One of the first milestones of human development is the understanding of the permanence of objects. It usually takes less than a year for a baby to realize that when an adult hides his face behind his hands in a game of hide-and-seek, he does not cease to exist. There is more to the world around us than 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, regardless of what others think. A red billiard ball is red, even in the dark and even if no one is checking its color. Redness is a property that belongs to the ball, not to some device that measures the ball, nor to the mind of some person who operates the measuring device. However, quantum mechanics, a fully validated theory of how the world works, seems to suggest otherwise. The mathematics of the theory assigns redness to the billiard ball at the moment you measure it. Before that, the theory just gives you a set of probabilities that tell you what color the ball might be. The key founders of quantum mechanics suggested that this mathematics reflects the nature of reality: that, in some sense, there is no single, objective reality. In 1935, Einstein rebelled against this surreal reading of quantum theory and proposed one of the most famous thought experiments of the 20th century. But his rebellion was doomed to failure. In 1964, physicist John Stewart Bell demonstrated with his own thought experiment that reality had to be stranger than Einstein supposed. Later work would test Bell’s thought experiment and prove him right. Then, 50 years after Bell, Matthew Pusey, then a graduate student at Imperial College London, took on what was left of Einstein’s notion of reality and, without originally intending to, dealt the fatal blow. Matthew Pusey addressed the debate over the nature of reality in his doctoral thesis at Imperial College London. Courtesy of Matthew Pusey The paradigm shift theorem came to be known as PBR by 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 dismisses our most sensible understanding of reality. “From the moment [PBR] “When the PBR theorem came out, Pienaar remembers thinking, ‘That’s it.’ The program is dead.” This is the story of how Einstein, Bell and finally Pusey and their collaborators ended reality as we know it. Through three theorems they demonstrated that quantum mechanics is incompatible with the world we perceive. And since experiment after experiment (including a recent PBR discovery) has established that quantum mechanics is the law of the land, something in 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 the development of the theory of quantum mechanics, Einstein turned around and attacked it. I couldn’t accept what he said about reality. Einstein knew deep down what reality was. It was the sum total of all objective facts (such as the color, position, and speed of a billiard ball) that exist in the objects they describe, independently of perception and mind. But quantum mechanics, as developed by Werner Heisenberg and Erwin Schrödinger in the mid-1920s, did not deal with facts. Quantum mechanics addresses the possibilities. Their mathematics provides a “quantum state”, also known as a 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 that the ball is red and a 23% chance of finding that it is green. Albert Einstein’s argument that particles should have real properties that quantum mechanics cannot capture became known as the Einstein-Podolsky-Rosen paradox. The question that has perplexed physicists for a century is: What is the nature of the ball before the experiment is carried out? Does the ball have different colors? Colorless? Does the concept of color have no meaning? In short, is there a reality that supports the permanence of the object we experience? For Einstein, the answer was obvious: of course reality exists; the wavefunction just doesn’t capture it completely. From this point of view, the wave function is like a weather forecast. Let’s say you wake up and your weather app tells you there’s a 77% chance it will rain today, but you haven’t looked outside to see what’s happening. Then you raise the blinds and see raindrops falling. At that moment you are 100% sure that it is raining. This change from 77% to 100% is a change in certainty that occurs purely in your head. It does not reflect any real weather changes. 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”), devised a thought experiment to demonstrate that the quantum state exists only as an information artifact. To understand the thought experiment, you need to know one peculiar thing about the wave function: its mathematical structure does not allow an object to have a position and a momentum at the same time. The key founders of quantum mechanics suggested that this mathematics reflects the nature of reality: that, in some sense, there is no single, objective reality. The EPR experiment involved bouncing two particles off each other and, some time later, making a measurement, such as how far a particle had traveled. This first measurement would also tell you how far the associated particle had traveled. EPR argued that the position of the associated particle had to exist, because it could be known. They argued that the associated particle also had momentum, because that property could be known in a similar way. Crucially, their experiment involved measuring only one particle once, while the other remained intact. This showed, the scientists argued, that the second particle had established and real properties, even before measurement. And since the wave function does not allow that, the wave function must not reflect the literal state of the particle; the quantum state is not real. To reach this conclusion, scientists had made two seemingly unquestionable assumptions. First, they assumed that it was possible for a particle to have a literal state, the “reality” assumption. 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 “locality” assumption. But just a few months after the EPR, Heisenberg’s mentor, Niels Bohr, attempted to put an end to 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 one abstract quantum physical description.” Their argument was similar to claiming that the weather forecast was accurate but that the atmosphere itself was nonexistent or indescribable. This denial of objective reality became known as the Copenhagen interpretation.
