovelli. In the meantime, experiment after experiment has showed that both Einstein’s relativity and quantum theory are correct, to astounding degrees of accuracy. Neither will give ground to the other. Now physicists working on unifying the two theories are beginning to hear what nature has been telling them. The revelation began with the concept of ‘emergence’. This is the recognition that a qualitatively new substance can emerge from a quantitative process of accumulation – a concept that reaches back to the ancient philosophy of dialectics. Not that a philosophical insight on its own is sufficient to take science forward. Even Einstein’s closest collaborator at Princeton University, David Bohm, stumbled as he attempted to apply dialectics to the problems Einstein outlined. The material understanding of the universe lay undiscovered. In fact, it was Bohm’s investigations that motivated John Bell. It was decades of experimental results that finally inspired the necessary theoretical insights. The discussion of the nature of time over the last 15 years, reviewed by Rovelli, has benefitted from the concept of emergence. Neither Newton’s equations nor those of quantum mechanics have any arrow of time. Events they describe can theoretically be reversed in time. So why doesn’t nature go backwards as well as forwards in time? In fact, positrons (positively charged electrons) can be thought of as electrons moving backward in time. But it turns out that the arrow of time emerges at the macroscopic level. Time emerges because the universe, taken as a whole, is slowly unwinding, despite its beautiful structures of spiral galaxies, solar systems and the emergence of life on this planet. The main cause is the heat loss of stars like our sun. As clocks tick away their springs must unwind. So it is with all nature on earth, nurtured by the sun, which burns up its fuel keeping life developing on our planet. The forward march of time emerges at larger scales. Similarly, the continuous space we experience emerges from the grains of space at larger scales too. And what is gravity but the warping of spacetime? So gravity is emergent, along with space and time. This, then, is the domain of Einstein’s general theory of relativity. Gravity has no place in the subterranean quantum world. Relativity and quantum mechanics stand independent of one another. The two theories cannot be unified because they deal with different levels of reality. This is a conceptual revolution. The greatest challenge in physics melts away into nothing. One result, Rovelli demonstrates, is that the infinities which plagued both Einstein’s theory and quantum mechanics now disappear in a ‘tremendous result’ for this insight, for which loop quantum gravity partisans like Carlo Rovelli can take credit. We can now pass ‘beyond the Big Bang’, not to negate it but to develop it further: for instance, to investigate what might have happened before the Big Bang. Difficult at times, ranging from a penetrating discussion of the atomists Democritus and Lucretius of the ancient Greek and Roman world, to modern information theory, this book is a rewarding read. |
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