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Nearly 60 years ago, the Nobel Prize\u2013winning physicist Eugene Wigner captured 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 such as an atom while Wigner stood outside. Quantum mechanics famously allows particles to occupy many locations at once\u2014a so-called superposition\u2014but the friend\u2019s observation \u201ccollapses\u201d the particle to just one spot. Yet for Wigner, the superposition remains: The collapse occurs only when he <\/em>makes a measurement sometime later. Worse, Wigner also sees the friend in a superposition. Their experiences directly conflict.<\/p>\n\n\n\n Now, researchers in Australia and Taiwan offer perhaps the sharpest demonstration that Wigner\u2019s paradox is real. In a study published this week in Nature Physics, they transform the thought experiment into a mathematical theorem that confirms the irreconcilable contradiction at the heart of the scenario. The team also tests the theorem with an experiment, using photons as proxies for the humans. Whereas Wigner believed resolving the paradox requires quantum mechanics to break down for large systems such as human observers, some of the new study\u2019s authors believe something just as fundamental is on thin ice: objectivity. It could mean there is no such thing as an absolute fact, one that is as true for me as it is for you.<\/p>\n\n\n\n \u201cIt\u2019s a bit disconcerting,\u201d says co-author Nora Tischler of Griffith University. \u201cA measurement outcome is what science is based on. If somehow that\u2019s not absolute, it\u2019s hard to imagine.\u201d<\/p>\n\n\n\n For physicists who have dismissed thought experiments like Wigner\u2019s as interpretive navel gazing, the study shows the contradictions can emerge in actual experiments, says Dustin Lazarovici, a physicist and philosopher at the University of Lausanne who was not part of the team. \u201cThe paper goes to great lengths to speak the language of those who have tried to merely discuss foundational issues away and may thus compel at least some to face up to them,\u201d he says.<\/p>\n\n\n\n Wigner\u2019s thought experiment has seen renewed attention in recent years. In 2015, \u010caslav Brukner of the University of Vienna tested the most intuitive way around the paradox: that the friend inside the lab has, in fact, seen the particle in one place or another, and Wigner just doesn\u2019t know what it is yet. In the jargon of quantum theory, the friend\u2019s result is a hidden variable.<\/p>\n\n\n\n Brukner ruled out that conclusion in a thought experiment of his own, using a trick\u2014based on quantum entanglement\u2014to bring the hidden variable out into the open. He imagined setting up two friend-Wigner pairs and giving each a particle, entangled with its partner in such a way that their attributes, upon measurement, are correlated. Each friend measures the particle, each Wigner measures the friend measuring the particle, and the two Wigners compare notes. The process repeats. If the friends saw definite results\u2014as you might suspect\u2014the Wigners\u2019 own findings would show only weak correlations. But instead, they find a pattern of strong correlations. \u201cYou run into contradictions,\u201d Brukner says. His experiment and a similar one in 2016 by Daniela Frauchiger and Renato Renner of ETH Z\u00fcrich led to an outpouring of papers and heated discussion at conferences.<\/p>\n\n\n\n But in 2018, Richard Healey, a philosopher of physics at the University of Arizona, pointed out a loophole in Brukner\u2019s thought experiment, which Tischler and her colleagues have now closed. In their new scenario they make four assumptions. One is that the results the friends obtain are real: They can be combined with other measurements to form a shared corpus of knowledge. They also assume quantum mechanics is universal, and as valid for observers as for particles; that the choices the observers make are free of peculiar biases induced by a godlike superdeterminism; and that physics is local, free of all but the most limited form of \u201cspooky action\u201d at a distance.<\/p>\n\n\n\n Yet their analysis shows the contradictions of Wigner\u2019s paradox persist<\/a>. The team\u2019s tabletop experiment, in which they created entangled photons, also backs up the paradox. Optical elements steered each photon onto a path that depended on its polarization: the equivalent of the friends\u2019 observations. The photon then entered a second set of elements and detectors that played the role of the Wigners. The team found, again, an irreconcilable mismatch between the friends and the Wigners. What is more, they varied exactly how entangled the particles were and showed that the mismatch occurs for different conditions than in Brukner\u2019s scenario. \u201cThat shows that we really have something new here,\u201d Tischler says.<\/p>\n\n\n\n It also indicates that one of the four assumptions has to give. Few physicists believe superdeterminism could be to blame. Some see locality as the weak point, but its failure would be stark: One observer\u2019s actions would affect another\u2019s results even across great distances\u2014a stronger kind of nonlocality than the type quantum theorists often consider. So some are questioning the tenet that observers can pool their measurements empirically. \u201cIt could be that there are facts for one observer, and facts for another; they need not mesh,\u201d says study co-author and Griffith physicist Howard Wiseman. It is a radical relativism, still jarring to many. \u201cFrom a classical perspective, what everyone sees is considered objective, independent of what anyone else sees,\u201d says Olimpia Lombardi, a philosopher of physics at the University of Buenos Aires.<\/p>\n\n\n\n And then there is Wigner\u2019s conclusion that quantum mechanics itself breaks down. Of the assumptions, it is the most directly testable, by experiments that are probing quantum mechanics on ever larger scales. But the one position that doesn\u2019t survive the analysis is to have no position, says another co-author at Griffith, Eric Cavalcanti. \u201cMost physicists, they think: \u2018That\u2019s just philosophical mumbo-jumbo,\u2019\u201d he says. \u201cThey will have a hard time.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":" by George Musser (8-17-20) Nearly 60 years ago, the Nobel Prize\u2013winning physicist Eugene Wigner captured 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 … Continue reading
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