OhOne of the first steps in human development is understanding the permanence of objects. It usually takes less than a year for a baby to realize that when an adult hides their face behind their hands in a game of hide-and-seek, they do not cease to exist. The world around us is not limited to 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, whatever one thinks about them. A red billiard ball is red, even in the dark and even if no one checks its color. Redness is a property belonging to the ball, not to a device measuring the ball, nor to the mind of a person using the measuring device.
However, quantum mechanics, a fully validated theory of how the world works, seems to suggest the opposite. The mathematics of the theory assigns a redness to the billiard ball at the time you measure it. Before that, the theory only gives you a set of probabilities telling you the color of the ball. The principal founders of quantum mechanics suggested that this mathematics reflects the nature of reality – that, in some sense, a single, objective reality does not really exist.
In 1935, Einstein rebelled against this surrealist reading of quantum theory, proposing 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, through his own thought experiment, that reality must be stranger than Einstein thought. 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, grabbed hold of what remained of Einstein’s notion of reality and, without initially intending it, delivered 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 became known as PBR, to Pusey and his academic accomplices, Jonathan Barrett and Terry Rudolph. Since the publication of the PBR in 2012, many theorists have come to accept that it excludes our most common-sense understanding of reality.
“As soon as the PBR came out, people knew it would be a turning point,” said Jacques Pienaar, a physicist now at the Federal University of Rio de Janeiro in Brazil.
At the time, Pienaar, like Pusey, was following in Einstein’s footsteps, embarking on theories intended to show that objective reality exists and that quantum mechanics only describes our incomplete knowledge of it. But when the PBR theorem came up, Pienaar remembers thinking, “That’s it. The program is dead.”
This is the story of how Einstein, Bell, and ultimately 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 PBR achievement – has established that quantum mechanics is the law of the land, something of reality must disappear.
The question that remains is: how much should we give up?
Einstein and reality
After playing a pioneering role in the development of the theory of quantum mechanics, Einstein turned around and tackled this theory. He couldn’t accept what that said about reality.
Einstein knew deep down what reality was. It was the sum total of all the objective facts – like the color, position and speed of a billiard ball – that exist in the objects they describe, independent of perception and the mind. But quantum mechanics, as developed by Werner Heisenberg and Erwin Schrödinger in the mid-1920s, was not concerned with facts.
Quantum mechanics deals with possibilities. Its mathematics gives you 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 the ball red and a 23% chance 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 preoccupied physicists for a century is: what is the nature of the ball before the experiment takes place? Does the ball have several colors? No color? Does the concept of color have no meaning? In short, is there a reality that supports the permanence of the objects we experience?
For Einstein, the answer was obvious: of course, reality exists; the wave function just doesn’t capture it completely.
From this point of view, the wave function is similar to a weather forecast. Let’s say you wake up and your weather app tells you there’s a 77% chance of rain today, but you haven’t looked outside to see what’s happening. Then you lift the blinds and see raindrops falling. At this point you are 100% sure it is raining. This shift from 77% to 100% is a shift in certainty that happens only in your head. This does not reflect a real weather 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”), proposed a thought experiment to prove that the quantum state exists only as an information artifact. To understand the thought experiment, you need to know one special thing about the wave function: its mathematical structure does not allow an object to have position and momentum at the same time.
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 the distance traveled by the partner particle. EPR argued that the position of the partner particle must exist, because it could be known. They argued that the partner particle also possessed momentum, because this property was knowable in the same way.
Basically, their experiment involved measuring a single particle once, while the other remained intact. This showed, according to the scientists, that the second particle had definite and real properties, even before the measurement. And since the wave function does not allow this, the wave function must not reflect the literal state of the particle; the quantum state is not real.
To reach this conclusion, scientists formulated two seemingly unassailable hypotheses. First, they assumed that it was possible for a particle to have a literal state, the “reality” hypothesis. 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” hypothesis.
But just months after the EPR, Heisenberg’s mentor, Niels Bohr, attempted to put an end to Einstein’s argument by attacking his first hypothesis. “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 amounted to asserting that the weather forecasts were accurate but that the atmosphere itself was either non-existent or indescribable. This denial of objective reality became known as the Copenhagen interpretation.
Gn world
