### The Strangest Man

#### A review of The Strangest Man by Rob Mason

Paul Dirac is an enigma. Unquestionably the greatest British theoretical physicist of this century, a Nobel Laureate at the age of 31, he ranks alongside Newton and Maxwell.

In 2009, Graham Farmelo published ‘The Strangest Man’, which won the 2009 Costa Prize for Biography and the 2009 'Los Angeles Times Science and Technology Book Prize'. The book was chosen by Physics World as the physics book of the year in 2009, when it was selected as one of Nature’s books of the year.

"I found the best ideas usually came, not when one was actively striving for them, but when one was in a more relaxed state… I used to take long solitary walks on Sundays, during which I tended to review the current situation in a leisurely way. Such occasions often proved fruitful, even though (or perhaps, because) the primary purpose of the walk was relaxation and not research".—* Paul A. M. Dirac*

One idea that Dirac ostensibly conceived is now the basis of all modern descriptions of the fundamental constituents of the universe. Such descriptions are based on the nineteenth – century concept of a “field,” which had superseded Newton’s vision that nature‘s basic particles move under the influence of forces exerted by other such particles, often over long distances. Physicists replaced the notion that the Sun and the Earth exert gravitational forces on each other by the more effective picture that the Sun, the Earth and all the other matter in the universe collectively give rise to a gravitational field which pervades the entire universe and exerts a force on each particle, wherever it is located. Likewise, electromagnetism and Einstein’s theory of gravity are examples of classical ‘field’ theory, each featuring a field that varies smoothly throughout space and time, not mentioning individual quanta. Such classical theories describe the universe in terms of a smooth, underlying fabric. Yet, according to quantum theory, the universe is fundamentally granular: it is made of tiny particles such as electrons and photons. Loosely speaking, the texture of the underlying fields should, according to classical ideas, be rather like a smooth liquid, whereas quantum theory suggests that it would be like a vast collection of separate grains of sand. To find a quantum version of Maxwell’s classical electromagnetism was one of the theoreticians most pressing problems, and Dirac’s next innovation was to solve it.

Dirac found a way of mathematically describing the creation and destruction of photons, both commonplace processes. Particles of light are continually created in vast numbers all over the universe in stars and also here on Earth, when an electric light is switched on, a match is struck, a candle is lit. Likewise, photons are continually destroyed – annihilated – for example, when they disappear into human retinas and when leaves convert sunlight to life giving energy. Neither of these processes of creation and annihilation can be understood using Maxwell’s classical theory, which has no way of describing things that appear out of nowhere or disappear into oblivion. Nor did ordinary quantum mechanics have anything to say in detail about the process of emission or absorption. Yet Dirac showed that this wizardry can be described in a new type of theory, a compact, mathematical description of the creation and destruction of photons. He associated each creation with a mathematical object, a creation operator, which is closely related to, but quite distinct from another object associated with an annihilation, an annihilation operator.

Dirac had begun to set out a quantum version of Maxwell’s unified field theory of electricity and magnetism. He had learned about that theory only three years before, in Cunningham’s lectures in Cambridge, and was now standing on Maxwell’s shoulders. So far as Dirac was concerned, his theory put an end to the hand-wringing about the ostensible conflict between two theories of light: a wave theory seemed to account for propagation, while a particle theory was needed to explain the interaction with matter. The new theory avoided the embarrassment of having to choose between the wave and particle description and replaced the two sharply contrasting pictures with a single, unified theory. Evidently pleased with himself, Dirac wrote that the pictures were in complete ‘harmony.’

In his paper, Dirac applied his theory and compared his results with the successful predictions Einstein had made a decade before, in 1916. Einstein had used old quantum ideas to calculate the rate at which atoms can emit and absorb light, producing formulae that appeared to describe these processes successfully. The question Dirac had to answer was: does the new theory compare favourably with Einstein’s?

Einstein’s theory had accounted for the interaction of light and matter in terms of three fundamental processes. Two of them were familiar enough: the emission and absorption of a photon by an atom. But Einstein also predicted a previously unknown way of ‘persuading’ an atom to jump from one energy level to a lower one, by stimulating it with another photon whose energy levels exactly equal to the difference between the two energy levels. The result of this process of ‘stimulated emission’ is that two photons emerge from the atom: the original one and another one given out when the atom jumps to the lower level. This process takes place in the ubiquitous laser – there is at least one in every in every CD and DVD player and in every bar-code reader – and so is the most common technological application of Einstein’s science. Dirac’s theory produced exactly the same formulae as Einstein’s and had the other advantage that it was more general and mathematically more coherent. As he probably realised, he had gone one better than Einstein

*Notes from the Book: "The Strangest Man by Graham Farmelo*

'In science one tries to tell people, in such a way as to be understood by everyone, something that no one ever knew before. But in the case of poetry, it's the exact opposite!'. *Paul Dirac*

https://www.youtube.com/watch?v=YfYon2WdR40

http://www.nytimes.com/2009/09/13/books/review/Gilder-t.html