The quantum paradox points to fragile foundations of reality | Science

Almost 60 years ago, Nobel Prize-winning physicist Eugene Wigner captured one of the many quirks of quantum mechanics in a thought experiment. He imagined a friend of his, sealed in a lab, measuring a particle like an atom while Wigner stood outside. Quantum mechanics allows particles to occupy multiple locations at once – a so-called superposition – but the friend’s observation “collapses” the particle in one location. Yet for Wigner, the superposition remains: collapse only occurs when he take a measurement a little later. Worse, Wigner also sees the friend in overlay. Their experiences are directly in conflict.

Now, researchers in Australia and Taiwan offer perhaps the clearest demonstration that Wigner’s paradox is real. In a study published this week in Physics of nature, they transform the thought experience into a mathematical theorem which confirms the irreconcilable contradiction at the heart of the scenario. The team is also testing the theorem with an experiment, using photons as proxies for humans. While Wigner believed that solving the paradox required quantum mechanics to decompose for large systems such as human observers, some of the authors of the new study believe that something so fundamental is on thin ice: objectivity. . It could mean that there is no absolute fact, a fact as true for me as it is for you.

“It’s a little baffling,” says Griffith University co-author Nora Tischler. “A measurement result is what science is based on. If it’s not absolute, it’s hard to imagine.

For physicists who dismissed thought experiments like Wigner’s as an interpretive navel observation, the study shows that contradictions can emerge in actual experiments, says Dustin Lazarovici, a physicist and philosopher at the University of Lausanne who does was not part of the team. “The document goes to great lengths to speak the language of those who have simply tried to discuss the fundamental issues and therefore may force at least some to face them,” he says.

Wigner’s thought experiment has received renewed attention in recent years. In 2015, Časlav Brukner from the University of Vienna tested the most intuitive way around the paradox: that the friend inside the lab actually saw the particle in one place or another, and Wigner just doesn’t know what it is yet. . In quantum theory jargon, the friend’s outcome is a hidden variable.

Brukner dismissed this conclusion in his own thought experiment, using a trick – based on quantum entanglement – to bring out the hidden variable. He imagined setting up two ami-Wigner pairs and giving each one a particle, entangled with its partner in such a way that their attributes, when measured, are correlated. Each friend measures the particle, each Wigner measures the friend measuring the particle, and the two Wigners compare the notes. The process repeats. If Friends saw definitive results – as you might expect – the Wigners’ own results would show only weak correlations. But instead, they find a pattern of strong correlations. “You run into contradictions,” Brukner says. Her experience and a similar experience in 2016 by Daniela Frauchiger and Renato Renner from ETH Zürich led to a wave of articles and lively discussions at conferences.

But in 2018, Richard Healey, a physics philosopher at the University of Arizona, pointed to a flaw in Brukner’s thought experiment, which Tischer and his colleagues have now closed. In their new scenario, they make four assumptions. The first is that the results obtained by friends are real: they can be combined with other measures to form a shared body of knowledge. They also assume that quantum mechanics is universal and as valid for observers as it is for particles; that the choices that observers make are free from particular biases induced by divine superdeterminism; and that the physics is local, free from anything but the most limited form of “spooky action” at a distance.

Yet their analysis shows that the contradictions of Wigner’s paradox persist. The team’s tabletop experiment, in which they created entangled photons, also confirms the paradox. Optical elements oriented each photon on a trajectory which depended on its polarization: the equivalent of observations of friends. The photon then entered a second set of elements and detectors which acted as the Wigners. The team found, once again, an irreconcilable mismatch between Friends and Wigners. In addition, they varied exactly how much the particles were entangled and showed that the mismatch occurs for different conditions than in the Brukner scenario. “It shows that we really have something new here,” Tischler says.

This also indicates that one of the four hypotheses must give. Few physicists think superdeterminism could be to blame. Some see locality as the weak point, but its failure would be blatant: the actions of one observer would affect the results of another even over great distances – a type of non-locality stronger than the type quantum theorists consider often. Some therefore question the principle that observers can aggregate their measurements empirically. “There are facts for one observer and facts for another; they don’t need meshes, ”suggests study co-author and Griffith physicist Howard Wiseman. It is a radical relativism, still shocking for many. “Classically, what everyone sees is considered objective, independent of what others see,” says Olimpia Lombardi, physics philosopher at the University of Buenos Aires.

And then there’s Wigner’s conclusion that quantum mechanics itself is collapsing. Among the hypotheses, it is the most directly testable, by experiments which probe quantum mechanics at ever larger scales. But the only position that doesn’t survive analysis is having no position, says fellow Griffith co-author Eric Cavalcanti. “Most physicists think, ‘This is just philosophical mumbo-jumbo,’ he says. “They will have a hard time.”