PBR theorem challenges Einstein’s idea of objective reality
A theorem developed by Matthew Pusey, Jonathan Barrett and Terry Rudolph has challenged the idea that quantum mechanics reflects incomplete knowledge of an…
Physicists Matthew Pusey, Jonathan Barrett and Terry Rudolph developed a theorem that challenges the idea that reality exists independently of observation, a view associated with Albert Einstein. Known as PBR, the result has pushed researchers to reconsider whether quantum mechanics describes an objectively real world or only what can be known about it.
The theorem does not mean the physical world has disappeared. It addresses a specific question: whether a quantum state reflects incomplete knowledge of properties that already exist, or whether that state is fundamental to the description of a system. The distinction reaches into how physicists interpret the theory used to describe the behavior of the world at very small scales.
What does the PBR theorem challenge?
PBR challenges the view that a quantum state is simply a summary of an observer’s incomplete information about an independently existing reality. The theorem’s authors are Pusey, Barrett and Rudolph, whose work has led many theorists to reject that commonsense interpretation.
The issue begins with a familiar intuition: objects have properties whether or not anyone is looking. A billiard ball, for example, seems to have a color even when it sits in the dark. Einstein argued for this kind of objective reality, in which properties belong to objects rather than to a measurement or the person making it.
Quantum mechanics presents a different picture. Its mathematics assigns probabilities to possible measurement results. In the example described by the researchers, a quantum calculation can give a 77% chance of finding a ball red and a 23% chance of finding it another color. The theory’s account of the outcome depends on measurement, rather than simply assigning the ball a definite color beforehand.
That does not mean the theory has failed. The account describes quantum mechanics as thoroughly validated and says repeated experiments have established it as the prevailing account of how the world works. The tension lies between the theory’s successful predictions and the familiar idea that objects carry definite properties independent of observation.
How did the argument develop?
Einstein challenged the implications of quantum theory with a thought experiment in 1935. In 1964, physicist John Stewart Bell showed through another thought experiment that reality had to be stranger than Einstein had assumed. Later work tested Bell’s argument and supported it.
Matthew Pusey, then a graduate student at Imperial College London, later took up the remaining question of whether quantum mechanics could still be understood as incomplete knowledge of an objective reality. His work with Barrett and Rudolph produced the PBR theorem, published in 2012.
Jacques Pienaar, a physicist at the Federal University of Rio de Janeiro, said the theorem’s significance was apparent from its release. “From the moment [PBR] came out, people knew it would be a landmark,” Pienaar said.
Pienaar had been exploring theories that preserved objective reality while treating quantum mechanics as an incomplete description. He recalled his reaction to PBR: “That’s it. The program’s dead.”
Experimental work has since tested the theorem, including a recent realization of PBR described in the account. The result has sharpened the question for physicists: if quantum mechanics accurately describes the world, which parts of the ordinary picture of reality need to change?
Many theorists now accept that PBR rules out the commonsense view that quantum states merely encode incomplete knowledge of an objective reality.
