Through Two Doors at Once - Anil Ananthaswamy *****

It's sometimes hard to imagine that there's anything new to say about the basics of quantum physics, yet Anil Ananthaswamy manages this in a twofold manner (appropriately, given the title). Through Two Doors at Once does so by using the double slit experiment as a constant reference point throughout the book, and by bringing in a number of the more modern variants on the experiment which rarely feature in popular accounts of quantum theory.

Strictly, the book should probably be called 'Through Two Doors at Once and Spooky Action at a Distance plus Things That Have a Similar Effect', as it uses both the double slit experiment and the EPR entanglement thought experiment, plus modern experiments which don't, for example, involve slits but rather beam splitters that are their logical equivalent - but I have to admit, that would be a clumsy title.

Ananthaswamy gives us a good overview of the development of quantum physics - sometimes quite summary - but by making repeated use of the double slit, going all the way back to Thomas Young, but also looking at the quantum specifics, he both helps the reader get a better feel for just why quantum physics can seem strange and also what the different interpretations, from Copenhagen to Many Worlds, tell us about what we can and can't know of what's happening inside the experiment.

The part of the book covering interpretations is perhaps slightly less effective than the rest, because, in the end, unless you are an enthusiast for a particular interpretation, the diversity of ideas tends to obscure, rather than help get a better understanding. (We still have to come back to Feynman's crushing '[Y]ou think I’m going to explain it to you so you can understand it? No, you’re not going to be able to understand it... You see, my physics students don’t understand it either. This is because I don’t understand it. Nobody does.')

The best part of this section is the explanation of the Bohm/deBroglie interpretation where there is both a wave and a particle, though there is one minor problem here, as we are told that making a strong measurement in the Bohm model leaves particles where you don't expect them to be - but are not told why the strong measurement of the particle causing a scintillation on a screen does leave them where we expect them to be.

For me, the only real improvement would have been to put a bit more character into the historical context: it's rather dry and summary. So, for example, we are not told about Einstein's dismissive 'Ist mir Wurst' remark about EPR's confusing use of two measurements. Another example: John Wheeler is described as coining the term 'black hole', rather than giving us the more interesting actual story. There's enough to get the point, but it could have been made more engaging.

Overall, though, Anathanswamy cleverly comes at quantum physics from a different direction, and as a result, adds to the picture we get from most popular titles. We really get into why the double slit plus entanglement are often described as the central mysteries of quantum theory, and though they can still send the brain spinning, there's the best description of many of the more recent experiments I've seen - useful as they can seem a little pointless without this kind of in-depth picture. An excellent addition to the 'Quantum physics for the rest of us' shelf.

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Review by Brian Clegg