The theory of quantum mechanics can give contradictory answers when applied to large objects like people and quantum computers, suggesting it may not be a valid description of how nature works at all levels.
Standard quantum theory explains the behaviour of microscopic things like electrons and atoms. We know that in practice, it is very difficult to observe quantum behaviour in large objects without their fragile quantum state collapsing, but in principle the theory should still apply – physicists have even used quantum theory to understand black holes.
But this universal applicability is an assumption – and one that may not be correct. To put this idea to the test, Daniela Frauchiger and Renato Renner at ETH Zurich, Switzerland, came up with an elaborate thought experiment.
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Heads or tails?
It begins with a person, say Amy, tossing a coin and using the result to set the spin of a particle: heads, the spin is set to down; tails, the spin is set to up. Amy then sends the particle to Brian, who measures the spin. If Brian finds the spin to be up, he knows Amy got tails, else she got heads. The pair repeat this experiment over and over.
Meanwhile, Amy has a friend, Andy, who uses quantum theory to model Amy and her entire lab. Andy checks whether Amy and her lab are in a quantum superposition of two states — essentially, are Amy and the lab acting as a single quantum object? The result is a “yes” or “no” answer.
Brian has a friend, Bella, who applies quantum theory to Brian and his lab in just the same way. “If a theory is universally valid, then it should be possible to use the theory to also describe someone who uses the theory,” says Renner.
Quantum theory says that the spin of the particle as measured by Brian is linked, or correlated with the outcome of Amy’s coin toss. That means if Andy gets a “yes” for his measurement of a particular coin toss experiment, he can make a definitive statement about the spin of the particle Brian saw. And if Bella gets a “yes” for her measurement of that particular experiment, then she can say something definitive about whether Amy saw heads or tails.
Frauchiger and Renner were able to show that this thought experiment sometimes leads to contradictions. For example, the maths of quantum theory shows that in a small minority of cases, Andy can get a “yes” and know that Brian saw a particle with spin down and at the same time, Bella can get a “yes” and know that Amy saw a coin toss that came up tails. The two facts don’t match up.
According to Frauchiger and Renner, the thought experiment’s contradictory results show that some key assumption in the theoretical analysis must be wrong.
For example, the analysis assumes that it’s equally valid for Amy to describe her system using quantum theory as it is for Andy to describe Amy and her entire lab with the same theory — ditto Brian and Bella. Renner thinks that this assumption — that quantum theory is universal — is suspect.
“Why should the same theory still work on the scale of humans?” he says. “It could well be that there is some modification.”
The thought experiment is “very provocative”, says Robert Spekkens, of the Perimeter Institute of Theoretical Physics in Waterloo, Ontario, Canada.
“There’s been this long tradition in the field of foundations of quantum mechanics to talk about what quantum theory predicts about how one observer ought to describe another observer,” he says. “Their contribution has really reinvigorated those discussions.”
The experiment itself is impossible to do at the moment, because it involves making quantum measurements on systems that themselves are doing quantum measurements. However, Amy and Brian don’t need to be humans.
“They just need to be systems that are complex enough that they can apply quantum mechanics,” says Renner. “I’d rather wait until we have small quantum computers.”
Nature Communications
