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A carefully designed experiment uses Hardy’s paradox to illustrate lack of local realism.
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Hardy’s 30-year-old thought exercise has led many researchers to chip away at real versions.
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If the universe isn’t locally real, that has implications for what’s possible through quantum phenomena.
For over 100 years now, quantum mechanics has rattled the cage of everything we’ve believed we know about physics. Is everything just made of wiggles and waves if you look close enough? How far can one entanglement be stretched—is it long enough to enable quantum telecommunications around the world?
And those questions don’t let up when it comes to reality, or realism. We know that a quantum particle doesn’t have a true state until it is observed (à la Schrödinger’s cat), but the question behind that fact persists for all things in existence—does an object still have properties when those properties are not being observed?
At a certain point, this foundational question becomes… well, entangled… with an additional concept called locality. Locality describes whether or not an object is influenced by more than its own immediate physical surroundings. If larger or more complex forces are at play, that could affect principles like causality and even free will. Albert Einstein’s iconic description “spooky action at a distance” is about the opposite of locality. Even gravity is not action at a distance—it’s now described as a result of overlapping force fields of many sizes.
All this brings us to the idea of Hardy’s paradox. While it may sound dry, its implications have ramifications for how real our universe is… and what the term “reality” even means. And in new research, scientists in China say they’ve found a way to observe this paradoxical thought exercise in quantum physics without any of the loopholes that have potentially compromised past experiments. The results appear now as the highlighted Editor’s Suggestion in the peer-reviewed journal Physical Review Letters.
Lucien Hardy is a working physicist at the Perimeter Institute for Theoretical Physics in the suburbs of Toronto, Ontario. As a specialist in quantum foundations, Hardy has spent his long career trying to reach and refine the edges of the entire shape of quantum physics, including how the mathematical principles that support it interact with the very real and applied theories describing our universe.
In 1992, Hardy began formulating a paradox related to particles and antiparticles. Certain interactions in physics cause both a particle and its matching antiparticle to be created and thrown in opposite directions. These two are destined for each other, though, and just like Romeo and Juliet, their attachment inevitably causes them both to be annihilated—after the tiniest fraction of a second, they’re reunited and destroy each other. What Hardy posited is a scenario where the particle and antiparticle might coexist without annihilation.
Hardy knew that setting up and measuring such an interaction would introduce variables that would threaten the integrity of the interaction itself, and as a result, interactions like this could only be observed after the fact, using probability rather than observation. This, too, is a foundational question of quantum physics: how can a field of study that produces only probabilities work in conjunction with the observation-based paradigm of classical physics?
In order to properly poke this paradox with a proverbial stick, the scientists behind this new research—based primarily at the University of Science and Technology of China in the far eastern city of Hefei, which is also the home of the EAST nuclear fusion tokamak facility—have designed an experiment that they say takes out the loophole variables of other setups. At its core, it’s an elaborate series of mirrors, lasers, crystals, splitters, and plates, combined with a random number generator. To ensure the numbers are truly random, the digits were generated so quickly that they could not have been influenced by any “local hidden variables” associated with locality.
Over six hours of running this setup, with the goal of splitting photons and neutralizing any possibility of loopholes, the scientists say that their data is very clear (though, still probabilistic). “Based on a null hypothesis test,” they wrote in the study, “the p value that the possibility our results can be explained by local realistic theories doesn’t exceed 10−16348.” You’d be way more likely to win the lottery, even though that, too, is virtually impossible.
The scientists say this reinforces the growing consensus that local realism is not sufficient to explain the outstanding questions of quantum physics. Their conclusion is not new—the 2022 Nobel Prize in Physics was given to three scientists who used entangled photons to “overthrow reality as we know it,” Scientific American explained.
The biggest success for these new researchers is their experimental setup. By twisting and tuning their photon fountain, they kept enough efficiency and fidelity to measure what they needed to measure while stripping away the influence of local variables. The results do support local unreality, so to speak, but they also lay further groundwork for people who use these quantum phenomena to design information theory and applied systems. Those things, at least, are real and on the up and up.
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