In an unexpected twist, Einstein also failed to appreciate the significance of the cosmological constant that he discarded. Later researchers saw that it could exist even in a universe that was not static. In 1998 astronomers discovered that the expansion rate of the universe was increasing, driven by an unknown repulsive force. Physicists call this force dark energy, and it seems to behave just like the cosmological constant. Einstein’s cast-off mathematical adjustment may thus be crucial to understanding the fate of the universe.
Einstein’s efforts to find a unified theory were doomed because he knew nothing of two fundamental forces. Yet the spirit of his quest lives on.
Einstein’s problems with relativity were nothing compared with the passionate manner in which he rejected the legacy of his ideas in quantum physics. In the early years of the 20th century, when most physicists still thought of light as a wave, Einstein was one of the few who believed in particles of light, or photons, and he tried to visualize how light could have both wave and particle aspects. During a 1909 lecture in Salzburg, Austria, he said, “I more or less imagine each such singular point [of light] as being surrounded by a field of force which has essentially the character of a plane wave.” In 1921 he won a Nobel Prize—his only one—for explaining the photoelectric effect, in which particles of light eject electrons from the surface of a substance. This is the principle exploited in solar cells today.
Despite his early advocacy of the quantum nature of reality, by the 1920s Einstein had begun voicing serious doubts about the theory. He objected to the seeming randomness of quantum mechanics, famously asserting that God does not play dice with the universe. The statistical nature of reality at the subatomic level is now well established, however. Einstein was wrong.
A fundamental reason for Einstein’s distrust of quantum physics concerned a startling property that he recognized before anyone else: It allows for instantaneous interactions between two objects, no matter how far apart they might be. In this case his insight was correct, but his understanding missed the mark.
In a 1935 paper titled “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” Einstein and two colleagues, Nathan Rosen and Boris Podolsky, presented what they considered a fatal blow to the prevailing interpretation of quantum theory. They argued that in principle it would be possible to carefully prepare two quantum particles—say, two electrons—in such a way that the properties of the two electrons were linked together, or “entangled.” According to quantum theory, a particle can simultaneously occupy many different states at once, settling into a single state only when it is observed. With two entangled particles, Einstein and his colleagues argued, quantum mechanics predicts that observing the properties of one electron would instantaneously determine the properties of the other, even if the two electrons were at opposite ends of the universe. In other words, quantum mechanics allowed for instantaneous interactions, a violation of the laws of special relativity, not to mention common sense.
Einstein was convinced that what he called “spooky action at a distance” was the result of some as yet undiscovered laws of nature, and that a better theory would explain the mystery without resorting to faster-than-light physics. But since Einstein’s death in 1955, the spooky instantaneous interactions he decried have repeatedly been shown to be real. They may even open the door to a form of teleportation (see “Teleportation? Very Possible. Next Up: Time Travel.” by Michio Kaku). Whether a more fundamental theory will ever replace quantum mechanics is still an open question, but the consensus today is that any such theory will retain the weird interconnectedness that Einstein found so objectionable.
In quantum mechanics as in cosmology, Einstein was so confounded by ideas that other researchers derived from his theories that he sought a way out. “The idea that you could have something here that could instantaneously affect something on the other side of the moon or the other side of the galaxy was to him something like telepathy, not physics,” says Antony Valentini, a physicist at Imperial College London. “He saw that if you believed quantum theory was a final theory, that is where it took you. So he thought, ‘Forget quantum theory; let’s try to develop a better theory.’”
Einstein spent much of the last three decades of his life searching for this alternative to quantum mechanics. He never succeeded. “So the story for most of the 20th century was that people thought Einstein’s later efforts were silly—what on earth was he doing, giving up work on quantum theory?” Valentini says. “But they didn’t see what he had seen. It’s not that other people were more brave in accepting quantum mechanics; they just didn’t see what was coming.”
We now know that Einstein’s efforts to find an all-encompassing “unified field” theory were doomed because he knew nothing about two fundamental forces within the nuclei of atoms, forces that came to be understood only after his death. Yet the search for a theory that would unify all of physics continues to occupy thousands of researchers around the world. The spirit, if not the style, of Einstein’s quest for a unified field theory lives on.
Still, Einstein’s missed connections serve as a warning to today’s physicists. Are they, too, oblivious to the limits of their vision, working with an incomplete understanding of some fundamental aspect of nature? “Yes, it’s perfectly possible,” Rovelli says. “If you look at the history of physics, almost every epoch has the feeling that ‘we know everything now.’ I think what we don’t know is probably huge. We are still far away from knowing all the ingredients.”
Perhaps if Einstein had lived longer he might have come to accept quantum mechanics, making peace with his most unwelcome scientific progeny. “It’s difficult to say, because Einstein changed his mind so many times on so many subjects,” Rovelli says. “That’s a sign of a great scientist. A great scientist is not somebody who believes his own ideas; it’s somebody who does not believe his own ideas. He’s ready to change his mind. What Einstein did was bring back science to its true soul, which is to change our view of the world, not just explain things. Einstein reminded us that what we don’t know is much more than what we know.”