The equation was “achingly beautiful”, as theoretical physicist Frank Wilczek later described it: like Einstein’s equation of general relativity, the Dirac equation was universal yet fundamentally simple; nothing in it could be changed without destroying its power.
The Strangest Man, by Graham Farmelo
Dirac’s equation, published in 1928, describes the relativistic and quantum behavior of an electron and predicts the existence of electron’s antiparticle – the positron. Although Dirac made other important contributions to physics, as the Nobel Prize Award summarized:
The importance of Dirac’s work lies essentially in his famous wave equation, which introduced special relativity into Schrödinger’s equation. Taking into account the fact that, mathematically speaking, relativity theory and quantum theory are not only distinct from each other, but also oppose each other, Dirac’s work could be considered a fruitful reconciliation between the two theories.
If Dirac’s greatest achievement can be distilled into one short equation he published in his mid-twenties, what could be so interesting in hundreds of pages of his biography?
Farmelo’s answer is that P.A.M. Dirac (1902-1984) was a very strange man. Arguably anyone who dedicates his life to search for a beautiful quantum field theory must have, to put it politely, an unusual psyche. Dirac was considered odd even by theoretical physicists’ standards, not only by cynics like Pauli, but also by fatherly figures like Einstein and Bohr.
The biography with the long title The Strangest Man, the Hidden life of Paul Dirac, Mystic of the Atom (US edition; the UK edition is titled The Strangest Man, the Hidden life of Paul Dirac, Quantum Genius), illustrates Dirac’s unusual behavior and outlook through his relations with his nearest family, his approach to research, his quick ascent as mathematical-physicist (Dirac “was not quite thirty and just a few months older than Newton’s age” when he took the Chair of Lucian Professor in Cambridge University), his attitude toward other physicists and their work. For me, with the exception of lengthy description of Dirac’s childhood and shorter posthumous speculations about Dirac’s possible autism, the book was a fascinating read.
Most of all I enjoyed how Farmelo described the different places Dirac visited and worked at and the physicists whose names one mostly encounters as a part of some principle or equation. In the book, greats like Pauli, Heisenberg, Bohr and first and foremost Dirac are not just formidable geniuses whose work changed the course of history, but they are also actual people faced with a rapidly changing world. Through Dirac’s travels between Copenhagen, Göttingen, Princeton and other hubs of physics, Farmelo shows that even those with secured positions in the most renowned universities, did not work in vacuum. At the beginning of Dirac’s career, aspiring theoretical physicists congregated to Göttingen. In the 1930s, Einstein, Pauli and many others who had to flee Germany, moved to the Institute of Advanced Studies in Princeton. The United States became a magnet for scientific talent before and after WWII, but the Cold War, once again, changed the atmosphere. In 1955, Dirac’s application for an American visa was refused. In 1971, two years after his retirement as the Lucian Professor in Cambridge, Dirac moved to Florida State University.
Dirac’s biography offers many anecdotes about this reticent and private man, whose formidable intuition led to creation of quantum field theory and to prediction of a new type of matter – the antimatter. It is worth to notice that Dirac stuck to his results when they were in vogue and when no one else believed in them. Throughout a meeting in Copenhagen in April 1932,
Dirac had to put up with Bohr’s hostile questioning and the taunts of other colleagues. Dirac appeared to take it all on the chin; according to one colleague, during the meetings that week he did not utter a word. In the final session of the meeting, Bohr lost patience and put him on the spot:
“Tell us, Dirac, do you really believe in that stuff?”
The room went silent, and Dirac stood briefly to intone his twelve-word reply:
“I don’t think anybody has put forward any conclusive argument against it.”
The much criticized “theory of holes” predicted the existence of a new, and then unsupported by any experimental evidence, “counterpart”of an electron—a positive charged particle with the same mass as electron. The positron was discovered in 1932 by C. Anderson, when he studied tracks of cosmic ray particles in a cloud chamber (Nobel Prize for 1936). Also in 1932, Blackett showed that pairs of electrons and positrons could be formed out of photons with sufficiently high energy (Nobel Prize 1948). Dirac received the Nobel Prize in 1933.
Commemorative marker in Westminster Abbey, London
Dirac’s equation (second line from below on the commemorative marker) looks deceptively unintimidating, but only the electron mass (denoted by m) has a meaning for a non-specialist. For those interested, an explanation and derivation of the equation can be found in those videos: