“This onslaught came down on us as a bolt from the blue,” recalled Léon Rosenfeld, a young colleague of the physicist Niels Bohr. They were shocked, shocked, by Einstein’s famous 1935 critique of their interpretation of quantum mechanics. But why the surprise? Einstein’s argument did not originate in 1935 or even with his storied confrontation with Bohr in Brussels in 1927. It went all the way back to 1909 and came out of the same physical intuition that had led Einstein to create quantum theory to begin with.
This short animation dramatizes one of the arguments Einstein made that year, which physicist John Cramer has called the “bubble paradox.” Experiments with x-rays had shown that radiation is beamed: emitted by one atom, absorbed by another. But according to classical electromagnetic theory, an atom should emit radiation as a wave spreading out in every direction, like an inflating soap bubble. If so, how could a single other atom absorb all the radiation? The bubble would need to pop, concentrating all its energy in one place, while ceasing to exist elsewhere. Physics has no explanation for such an event. The bubble is expanding at the speed of light, so any communication between opposite sides would have to occur faster than light—in violation of the theory of relativity and, more generally, the principle of locality.
The mystery goes away if light energy is packaged into particles, or quanta. Classical electromagnetic theory itself goes pop: it captures the collective effects of zillions of particles, but falls to pieces on the particle-by-particle level, where it must give way to quantum mechanics. Einstein wrote:
According to our prevailing theory, an oscillating ion generates a spherical wave that propagates outwards.… Newton’s emission theory of light seems to contain more truth with respect to this point than the oscillation theory since, first of all, the energy given to a light particle is not scattered over infinite space, but remains available for an elementary process of absorption.
As philosopher Guido Bacciagaluppi and physicist Antony Valentini have written, Einstein recycled this argument in his debate with Bohr in 1927. According to the orthodox interpretation of quantum mechanics that Bohr was championing, the quantum wavefunction of a particle abruptly collapses—just like the popped bubble. Such a process would be nonlocal, again in apparent violation of relativity theory. For Einstein, the natural conclusion was that quantum mechanics was as incomplete as classical electromagnetic theory. “It’s remarkable that Einstein had such a simple and compelling argument already in 1909 and that people failed to understand it for decades,” Valentini tells me.
It was Einstein, rather than quantum theory’s self-appointed defenders such as Bohr, who was truer to the original spirit of the theory. For all the force of his argument, though, it did turn out later that quantum mechanics really is nonlocal.