One of a human’s earliest developmental milestones is an understanding of object permanence. It generally takes less than a year for a baby to realize that when an adult hides their face behind their hands in a game of peek-a-boo, they don’t cease to exist. There’s more to the world around us than what we perceive.
This suggests — or at least, it suggested to Albert Einstein — that the world is “real,” in the sense that things in the world have properties that are intrinsically true no matter what anyone thinks. A red billiard ball is red, even in the dark, and even if no one is checking its color. The redness is a property belonging to the ball, not to some device measuring the ball, and not to the mind of some person operating the measuring device.
Yet quantum mechanics, a thoroughly validated theory of how the world works, seems to suggest otherwise. The theory’s math assigns redness to the billiard ball at the moment you measure it. Before then, the theory gives you only a set of probabilities telling you what color the ball might be. Key founders of quantum mechanics suggested that this math reflects the nature of reality — that in some sense, a single, objective reality doesn’t really exist.
In 1935, Einstein rebelled against this surreal reading of quantum theory, proposing one of the 20th century’s most famous thought experiments. But his rebellion was doomed. In 1964, the physicist John Stewart Bell showed with a thought experiment of his own that reality had to be stranger than Einstein assumed. Subsequent work would put Bell’s thought experiment to the test — and prove him right.
Then, 50 years after Bell, Matthew Pusey, at the time a graduate student at Imperial College London, took on what remained of Einstein’s notion of reality and — without originally intending to — delivered the killing blow.
Matthew Pusey took on the debate about the nature of reality in his doctoral thesis at Imperial College London.
Courtesy of Matthew Pusey
The paradigm-shifting theorem became known as PBR, for Pusey and his academic accomplices, Jonathan Barrett and Terry Rudolph. Since the publication of PBR in 2012, many theorists have come to accept that it rules out our most commonsense understanding of reality.
“From the moment [PBR] came out, people knew it would be a landmark,” said Jacques Pienaar, a physicist now at the Federal University of Rio de Janeiro in Brazil.
At the time, Pienaar, like Pusey, had been walking in Einstein’s footsteps, dabbling in theories designed to show that objective reality exists and that quantum mechanics merely describes our incomplete knowledge of it. But when the PBR theorem appeared, Pienaar remembers thinking, “That’s it. The program’s dead.”
This is the story of how Einstein, Bell, and finally Pusey and his collaborators killed reality as we know it. In three theorems, they proved that quantum mechanics is incompatible with the world we perceive. And since experiment after experiment — including a recent realization of PBR — has established that quantum mechanics is the law of the land, something about reality has to go.
The question that remains is: How much do we have to give up?
Einstein and Reality
After playing a pioneering role in developing the theory of quantum mechanics, Einstein turned around and attacked it. He couldn’t accept what it said about reality.
Einstein knew in his bones what reality was. It was the sum total of all objective facts — like the color, position, and velocity of a billiard ball — that exist in the objects they describe, independent of perception and mind. But quantum mechanics, as developed by Werner Heisenberg and Erwin Schrödinger in the mid-1920s, didn’t deal in facts.
Quantum mechanics deals in possibilities. Its mathematics gives you a “quantum state,” also known as the wave function. This wave function can tell you, for example, that in an experiment measuring the color of a particular billiard ball, you will have a 77% chance of finding the ball to be red and a 23% chance of finding it to be green.
Albert Einstein’s argument that particles should have real properties that quantum mechanics can’t capture became known as the Einstein-Podolsky-Rosen paradox.
The question that has stumped physicists for a century is: What is the nature of the ball before the experiment takes place? Does the ball have multiple colors? No color? Does the concept of color not have a meaning? In short, is there a reality to support the object permanence we experience?
To Einstein, the answer was obvious: Of course reality exists; the wave function just doesn’t fully capture it.
In this view, the wave function is like a weather forecast. Say you wake up and your weather app tells you there’s a 77% chance that it will rain today, but you haven’t looked outside to see what’s going on. Then you raise the blinds and see raindrops falling. In that moment you become 100% certain that it’s raining. This shift from 77% to 100% is a shift in certainty that occurs purely in your head. It doesn’t reflect some real meteorological change. The atmosphere is real, and your (un)certainty about its state is something else — a reflection of the information in your mind.
Einstein, along with his collaborators Boris Podolsky and Nathan Rosen (collectively, “EPR”), came up with a thought experiment to prove that the quantum state exists only as an artifact of information. To understand the thought experiment, you need to know one peculiar thing about the wave function: Its mathematical structure doesn’t allow an object to have a position and a momentum at the same time.
The EPR experiment involved bouncing two particles off each other and, sometime later, making a measurement, such as how far one particle had traveled. This first measurement would also tell you how far the partner particle had traveled. EPR argued that the position of the partner particle had to exist, because it could be known. They argued that the partner particle also had a momentum, because that property was knowable in a similar way.
Crucially, their experiment involved measuring only one particle one time, while the other remained untouched. This showed, the scientists argued, that the second particle had properties that were set and real, even before measurement. And since the wave function doesn’t allow for that, the wave function must not reflect the literal state of the particle; the quantum state is not real.
To reach this conclusion, the scientists had made two seemingly unassailable assumptions. First, they assumed that it was possible for a particle to have a literal state, the assumption of “reality.” And second, they assumed that this literal state existed within the particle and nowhere else, so the only way to change it would be to interact with the particle, the assumption of “locality.”
But just months after EPR, Heisenberg’s mentor, Niels Bohr, tried to shut down Einstein’s argument by attacking its first assumption. “There is no quantum world,” Bohr would later say, according to his assistant Aage Petersen. “There is only an abstract quantum physical description.” His argument was akin to asserting that the weather forecast was accurate but that the atmosphere itself was either nonexistent or indescribable. This denial of objective reality became known as the Copenhagen interpretation.
