Paul Dirac鈥檚 prediction of antimatter was the greatest triumph of modern physiocs. Applying relativity to quantum theory, he argued in 1931 that there should exist an anti-electron, and that it would have the same mass as the electron but the opposite charge. The following year, an experimenter who knew nothing of Dirac鈥檚 prediction was the first to observe the anti-electron among the cosmic rays raining down from the californian summer sky. This discovery established Dirac as a hero of modern science. Imagine how he must have felt 50 years later when he learned that someone else had foreseen the positive electron almost a decade before him. Dirac, however, had done something similar himself. In 1956, two Chinese-American physicists made a prediction that, when confirmed the following year, became the most sensational discovery of the decade. Dirac had forseen it seven years earlier.
ANTIMATTER has been around since the beginning of time. But physicists only became aware of it 70 years ago. That first awareness dawned not in a lab but in the mind of the English theoretical physicist Paul Dirac, who predicted the existence of the anti-electron. What he didn鈥檛 find out until 1980, near the end of his life, was that his brilliant prediction had already been made by someone else.
Dirac predicted the anti-electron using an equation for the behaviour of the electron, which he had conjured up at Cambridge University when he was a 25-year-old researcher in quantum physics. Almost from nowhere, he concocted a mathematically beautiful equation that gave a quantum description of the electron that was consistent with Einstein鈥檚 special theory of relativity. After a few pages of algebra, he demonstrated that the equation accounted exquisitely for both the electron鈥檚 spin and its magnetism.
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But there was a problem with the Dirac equation: although its description of the electron was uncannily accurate, it seemed to predict the seemingly impossible 鈥� that electrons can have negative kinetic energy. For most of Dirac鈥檚 peers, this prediction was an unconscionable embarrassment, but he persevered with it. And by the end of 1929, he had come up with an ingenious way out of the difficulty.
His proposal relied on the exclusion principle discovered a few years earlier by Wolfgang Pauli. This says that no two electrons can occupy the same quantum state.
Dirac鈥檚 idea was that 鈥渆mpty space鈥� actually contains electrons that obey the negative-energy solutions of the equation. He suggested that the negative-energy states are normally 鈥渇ull鈥�, just like the electrons that fill up the low-energy states of heavy atoms, according to the Pauli principle. This means that positive-energy electrons can鈥檛 make transitions to these negative-energy states, explaining why ordinary electrons don鈥檛 continually disappear into space.
For many, this tale was too far-fetched to be believable. But Dirac was not finished yet. He proposed that if a negative-energy electron were for some reason 鈥渄isplaced鈥�, experimenters should be able to observe it as a positive-energy particle with a positive charge. (The empty space will have become less negative, and therefore more positive.) At first, he identified these holes as protons, but by 1931 he had changed his mind and come to a much more radical conclusion: 鈥淎 hole, if there was one, would be a new kind of elementary particle, unknown to experimental physics, having the same mass and opposite charge to the electron.鈥� Despite Dirac鈥檚 reputation as an ace theorist, his prediction was widely ignored. But not for long.
A year later, on 2 August 1932, the American experimenter Carl Anderson observed among the cosmic rays raining down on Pasadena a 鈥減article鈥� that seemed to have the opposite charge to that of the electron but a 鈥渃omparable mass鈥�. Anderson and Dirac were oblivious of each other鈥檚 work and so neither the theoretician nor the experimenter realised they were the first people to catch a glimpse of antimatter.
It was not until the next year back at Cambridge that Patrick Blackett and Giuseppe Occhialini put two and two together to conclude that Anderson had verified Dirac鈥檚 prediction of the anti-electron and so become the first person to observe antimatter. By then, the editor of the journal that published Anderson鈥檚 photograph of the anti-electron鈥檚 track had dubbed the particle a 鈥減ositron鈥�.
By the time Dirac came to collect his Nobel prize in physics, towards the end of 1933, he was in the enviable position of the theorist who has been proved right after his colleagues had almost unanimously dismissed his most imaginative work as misguided and even perverse. His hole theory was soon superseded by quantum field theory (which he did much to invent and came to abominate), so one can only wonder at his ability to use a wrong theory to produce one of the most triumphant predictions of modern science.
From the early 1970s until his death in 1984, Dirac toured the world lecturing on the importance of mathematical beauty in the equations of fundamental physics and how he used his beautiful equation to predict antimatter. It was during a conference in May 1980 at Fermilab, near Chicago, that he learned he had not, after all, been the first to make the most celebrated prediction of modern physics.
Dirac had travelled up from his home in Tallahassee in Florida with a nasty bout of flu to attend a conference on the history of particle physics, where he gave a talk on one of his favourite subjects, the history of quantum field theory. The bombshell came when he had finished speaking. The Nobel-prizewinning experimenter Leon Lederman remembers the occasion vividly. Speaking to 麻豆传媒 about it last week, he recalled what he describes as 鈥渢he quintessential Dirac talk: it lasted over an hour, he used no notes and the content poured out like heavy cream鈥�.
Lederman remembers that when the floor was thrown open to questions, the veteran physicist Victor Weisskopf made a quite astonishing remark. Albert Einstein, he said, had speculated about the existence of a positive electron in 1922. 鈥淒irac waved his hand dismissively, adding 鈥楬e was lucky鈥�,鈥� says Lederman. Surprisingly, no one in the audience probed further to ask what had led Einstein to make this prediction and, as Weisskopf died in April this year, we may never know.
Einstein would surely have agreed with Dirac鈥檚 response. It is one thing to guess that a particle might exist, quite another to deduce its existence from well-founded principles. The credit should go to the person who makes the advance and appreciates its meaning and significance. This is why Einstein is rightly regarded as the principal author of relativity theory, even though the French mathematician Henri Poincar茅 foresaw many elements of the theory several years before him.
If Dirac felt a little irritated when he first heard of Einstein鈥檚 suggestion of the positive electron, he might have consoled himself with the thought of his similar coup of far-sightedness rather later. This concerned the prediction in 1956 by the great Chinese-American physicists Tsung Dao (T. D.) Lee and C. N. 鈥淔rank鈥� Yang of a fundamental difference between left and right when subatomic particles interact through the weak force (the force responsible for radioactive beta decay). When Chien-Shiung Wu of Columbia University confirmed their prediction the following year, the news featured on the front page of The New York Times, caused pandemonium in the normally equable world of physics, and earned Lee and Yang a Nobel prize.
Dirac was one of the few who were not dumbfounded by the discovery. In 1949 he had written that, in his view there was no need to believe in the almost universally held prejudice that nature has a perfect left-right symmetry at the fundamental level. A year earlier, he had made the same point during the oral examination of Kenneth Le Couteur鈥檚 PhD. Dirac asked the hapless student why the fundamental equations of physics should necessarily feature no difference between left and right. When the student showed that, like almost every other physicist in the world at the time, he could not give a reasoned answer, Dirac looked at him sadly. With his customary verbal economy, he remarked, 鈥淲ell, it is not so for living matter.鈥�
With characteristic modesty and taciturnity, Dirac spent no time arguing for the precedence of his discoveries. Nor was this necessary, as his peers well knew that his was a unique talent, an extraordinary ability to divine the workings of the Universe as if he were privy to what Stephen Hawking has described as 鈥渢he mind of God鈥�.