About 60 years ago or so a Nobel winning physicist, Eugene Wigner posed a thought experiment that demonstrated an oddity of quantum mechanics. Here's how it goes. Imagine two friends in a lab, measuring an atom, let's call them Jack and Jill. Jill is inside a sealed room with the said atom while Jack is sitting outside.
As famously known, the concept of superposition dictates that a quantum system can exist in multiple states, that is until it's observed by an observer. So, Jill is inside and is observing the atom.
This observation collapses the particle into a single state. But as Jack is outside and has no means of communication with Jill, this collapse did not take place in his reality. He has to go in and make an observation to determine the state. What's worse, since he can't observe Jill, she is also in a superposition. Oh no! Conflicting experiences.
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Researchers in Taiwan and Australia have come up with a way to demonstrate that Wigner's paradox is real indeed. In their publication published on Nature Physics, the team transforms the thought experiment into a mathematical theorem that validates the contradictory nature of the scenario. The team also devised an experiment where they put photons in lieu of human observers.
Wigner believed that quantum mechanics had to break down for complex systems like human observers for us to resolve this paradox. This study, according to some authors in the study puts something gravely fundamental at stake, objectivity. It might just be the case that there's nothing we can deem as an absolute fact and that what might carry truth in Jill's case might not apply to Jack's reality.
One of the co-authors Nora Tischler from Griffith University says “It’s a bit disconcerting, a measurement outcome is what science is based on. If somehow that’s not absolute, it’s hard to imagine.”
Wigner's thought experiment has recently seen a newfound interest in 2015. Časlav Brukner from the University of Vienna tested the most obvious solution to the paradox by stating that Jill can, indeed, observe the atom in a single state and place, it's just that Jack has no idea what is going on. To put it more science-y, the position of the atom is merely a hidden variable for Jack.
He imagined an alternate reality with two Jills, where each Jill has an atom they observe, which are entangled with each other, so when observed, their properties correlate. Each Jill take measurements and compare their findings. In this scenario, their observations correlate strongly.
In 2018, a physics philosopher from the University of Arizona, Richard Healey pointed out a loophole in Brukner's experiment. Which is now closed in this latest publication from Tischer and her team. In the renewed scenario, they make four assumptions.
- The results obtained by Jills are real
- They can be combined into a consistent corpus
- Quantum mechanics are universal, they apply to both observers and particles
- The Jills have no bias
Under these assumptions, the paradox still persists. Optical elements drove each photon towards a path-dependent on its polarization (this is the equivalent of Jacks' observations). Then each photon went through a second set of measurement processes (and this one's the Jills). The team found a mismatch between Jack and Jill's data here.
One of the four assumptions has to cave in. Griffith says “There are facts for one observer, and facts for another; they need not mesh.” And physics philosopher Olimpia Lombardi from University of Buenos Aires adds “From a classical perspective, what everyone sees is considered objective, independent of what anyone else sees,”
Another co-author Eric Cavalcanti remarks “Most physicists, they think: ‘That’s just philosophical mumbo-jumbo. They will have a hard time.”