Buried in Albert Einstein’s mail one spring day in 1953 lay a letter from an ordinary mortal, a 20-year-old high school dropout named John Moffat. Two more disparate correspondents would be hard to imagine. Moffat was an impoverished artist and self-taught physicist. Einstein was a mythic figure—the world’s most famous scientist. Moffat was living with his British father and Danish mother in Copenhagen. Einstein was at the Institute for Advanced Study in Princeton, New Jersey. Yet both men were outsiders. In his later years, Einstein had become increasingly isolated from the physics community, refusing to embrace the strange but powerful theory of quantum mechanics—with its particles that are also waves and that exist in no specific place until they’re observed. Nature, he argued, couldn’t be so perverse. So for nearly 30 years he had pursued a quixotic goal: the creation of a unified field theory to describe all the forces of nature and to demystify the quantum world.
That was the occasion for Moffat’s letter. He thought he could offer Einstein some constructive criticism. “I wrote him to say that I wasn’t happy about what he was doing,” Moffat recalls. There was nothing unusual about this. Plenty of people sent letters to Einstein, not all of them rational. But in Moffat’s case something unexpected happened: Einstein wrote back.
“Dear Mr. Moffat,” the reply began. “Our situation is the following. We are standing in front of a closed box which we cannot open, and we try hard to discover about what is and is not in it.” That closed box is the universe, of course, and no one had done more to pry off the lid than Einstein. Yet in the eyes of nearly all his colleagues he had contributed almost nothing of importance to physics for almost 20 years.
Were they right? Did he squander his genius by chasing vainly after an ultimate theory? That is the conventional view. But at least a few physicists now argue that Einstein was far ahead of his time, raising questions that will challenge researchers for decades. “It’s often said that Einstein wasted his time later in life,” says Moffat, who went on to become a theoretical physicist. “This, of course, is erroneous. Einstein never wasted his time.”
Einstein’s split with mainstream physics came at the very height of his career. In 1927, when he was 48, the world’s leading physicists gathered at a conference in Brussels to debate an issue that remains contentious to this day: What does quantum mechanics have to say about reality? Einstein had won the Nobel Prize in physics for research that showed that light consists of particles of energy—research that laid the groundwork for quantum mechanics. Yet he dismissed the new theory out of hand. At the conference, he clashed with the great Danish physicist Niels Bohr, launching a feud that would last until Einstein’s death in 1955.
Bohr championed the strange new insights emerging from quantum mechanics. He believed that any single particle—be it an electron, proton, or photon—never occupies a definite position unless someone measures it. Until you observe a particle, Bohr argued, it makes no sense to ask where it is: It has no concrete position and exists only as a blur of probability.
Einstein scoffed at this. He believed, emphatically, in a universe that exists completely independent of human observation. All the strange properties of quantum theory are proof that the theory is flawed, he said. A better, more fundamental theory would eliminate such absurdities. “Do you really believe that the moon is not there unless we are looking at it?” he asked.
“He saw in a way more clearly than anyone else what quantum mechanics was really like,” British physicist Julian Barbour says. “And he said, ‘I don’t like it.’” In the years after the conference in Brussels, Einstein leveled one attack after another at Bohr and his followers. But for each attack Bohr had a ready riposte. Then in 1935 Einstein devised what he thought would be the fatal blow. Together with two colleagues in Princeton, Nathan Rosen and Boris Podolsky, he found what appeared to be a serious inconsistency in one of the cornerstones of quantum theory, the uncertainty principle.
Formulated in 1927 by the German physicist Werner Heisenberg, the uncertainty principle puts strict limits on how accurately one can measure the position, velocity, energy, and other properties of a particle. The very act of observing a particle also disturbs it, Heisenberg argued. If a physicist measures a particle’s position, for example, he will also lose information about its velocity in the process.
Einstein, Podolsky, and Rosen disagreed, and they suggested a simple thought experiment to explain why: Imagine that a particle decays into two smaller particles of equal mass and that these two daughter particles fly apart in opposite directions. To conserve momentum, both particles must have identical speeds. If you measure the velocity or position of one particle, you will know the velocity or position of the other—and you will know it without disturbing the second particle in any way. The second particle, in other words, can be precisely measured at all times.
Einstein and his collaborators published their thought experiment in 1935, with the title “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” The paper was in many ways Einstein’s swan song: Nothing he wrote for the rest of his life would match its impact. If his critique was right, quantum mechanics was inherently flawed.
Bohr argued that Einstein’s thought experiment was meaningless: If the second particle was never directly measured, it was pointless to talk about its properties before or after the first particle was measured. But although quantum physics eventually carried the day, it wasn’t until 1982, when the French physicist Alain Aspect constructed a working experiment based on Einstein’s ideas, that Bohr’s argument was vindicated. In 1935 Einstein was convinced that he had refuted quantum mechanics. And from then until his death 20 years later, he devoted nearly all his efforts to the search for a unified field theory.