| Albert Einstein may be the most famous physicist of all time. Most people know him as the originator of general relativity, a revolutionary theory that recast the force of gravity as curves in space and time. He's also known as a skeptic of quantum mechanics who sought an alternative to the strange theory of atoms.
That is a true, but flattened, account of Einstein's relationship with quantum theory. In reality, Einstein contributed multiple load-bearing columns to the edifice that would become quantum mechanics and deserves to be remembered as a quantum pioneer alongside Max Planck, Niels Bohr, Werner Heisenberg and Erwin Schrödinger. Einstein did struggle to accept quantum mechanics, but his resistance did not stem from flippant dismissal or a lack of understanding. On the contrary, Einstein contributed so much to its development precisely because he so deeply intuited the theory's radical implications.
"I have thought a hundred times as much about the quantum problems as I have about general relativity theory," he once told the German physicist Otto Stern.
Planck and Einstein kicked off the quantum era together. First, Planck proposed in 1900 that matter absorbs energy in chunks. Five years later, Einstein dropped two revolutionary papers that furthered the case that the world is fundamentally "quantized." One paper established that matter is made of atoms, ending centuries of debate. The other ran with Planck's idea, arguing that light also carries energy in chunks. This idea was controversial, but Einstein was eventually vindicated. He won his only Nobel Prize for establishing the quantum nature of light, rather than for his better-known theory of general relativity.
Einstein's quantum discomfort began in the 1920s, as it became clear that the theory put new and unwelcome limits on our knowledge. A particle doesn't have a position, for instance; it merely has probabilities of being observed in different positions. This led to years of friendly debate between Einstein and Bohr as they traded increasingly sophisticated thought experiments that sharpened physicists' understanding of the theory.
Then, in 1935, Einstein, together with Boris Podolsky and Nathan Rosen (collectively, "EPR"), proposed a quantum thought experiment with seemingly unthinkable implications. If one took quantum mechanics seriously, this experiment would create a "spooky" connection in which a single property would reside among multiple particles. This phenomenon, EPR argued, was clearly impossible, and so quantum mechanics must only be a step toward a more complete theory. "There is no doubt that quantum mechanics has seized hold of a beautiful element of truth, and that it will be a test stone for any future theoretical basis," Einstein wrote in 1936.
What's New and Noteworthy
On that point, Einstein was mistaken. A refined thought experiment developed in the 1960s all but guaranteed that the spooky connection Einstein and his colleagues had identified, which came to be called "entanglement," was hardwired into the nature of reality. That experiment has since been carried out, and under increasingly stringent conditions. Physicists have even measured the different speeds at which entanglement can spread through chains of particles.
Through his skepticism, Einstein had discovered a profound and potent physical phenomenon. Schrödinger described entanglement as "the characteristic trait of quantum mechanics." It appears to give quantum computers their power, at least in part, and it enables robust encryption schemes.
While Einstein may have lost the battle over entanglement, some physicists hold out hope that his perspective may yet win the war for the soul of quantum mechanics. The theory is here to stay, and it may very well be a complete description of the world. But there is a growing belief that Bohr and Heisenberg's take on the theory, known as the Copenhagen interpretation, leaves something to be desired. At a conference held this summer to celebrate the centennial of quantum mechanics, a hot topic was the ongoing hunt for a reinterpretation of the theory that might at least partially restore the intuitive, concrete picture of reality Einstein hungered for.
In an ironic twist, Einstein's two great and seemingly antagonistic contributions to 20th-century physics — quantum mechanics and general relativity — have become increasingly intertwined in the 21st century. Physicists have discovered how to describe a negatively curved space-time fabric as a collection of quantum particles and vice versa. This translation suggests that the physical laws for space-time and gravity can be thought of as remixed versions of the quantum laws for particles, and that quantum phenomena are funhouse-mirror images of gravitational phenomena.
In particular, the entanglement that Einstein and his EPR co-authors couldn't accept may be a reflection of a theoretical space-time geometry he helped discover (also with Nathan Rosen): a wormhole, technically known as an Einstein-Rosen (ER) bridge. Both entanglement and wormholes concern an enigmatic connection between distant locations. Some theorists suspect that the similarity is more than a metaphor — that "ER" may equal "EPR" in some literal sense that they have yet to fully grasp. And so physicists are still trying to untangle the web of Einstein's insights, 120 years after his first revolutionary papers. | 
