Monday, December 28, 2015

Emergent spacetime: a review of George Musser's book

George Musser is profoundly disserved by his book’s cover. Featuring garish colours, a clichéd title and fatuous sub-title, it is easy to assume that this is a sensationalist popularisation for dummies. You could not be further from the truth. George Musser is a contributing editor for Scientific American and the author of ‘The Complete Idiot’s Guide to String Theory’ – he has interviewed the world’s leading physicists and produced a wonderfully clear account of how our familiar spacetime might be emergent (Amazon link).

He starts with the classic experiment: produce two photons in correlated polarisation states. Set them going in opposite directions. If the polarisation of one photon is measured, its value instantly determines the polarisation state of the other, no matter how far away it has flown. This is quantum non-locality and it tells us that something is wrong with our understanding of spacetime as a smooth continuum with light cones determining cause and effect.

Physicists tend to hate this kind of observation. Given that quantum theory itself defies any straightforward interpretation as a theory of ‘reality’ it seems that non-locality is just one more piece of ontological weirdness. Better to shut up and calculate: we know the theory work incredibly well and we know how to interpret the answers (as probabilities).

The sense that ‘reality’ is real and should make sense in its own terms is a powerful intuition. It has frequently been use to highlight conceptual weaknesses in otherwise successful theories. Musser recounts just how much trouble Newton’s contemporaries (and Newton himself) had with the apparently infinite speed of gravity in his theory – this is also a kind of non-locality. It was nineteenth century field theories (Faraday, Maxwell) followed by General Relativity which (briefly) restored locality to physics.

Quantum non-locality is something else. Musser writes (p. 125), “Instead of thinking of quantum non-locality as an effect which operates within space, I think we need to take it as a sign that space itself is a doomed concept.”  What would a theory of emergent spacetime look like? There are a number of ideas; naturally none are fully worked out.

Fay Dowker talks about causal sets - space is built out of discrete units, ordered in a complex network whose structure creates space. Fotini Markopoulou has a similar networked theory of ‘atomic grains of space’ in an approach punningly-termed quantum graphity; the link density is determined by the available energy, from which emerges space as we know it. String theory has a model of emergent-space based on matrix models: the matrices catalogue the web of interactions between D0-branes, dimension zero building blocks of space. Leonard Susskind is associated with this line of research.

Musser explains these various theories in some detail, as best he can, describing their applications to black hole modelling and the early universe. AdS/CFT makes its obligatory appearance with yet another brave attempt to explain the holographic principle. But these diverse approaches deal mainly in space, treating time asymmetrically.

The book finishes with the Amplituhedron. Built on the foundations of S-matrix theory and twisters with a dash of string theory, the amplituhedron is a geometric structure used for calculating transition probabilities for particle interactions. Each particle contributes a polyhedron vertex with its momentum setting the size of the corresponding polyhedral face. The interior volume gives the resulting amplitude.
‘“There are no fields, no particles, no interactions,” Trnka says. The locality we observe in daily life is a consequence of the way the faces fit together – specifically that they form a closed shape, as opposed to disconnected planes.’
I don’t think the reader is expected to fully grasp this.

The idea that the 13.8 billion light year observable universe is an emergent artefact of an underlying non-spatial non-temporal quantum reality is surely the most mind-blowing concept of modern physics. Yet there are excellent reasons for taking it seriously. George Musser has written a clear, accessible and intelligent review of how this might be possible – it’s as near as most of us are ever going to get to understanding it – and he is to be congratulated.

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