Albert Einstein spent decades fighting the foundational math of quantum mechanics because it stripped physical objects of independent properties, but a 2012 theorem known as PBR—devised by Matthew Pusey, Jonathan Barrett, and Terry Rudolph—has systematically dismantled the physicist’s final defenses of an objective, mind-independent reality.
The Billiard Ball Paradox That Stymied 20th-Century Physics
Human infants grasp object permanence before their first birthday, understanding that a caregiver’s face behind two hands does not cease to exist during a game of peek-a-boo. Albert Einstein relied on that exact intuition to anchor his view of the physical universe. To Einstein, a red billiard ball remains red in total darkness, independent of any measuring device or human observer.
Yet the mathematics developed by Werner Heisenberg and Erwin Schrödinger in the mid-1920s rejected fixed properties entirely. Instead of assigning a definitive color or position to a particle, quantum mechanics deploys a wave function. This mathematical construct delivers only probabilities—such as a 77% chance of finding a billiard ball red and a 23% chance of finding it green.
Einstein argued that the wave function was merely an incomplete weather forecast rather than a literal description of reality. Just as checking a window turns a meteorological probability into a 100% certainty of rain without altering the atmosphere outside, Einstein believed measurements simply revealed pre-existing states.
In 1935, Einstein, Boris Podolsky, and Nathan Rosen came up with a thought experiment to prove that particles must possess real, independent properties. They rested their argument on two pillars: reality, which assumes particles have literal states, and locality, which dictates that those states reside entirely within the particle unless acted upon directly.
How Bell and Pusey Dismantled Local Realism
Niels Bohr countered the EPR paradox by insisting that no underlying quantum world existed, only an abstract physical description, giving rise to the Copenhagen interpretation. Decades later, experiment after experiment has established that quantum mechanics is the law of the land, forcing physicists to choose between Einstein’s intuition and empirical mathematics.
In 1964, physicist John Stewart Bell deployed a thought experiment demonstrating 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. Working alongside Jonathan Barrett and Terry Rudolph, Pusey tackled the remnants of Einstein’s reality framework in his doctoral thesis.
The resulting theorem, published in 2012 and designated as PBR, delivered what many in the field recognized as a landmark. Jacques Pienaar, a physicist now at the Federal University of Rio de Janeiro, recalled realizing the gravity of the publication immediately. Pienaar had been exploring theories designed to prove objective reality existed, but upon reading the PBR theorem, he concluded that the program was dead.
The Repercussions for Modern Quantum Theory
The PBR theorem rules out our most commonsense understanding of reality. Since repeated experiments confirm that quantum mechanics accurately describes the universe, physicists face an uncomfortable mathematical reality.
The math assigns physical properties only at the exact moment of measurement. Before measurement occurs, only probabilities exist.
With PBR closing off traditional loopholes, the foundational debate has shifted from whether objective reality exists to precisely how much of our commonsense framework must be abandoned to accommodate the laws of quantum mechanics.
