*Everything that can happen does happen*, which is a phrase they reiterate throughout the book. Cox is best known as a TV science presenter for BBC. His series on the universe can be highly recommended. His youthful and conversational delivery, combined with an erudite knowledge of physics, makes him ideal for television. The same style comes across in the book despite the inherent difficulty of the topic.

In the last chapter, an epilogue, he mentions writing in September 2011, so this book really is hot off the press. Whilst the book is meant to cater for people with a non-scientific background, I’m unsure if it succeeds at that level and I’m not in a position to judge it on that basis. I’m fairly well read in this area, and I mainly bought it to see if they could add anything new to my knowledge and to compare their approach to other physics writers I’ve read.

They reference Richard Feynman (along with many other contributors to quantum theory) quite a lot, and, in particular, they borrow the same method of exposition that Feynman used in his book,

*QED*. In fact, I’d recommend that this book be read in conjunction with Feynman’s book even though they overlap. Feynman introduced the notion of a one handed clock to represent the phase, amplitude and frequency of the wave function that lies at the heart of quantum mechanics (refer my post on

**Schrodinger’s equation**, May 2011).

Cox and Forshaw use this same analogous method very effectively throughout the book, but they never tell the reader specifically that the clock represents the wave function as I assume it does. In fact, in one part of the book they refer to clocks and wave functions independently in the same passage, which could lead the reader to believe they are different things. If they are different things then I’ve misconstrued their meaning.

Early in their description of clocks they mention that the number of turns is dependent on the particle’s mass, thus energy. This is a direct consequence of Planck’s equation that relates energy to frequency, yet they don’t explain this. Later in the book, when they introduce Planck’s equation, they write it in terms of wavelength, not frequency, as it is normally expressed. These are minor quibbles, some might say petty, yet I believe they would help to relate the use of Feynman’s clocks to what the reader might already know of the subject.

One of the significant facts I learnt from their book was how Feynman exploited the ‘least action principle’ in quantum mechanics. (For a brief exposition of the least action principle refer my post on

**The Laws of Nature**, Mar. 2008). Feynman also describes its significance in gravity in

*Six-Not-So-Easy Pieces*: the principle dictates the path of a body in a gravitational field. In effect, the ‘least action’ is the difference between the kinetic and potential energy of the body. Nature contrives that it will always be a minimum, hence the description, ‘principle of least action’.

Now, I already knew that Feynman had applied it to quantum mechanics, but Cox and Forshaw provide us with the story behind it. Dirac had written a paper in 1933 entitled ‘The Lagrangian in Quantum Mechanics’ (the Lagrangian is the mathematical formulation of least action). In 1941, Herbet Jehle, a European physicist visiting Princeton, told Feynman about Dirac’s paper. The next day, Feynman found the paper in the Princeton library, and with Jehle looking on, derived Schrodinger’s equation in one afternoon using the least action principle. Feynman later told Dirac about his discovery, and was surprised to learn that Dirac had not made the connection himself.

But the other interesting point is that the units for ‘action’ in physics are mx

^{2}/t which are the same units as Planck’s constant,

*h*. In other words, the fundamental unit of quantum mechanics is an ‘action’ unit. Now, units are important concepts in physics because only entities with the same type of units can be added and subtracted in an equation. Physicists talk about dimensions, because units must have the same dimensions to be able to be combined or deducted. The dimensions for ‘action’, for instance, are 1 of mass, 2 of length and -1 of time. To give a more common example, the dimensions for velocity are 0 of mass, 1 of length and -1 of time. You can add and subtract areas, for example, (2 dimensions of length) but you can’t add a length to an area or deduct an area from a volume (3 dimensions of length). Obviously, multiplication and calculus allow one to transform dimensions.

One of the concepts that Cox and Forshaw emphasise throughout the book is the universality of quantum mechanics and how literally everything is interconnected. They point out that no 2 electrons can have exactly the same energy, not only in the same atom but in the same universe (the Pauli Exclusion Principle). Also individual photons can never be tracked. In fact, they point out a little-known fact that Planck’s law is incompatible with the notion of tracking individual photons; a discovery made by Ladislas Natanson as far back as 1911. No, I’d never heard of him either, or his remarkable insight.

Cox and Forshaw do a brilliant job of explaining Wolfgang Pauli’s famous principle that makes individual atoms, and therefore matter, stable. They also expound on Freeman Dyson’s and Andrew Leonard’s 1967 paper demonstrating that it’s the Pauli Exclusion Principle that stops you from falling through the floor. Dyson described ‘the proof as extraordinarily complicated, difficult and opaque’, which may help to explain why it took so long for someone to derive it.

They also do an excellent job of explaining how quantum mechanics allows transistors to work, which is arguably the most significant invention of the 20th Century. In fact, it’s probably the best exposition I’ve come across outside a text book.

But what comes across throughout their book, is that the quantum world obeys specific ‘rules’ and once you understand those rules, no matter how bizarre they may seem to our common sense view of the world, you can make accurate and consistent predictions. The catch is that probability plays a key role and deterministic interpretations are not compatible with the quantum universe. In fact, Cox and Forshaw point out that quantum mechanics exhibits true ‘randomness’ unlike the ‘chaotic’ randomness that is dependent on ultra-sensitive initial conditions. In a recent issue of

*New Scientist*, I came across someone discussing free will or the lack of it (in a book review on the topic) and espousing the view that everything is deterministic from the Big Bang onwards. Personally, I find it very difficult to hold such a philosophical position when the bedrock of the entire physical universe insists on chance.

Cox and Forshaw don’t have much to say about the philosophical implications of quantum mechanics except in one brief passage where they reveal a preference for the 'many worlds' interpretation because it does away with the so-called ‘collapse’ or ‘decoherence’ of the wave function. In fact, they make no reference to ‘collapse’ or ‘decoherence’ at all. They prefer the idea that there is an uninterrupted history of the quantum wave function, even if it implies that its future lies in another universe or a multitude of universes. But they also give tacit acknowledgement to Feynman’s dictum: ‘…the position taken by the “shut up and calculate” school of physics, which deftly dismisses any attempt to talk about the reality of things.’

In the epilogue, Cox and Forshaw get into some serious physics where they explain how quantum mechanics gives us the famous Chandrasekhar limit, developed by Subrahmanyan Chandresekhar in 1930, which determines how big a star can be before it becomes a neutron star or a black hole. The answer is 1.4 solar masses (1.4 times the mass of our sun). Mind you, it has to go through a whole series of phases in between and that’s what Cox and Forshaw explain, using some fundamental algebra along with some generous assumptions to make the exposition digestible for laypeople. But the purpose of the exercise is to demonstrate that quantum phenomena can determine limits on a stellar scale that have been verified by observation. It also gives a good demonstration of the scientific method in practice, as they point out.

This is a good book for introducing people to the mysteries of quantum mechanics with no attempt to side-step the inherent weirdness and no attempt to provide simplistic answers. They do their best to follow the Feynman tradition of telling it exactly as it is and eschew the magic that mysteries tend to induce. Nature doesn’t provide loop holes for specious reasoning. Quantum mechanics is the latest in a long line of nature’s secret workings, mathematically cogent and reliable, but deeply counter-intuitive.