Loop Quantum Gravity and the Search for the Structure of Space, Time and the Universe

*(2018) Jim Baggott, Oxford University Press, £20 / US$24.95, trdpbk, xvii + 421pp, ISBN 978-0-198-80911-1*

Loop quantum gravity is arguably one of the better (if not currently 'best') hypothetical theories that brings together quantum mechanics (the way things work at very small scales) and general relativity (the way gravity emerges as a property of [curved] space-time). While the hypothetical theory of loop quantum gravity is not a grand unified theory – while it uses time (as in space-time) it does not define time (though does give us a fundamental unit of time: the time light takes to travel a Planck length). Nonetheless, the notion of loop quantum gravity is very important avenue of exploration for particle as well as cosmological physicists.

Given there is little experimental evidence (just circumstantial evidence) for loop quantum gravity, there is much debate as to loop quantum gravity even though mathematically the hypothetical theory seems to stand up. Some of you may be aware of the temperature that this debate can reach from the argument between Leslie Winkle and Sheldon Cooper in *The Big Bang Theory*. So loop quantum gravity is a rival theory to superstring theory but without so many dimensions (loop quantum gravity just needs three of space and one of time – M-theory requires 11 – and without the need for supersymmetry). As such, as Carlo Rovelli (we'll come to him shortly) says loop quantum gravity is most parsimonious (fulfils Occam's Razor. It may though be possible to have a form of loop quantum gravity with more dimensions. In loop quantum gravity space at the infinitely smallest of levels is not 'smooth' but made up of a spin network of finite-sized chunks (excuse the biological terms slipping in – I can't help it). This has implications for cosmology, as there cannot be a Big Bang but instead a Big Bounce. Nor is there a singularity at the heart of a black hole but a Planck star.

Before I continue and review Jim Baggott's book, I should say that I am arguably not the best person to review this book. Yet again, like Schrodinger's cat, at the same time I am arguably well placed to review it. My training as an environmental scientist, with a career in the life and geo-sciences, means that I have an unbiased, non-physicist perspective of relevance to potential non-physicist readers, yet have a sufficient a science tool-kit to at least begin to explore the book's thinking.

Let's be clear, the topic is difficult with which to come to grips. Chemist, Jim Baggott has, with his book, *Quantum Space*, made a fair fist of explaining loop quantum gravity without the use of mathematical equations (well, there are one or two simple ones plus a few more complex ones in the footnotes and endnotes.) This book therefore is of relevance to those who are not physicists but who are seriously into popular science and have a hunger to learn. Jim Baggott – the author of*Mass* and *Origins* -- himself is not a physicist but is a chemist, and as physical chemistry has a substantive overlap with quantum mechanics I'm assuming it is through that route that he comes to the topic of loop quantum gravity. Having said that, I strongly suggest that non-physicists, seeking to learn about loop quantum gravity from Baggott's book, should take notes as they read as there is another dimension (interesting as it is) to the book that – while complimentary – gets in the way of focussed concept contemplation.

The book will also appeal to such a potential readership in that it is an account as to how physicists' thinking over recent decades came to the development of loop quantum gravity. Here, especially, it has some focus on two particular researchers: US American Lee Smolin and Italian Carlo Rovelli. This is the book's, aforementioned, other dimension. (By the way, Carlo Rovelli has a short, loop quantum gravity lecture series on YouTube and the first lecture (see afore link) has an important comment on Karl Popper which many (including scientists) get wrong.

Jim Baggott's book will have strong appeal to physicists who work in other areas of the discipline, yet who want to see how loop quantum gravity developed: Baggott's *Quantum Space* will be perfect for them.

There are also those of us somewhere less localised on the above potential-reader landscape. For example, the wonderful PBS Space Time channel currently has nearly 1**.**5 million subscribers and the betting is that a few thousand of these would be interested in Baggott's book. (Yes Oxford University Press, nearly one-and-a-half million: spending some promotional budget with PBS Space Time would be a sound investment.)

And if you found yourself following a number of the links in this review, then it could well be that Baggott's *Quantum Space* is for you.

Jonathan Cowie

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