Albert Einstein died in 1955. But, arguably, he died as a scientist three decades earlier. For it was then that the genie he let out of the bottle in 1905 finally escaped from his control. The genie was the “photon”, the bullet-like particle that Einstein invented to explain how light was able to knock electrons from metals and which he claimed was spat out and swallowed by atoms. His self-proclaimed “reckless hypothesis” would eventually overturn everything he ever believed in. The poignant moment when the tide of scientific history turned against the greatest physicist of the 20th century is marked by a long-forgotten paperthat Einstein delivered at the Brazilian Academy of Sciences on 7 May 1925.
NOT many people know that Albert Einstein visited Brazil. Only a very few biographies of the great man even mention the fact. The trip came about because in 1922, Einstein joined Marie Curie and Hendrik Lorentz in taking up a position with the League of Nations’ International Committee on Intellectual Cooperation, the forerunner of UNESCO. And, at an ICIC meeting, he ran into an Argentinian journalist and writer called Leopoldo Lugones, who begged him to lecture in his home country. Einstein accepted and, with just his trusty violin for company, he set sail from Hamburg on 5 March 1925.
Einstein’s three-month-long South American trip took him not only to Argentina but also to Uruguay, and eventually, at the end of his gruelling tour, he arrived in Brazil. Einstein was particularly pleased to be there because of the role Brazilian scientists had played in verifying his theory of gravity, the general theory of relativity. During the total eclipse of the sun in 1919, their confirmation that starlight passing the sun was bent by its gravity by precisely the amount Einstein predicted, helped propel him into the scientific stratosphere, fêted as the greatest scientist since Isaac Newton, a global superstar who outshone even Charlie Chaplin. “The problem conceived in my head was solved by the luminous sky of Brazil,” he told his Brazilian hosts.
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And, if Einstein was pleased to be in Brazil, Brazil was just as pleased to see Einstein: on the night of 7 May, the auditorium at the Brazilian Academy of Sciences was packed. Almost certainly, the scientists who had assembled in Rio de Janeiro expected Einstein to talk about relativity, the subject of every other talk on his South American tour. Einstein, however, had other ideas.
To Einstein, relativity was nothing more than a natural extension of classical, or 19th-century, physics. In his opinion, the only “revolutionary” thing he had ever done in his life was come up with the idea of the photon. And it was this he chose to talk about in Rio.
In fact, the term photon is a relatively modern one, coined by the American chemist Gilbert Lewis in the journal Nature only in 1926. Einstein instead talked of the quantum. This was the term used in 1900 by Max Planck, who explained the character of the light radiated by a hot body by assuming that its atoms vibrated with energies that were multiples of a fundamental chunk, or quantum, of energy. Planck never thought of quanta as anything more than mathematical conveniences. Einstein alone saw them as real – tiny bullets of energy which could fly through empty space and which were swallowed up and spewed out by atoms.
The idea was revolutionary because there was overwhelming evidence that light was a wave, like a ripple spreading on the surface of a pond. But a wave was spread out through space and a particle was a discrete entity. How could the two be compatible? This was the problem Einstein had been wrestling with for 20 years while the rest of the world’s physicists looked on – and remained unconvinced. In Rio, he confessed that he still had not found an answer.
The paper Einstein gave that night has survived because Arthur Getúlio das Neves, head of the Einstein reception committee, had it translated into Portuguese and published in the proceedings of the Brazilian Academy of Sciences. Neves died in 1928 and the manuscript was forgotten. However, in the 1990s, his grandson, Jorge Getúlio Veiga found the paper and sent a copy to the Einstein Archive. Only now has it finally been translated into English by Richard Campos, a historian at the City University of New York.
What Einstein discussed in Rio was a suggestion by the Danish physicist Niels Bohr that it might be possible to explain the properties of light without invoking the photon. The price would be abandoning the cherished idea that energy can never be created or destroyed and accepting that the conservation of energy was merely a statistical thing, something which nature observed only “on average”. In Berlin at that time, Hans Geiger and Walther Bothe were carrying out experiments to see whether the idea could explain the way light was scattered off electrons. The experiments were not complete, Einstein told his audience, but they already appeared to rule out Bohr’s idea, strengthening the case for the photon. And that meant the problem of understanding the photon remained.
Although Einstein had been unable to explain how a photon might behave as both a wave and a particle, he had shown that the equations that described a spread-out light wave needed to contain localised “knots”, which were absorbed and emitted by atoms like tiny bullets. But try as he might, he could not obtain a mathematical picture of the photon that explained its two contradictory properties.
The reason was that it was impossible – at least with the classical physics Einstein was using. And, across the Atlantic in Germany, one young man had realised that. His name was Werner Heisenberg, and he was preparing for a trip to the fog-shrouded island of Helgoland in the North Sea. Far from the bustle of university life, he planned to do some serious thinking to try to make sense of what nature was telling physicists about the microscopic world of atoms. There, just a month after Einstein’s Rio talk, in a creative surge to rival any of Einstein’s, Heisenberg would invent a new physics totally incompatible with classical physics, called quantum theory.
Quantum theory had existed before but only as an ad hoc mixture of classical physics and arbitrary “quantum postulates”. Heisenberg’s “new” quantum theory, with its matrices and its contempt for anything that was not “observable”, broke free of the constraints of classical physics forever and took physicists into uncharted territory the like of which they had never dreamed of.
What Einstein could not see – did not want to see – was that the photon could never be made compatible with classical physics because it was not a classical thing. It was something else, neither a particle nor a wave but something with absolutely no analogue in the everyday world, which in some circumstances could display the characteristics of a wave and in others the properties of a particle.
To his dying day, Einstein would never accept Heisenberg’s “quantum theory”, which replaced certainty with uncertainty, the 100 per cent predictability of classical physics with capricious, random unpredictability. “God does not play dice with the universe,” he petulantly declared. “Ironically, the price paid for the photon’s final assurance remained unacceptable to Einstein for the rest of his life,” says Campos.
The significance of the talk in Rio is that it shows Einstein still desperately hoping that the genie he set free in 1905 could be tamed with the old, classical, physics, at the very moment in Europe that the genie was on the verge of escaping from the bottle forever. It was the very last time Einstein spoke on the problem of the photon before the advent of the solution – modern quantum theory.
On the last day of his exhausting tour of South America, Einstein wrote: “Free at last, but more dead than alive.” It was almost a premonition. When he boarded the boat for Europe, Einstein was heading for oblivion as a front-rank scientist. Within weeks he would be outside the mainstream of physics. For the new quantum theory would turn out to be the most successful physical theory ever devised. And Einstein, who would never embrace it, would be reduced to firing pot-shots at it for the rest of his life.