Not just “philosophical mumbo-jumbo”: An experiment shows how facts can depend on the observer.
Davide Bonazzi / Salzman Art
By George Musser
About 60 years ago, Nobel Prize-winning physicist Eugene Wigner conquered one of the many oddities 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 famously enables the particles to occupy many locations at once – a so-called superposition – but the friend’s observation ‘disturbs’ the particle in only one place. Yet for Wigner, the superposition remains: The column only happens when hy makes a measurement a little later. Worse is that Wigner also sees the friend in a superposition. Their experiences conflict directly.
Now, researchers in Australia and Taiwan are perhaps offering the sharpest demonstration that Wigner’s paradox is real. In a study published this week in Nature Physics, they transform the thought experiment into a mathematical statement that confirms the incompatible contradiction at the heart of the scenario. The team also tested the proposition with an experiment, using photons as proxies for the humans. While Wigner believed that solving the paradox requires quantum mechanics for large systems such as human observers, some of the authors of the new study believe that something just as fundamental lies on thin ice: objectivity. It could mean that there is no such thing as an absolute fact, one that is as true to me as it is to you.
“It’s a little disturbing,” says co-author Nora Tischler of Griffith University. “A measurement outcome is science based on. If that is somehow not absolute, it is difficult to imagine. ”
For physicists who have rejected thought experiments such as Wigner’s as interpretive umbilical cord removal, the study shows the contradictions can arise in real experiments, says Dustin Lazarovici, a physicist and philosopher at the University of Lausanne who was not part of the team. “The paper has discussed a lot to speak the language of those who have tried to discuss only fundamental issues away and can therefore at least force some to address them,” he says.
Wigner’s thought experiment has received renewed attention in recent years. In 2015, Časlav Brukner of the University of Vienna tested the most intuitive way around the paradox: that the friend in the lab had in fact seen the partition somewhere, and Wigner just did not know what it was. In the jargon of quantum theory, the friend’s result is a hidden variable.
Brukner debuted that conclusion in his own thought experiment, using a trick – based on quantitative acceleration – to uncover the hidden variable. He proposed to set up two friend-Wigner pairs and give each one a part, scattered with his partner in such a way that their attributes, by measurement, correlated. Each friend measures the partition, each Wigner measures the friend who measures the particle, and the two Wigners compare notes. The process repeats. If the friends saw definitive results – as you might think – Wigners’ own findings would show only weak correlations. But instead they find a pattern of strong correlations. “You have opposites,” Brukner says. His experiment and a similar one in 2016 by Daniela Frauchiger and Renato Renner of ETH Zurich led to an outpouring of papers and heated discussion at conferences.
But in 2018, Richard Healey, a physicist of physics at the University of Arizona, pointed out for a while in Brukner’s thought experiment, which Tischer and her colleagues have now closed. In their new scenario, they make four assumptions. One is that the results that friends get are real: they can be combined with other measures to form a shared corpus of knowledge. They also assume that quantum mechanics is universal, and equally valid for observers as for particles; that the choices made by the observers are free from peculiar parties induced by a divine superdeterminism; and that physics is local, free of all but the most limited form of ‘spooky action’ at a distance.
Yet their analysis persists the contradictions of Wigner’s paradox. The team’s tablet experiment, in which they made entangled photons, also creates a backup of the paradox. Optical elements sent each photon on a path that depended on its polarization: the equivalent of friends’ observations. The photon then entered a second set of elements and detectors that play the role of the Wigners. The team found, again, an incompatible mismatch between the Friends and the Wigners. What’s more, they differ exactly in how familiar the particles were and showed that the mismatch occurs for different circumstances than in Brukner’s scenario. “That shows we really have something new here,” Tischler says.
It also indicates that one of the four assumptions must be given. Some physicists believe that superdeterminism could be to blame. Some see location as the weak point, but the failure would be severe: the actions of one observer would affect the great results of another – a stronger kind of nonlocality than the kind of quantum theorists often consider. That some doubt the tenet that observers can pool their measurements empirically. “There are facts for one observer, and facts for another; they do not have to be mesh, “suggests co-author and Griffith physicist Howard Wiseman. It’s a radical relativism that still lingers for a long time.” From a classical perspective, what everyone sees is considered objective, regardless of what anyone else sees. , “says Olimpia Lombardi, a philosopher of physics at the University of Buenos Aires.
And then there’s Wigner’s conclusion that quantum mechanics itself breaks. Of the assumptions, it is directly testable, by experiments examining quantum mechanics on ever larger scales. But the one position that the analysis does not survive is not having a position, says another co-author at Griffith, Eric Cavalcanti. “Most physicists think, ‘That’s just philosophical mumbo-jumbo,'” he says. “They’ll have a hard time.”
