The concept of loop quantum gravity

Book review: ‘Quantum Space’, by Jim Baggott

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The less well known alternative to string theory as a way of reconciling quantum mechanics and general relativity.

When it comes to understanding how the universe works, there are two established theories: quantum mechanics explains everything at a small scale, while general relativity handles the cosmological big picture. Both theories are, according to Jim Baggott, “extraordinarily successful”. The problem, as he outlines in the preface to ‘Quantum Space: Loop Quantum Gravity and the Search for the Structure of Space, Time and the Universe’ (Oxford University Press, £20, ISBN 9780198809111), is that they are mutually incompatible.

Such trifles aren’t normally sufficient to daunt the theoretical physicist and yet, given that “as far as we know we’ve only ever had one universe,” for the author there is a pressing need for a quantum theory of gravity.

Or, to put it in more easily understood terms, we need to find a new way of bringing together these two theories “that is compatible with physics on any scale.” There are two routes: string theory and loop quantum gravity (LQG). While everyone has heard of the former, few outside the world of physics will have heard much of the latter. ‘Quantum Space’ is Baggott’s fascinating attempt to push LQG into the limelight he feels it deserves.

As you’d expect from a heavyweight science writer, ‘Quantum Space’ isn’t to be taken lightly, despite at first glance seeming to occupy the fringes of current thinking. The idea that LQG is somehow a ‘green ink’ notion has nothing to do with the science itself, he says. It’s more to do with the fact that the string theorists have run a better media campaign, creating a bias that sometimes spins out into the widely held opinion that string theory is somehow the only valid option on the table when it comes to reconciling quantum mechanics with general relativity.

There are currently 30 teams in universities worldwide researching LQG, principal among which - and a duo whose ideas form the core of Baggott’s argument - are Lee Smolin and Carlo Rovelli. These are serious scientists. While others have left the field in despair, Smolin and Rovelli soldier on. Says the author: “I want to be straight with you about one thing. Like the string or M-theory framework, LQG is still a work in progress. It’s not finished and we don’t yet have all the answers.”

Given that even the most probing of scientific minds will have barely given loop quantum gravity a second thought, it’s hard to see how much traction Baggott will get with his evangelising of the subject. Chasing a theory of quantum gravity is, to use his words, “not for the faint of heart”.

And yet, science has a habit of granting good fortune to the brave underdog. It might just be that with ‘Quantum Space’ the author will realise his aim of at least giving string theory’s only and little-known competitor a hearing in our search for an understanding of the structure of space, time and the universe.

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