A quantum theorem first published in 2012, known as PBR, has convinced many theorists that quantum mechanics rules out the most commonsense understanding of reality. Under its assumptions, the theorem rules out the idea that the quantum state merely represents incomplete knowledge of an underlying real world.
The theorem, which came to be known as PBR after its authors Pusey, Barrett and Rudolph, builds on earlier work by John Stewart Bell to show, under its assumptions, that the quantum state cannot be interpreted as a mere statistical summary of an underlying objective reality, as Einstein had argued.
Einstein famously rejected the Copenhagen interpretation of quantum mechanics, which denies objective reality. In 1935, he and colleagues Boris Podolsky and Nathan Rosen devised a thought experiment to argue that particles must have real properties independent of measurement. The EPR paradox assumed both “reality” (particles have intrinsic properties) and “locality” (no instantaneous influence between distant particles).
In 1964, Bell showed that at least one of these assumptions must be wrong. Experiments in the 1980s confirmed that particles matched more often than Einstein's reality would allow, showing that the world could not be both real and local. However, some physicists still held out hope that reality could be preserved by treating the quantum state as a state of knowledge, like a weather forecast, rather than a physical object.
The PBR theorem closed that door. Using a thought experiment reminiscent of a card game, Pusey and his collaborators showed, under the game’s assumptions, that quantum states cannot overlap in the way that states of knowledge do. In the quantum version of the game, a player can always win, meaning that under those assumptions the quantum state must correspond to something real rather than just information.
"From the moment it 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. Pienaar had been working on theories that aimed to show objective reality exists, but after PBR he recalled thinking, "That's it. The program's dead."
Pusey himself had been trying to improve an earlier theory by Perimeter Institute physicist Robert Spekkens, which showed that many quantum phenomena could be explained by treating the quantum state as incomplete knowledge. After trying to upgrade Spekkens' theory, Pusey said, “I spent a few days trying to do that, and got fed up. So I decided to prove that it was impossible instead.”
Since PBR appeared, experiment after experiment, including a recent realization of the theorem, has established that quantum mechanics is the law of the land. The question that remains is how much of reality has to be given up.
Analysis
Why This Matters
- The theorem resolves a century-old debate about the nature of quantum reality, showing that Einstein's attempt to preserve a classical objective world cannot succeed.
- It forces physicists and philosophers to confront the possibility that the universe is fundamentally probabilistic and that properties do not exist independently of measurement.
- The result narrows the range of viable interpretations of quantum mechanics, pointing toward those that accept the wave function as a real physical object rather than a tool for prediction.
Background
Einstein never accepted the Copenhagen interpretation of quantum mechanics, which denied objective reality. In 1935, he and colleagues Podolsky and Rosen published the EPR paradox, arguing that particles must have real properties. John Bell's 1964 theorem showed that at least one of Einstein's assumptions, reality or locality, had to be wrong. Experiments confirmed Bell's prediction, but some physicists continued to explore the possibility that the quantum state represented incomplete knowledge. The PBR theorem, published in 2012, eliminated that possibility, showing that the quantum state cannot be reduced to a state of belief.
Key Perspectives
Albert Einstein / EPR: Argued that particles have real properties (position, momentum) independent of measurement, and that the quantum state merely reflects incomplete knowledge.
Niels Bohr / Copenhagen interpretation: Denied that objective reality exists at the quantum level, asserting that physical properties arise only through measurement.
John Stewart Bell: Proved that at least one of EPR's assumptions (reality or locality) must be false; experiments confirmed his result.
Pusey, Barrett, Rudolph (PBR): Showed that the quantum state cannot be interpreted as mere knowledge, ruling out Einstein's original vision of an objective underlying reality.
Robert Spekkens: Developed a model in which the quantum state represents incomplete knowledge, demonstrating that many quantum phenomena could be explained that way, though PBR later showed such models cannot fully replicate quantum mechanics.
What to Watch
- Future theoretical work will explore how much of everyday reality can be salvaged given that the quantum state must be real.
- Experimental realizations of PBR and related tests may further constrain possible interpretations.
- The development of alternative interpretations, such as Bohmian mechanics or the Many-Worlds interpretation, which accept the reality of the wave function while making different tradeoffs.
Checked
12 claims in this story checked against 1 source: 11 supported.
Show each claim
- SupportedThe PBR theorem dealt a fatal blow to Einstein’s view of reality, as physicists say.Quanta Magazine: “Then, 50 years after Bell, Matthew Pusey ... delivered the killing blow.”
- SupportedThe PBR theorem was published in 2012.Quanta Magazine: “Since the publication of PBR in 2012, many theorists have come to accept that it rules out our most commonsense understanding of reality.”
- SupportedMany theorists have come to accept that PBR rules out a commonsense understanding of reality.Quanta Magazine: “Since the publication of PBR in 2012, many theorists have come to accept that it rules out our most commonsense understanding of reality.”
- SupportedUnder its assumptions, PBR rules out the idea that the quantum state is merely incomplete knowledge of an underlying real world.Quanta Magazine: “PBR posed a quantum version of the game and asked whether the quantum states could overlap, and whether quantum states could therefore be states of knowledge.”
- Partly supportedPienaar said, “From the moment it came out, people knew it would be a landmark.”Quanta Magazine: ““From the moment [PBR] came out, people knew it would be a landmark,” said Jacques Pienaar”
- SupportedPienaar recalled thinking, “That’s it. The program’s dead.”Quanta Magazine: “But when the PBR theorem appeared, Pienaar remembers thinking, “That’s it. The program’s dead.””
- SupportedEinstein, Boris Podolsky and Nathan Rosen devised their thought experiment in 1935 to argue that particles have real properties independent of measurement.Quanta Magazine: “In 1935, Einstein rebelled against this surreal reading of quantum theory, proposing one of the 20th century’s most famous thought experiments.”
- SupportedThe EPR argument assumed reality and locality.Quanta Magazine: “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 ... the assumption of “locality.””
- SupportedIn 1964, Bell showed that at least one of EPR’s two assumptions had to be wrong.Quanta Magazine: “He had proved that at least one of EPR’s two key assumptions had to be wrong.”
- SupportedExperiments in the 1980s confirmed that particles match more often than Einstein’s reality would allow.Quanta Magazine: “Physicists carried out his experiments in the 1980s and confirmed that particles really do match more than Einstein’s reality would allow.”
- SupportedIn the quantum version of the game, the player could always win, implying that quantum states cannot overlap as states of knowledge do.Quanta Magazine: “PBR posed a quantum version of the game and asked whether the quantum states could overlap, and whether quantum states could therefore be states of knowledge. ... The surprise was that in the quantum version of the game, the player could always win.”
- SupportedPusey said, “I spent a few days trying to do that, and got fed up. So I decided to prove that it was impossible instead.”Quanta Magazine: ““I spent a few days trying to do that, and got fed up,” he said. “So I decided to prove that it was impossible instead.””
