Does quantum physics place limits on the extent to which we have free will? This long-debated question stems from the way different physicists interpret the mathematics of quantum theory, but now researchers say they have come up with a way to test the idea and finally settle the matter.
鈥淓veryone has their own opinion on what is going on behind quantum theory. But it shouldn鈥檛 matter what we think; the only thing that matters is what we can prove through mathematics and through experiments. And this is what we are doing,鈥� says at the University of Seville in Spain.
One of the strangest properties of quantum physics is 鈥渘on-locality鈥�. Here, if quantum objects are entangled, they can unexpectedly maintain coordinated behaviours across extremely large distances. The mechanisms that underlie this coordination are not yet understood, but the debates about them go back decades 鈥� and have involved ideas such as foregoing free will.
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A pivotal moment in studying non-locality came in 1964, when the physicist John Stewart Bell came up with a way of measuring it. Bell developed a way for two hypothetical experimenters, Alice and Bob, to study entangled particles and determine the extent to which they are correlated, even over large distances. Plugging their data into Bell鈥檚 鈥渋nequality鈥� equation reveals whether the particles are correlated in a non-local way that exists only in quantum physics.
Decades of experiments, including those awarded the 2022 Nobel prize in physics, have consistently found that Bell鈥檚 inequality is violated and that particles do maintain non-local quantum properties. But there is still room to ask why, which is where the work of Cabello and his colleagues 鈥� and , both at The University of Hong Kong 鈥� comes in.
The researchers focused on three assumptions baked into Bell鈥檚 scenario. These are known as 鈥渕easurement independence鈥�, 鈥減arameter independence鈥� and 鈥渙utcome independence鈥�. All three ensure that there aren鈥檛 any unaccounted-for correlations or coordination attempts between Alice鈥檚 experiment and Bob鈥檚 鈥� for instance, that their measurement devices are secretly connected or somehow communicating with each other, which would violate the assumption of parameter independence, or that the outcomes of all measurements are somehow predetermined, which would violate the assumption of outcome independence.
The assumption of measurement independence, however, is particularly significant because it can be linked to the notion that each experimenter has free will throughout the experiment. If there was a hidden law that made Alice act in a certain way whenever Bob acted in another, then the correlations in the data could be chalked up to this law rather than non-local correlations. The oddness of non-locality would be avoided, but at the cost of Alice and Bob not being truly free to make choices regarding how they use their measurement devices during the experiment.
Some past experiments have already focused on the role of human choice in Bell tests 鈥� for instance, in an experiment where Alice and Bob鈥檚 measurement settings were playing an online video game.
But now, Cabello and his colleagues are taking this a step further and considering whether some of the independence assumptions can be relaxed 鈥� for example, could Alice and Bob have only 鈥減artial鈥� free will? In other words, could their actions, on some occasions, be predetermined? The situation would be analogous to being able to choose whatever you want for breakfast most days, but occasionally the laws of physics intervene to force you to eat cereal. It sounds strange, but the researchers have come up with several new equations, similar to Bell鈥檚 inequality,听that would let experimenters test this idea.
at the University of Geneva in Switzerland says that the work could help eliminate some of the competing ideas for why quantum theory is non-local. 鈥淭he experimental violation of Bell鈥檚 inequality is an established fact. This is done now,鈥� he says. 鈥淏ut you always have assumptions if you want to have a theorem.鈥�
Cabello says that experimentally eliminating the possibility of partial free will would have implications for the philosophy of religion. 鈥淢any religions resolve the conflict between the concept of an omniscient God and God鈥檚 commandment not to commit sin by assuming human beings have partial free will,鈥� he says. But if partial free will is not possible, neither is this resolution.
There could also be repercussions for some of the more extreme interpretations of quantum theory. These include 鈥�superdeterminism鈥�, which claims that, despite appearances to the contrary, still-hidden laws of physics dictate everything that happens, including violations of Bell鈥檚 inequality. In the superdeterminist view, Bell鈥檚 protocol does not diagnose non-locality, but rather reflects the fact that much about the physical world is predetermined 鈥� a view that naturally raises the possibility that the laws of physics are at odds with unlimited free will.
鈥淲e鈥檝e either got to face the fact that the world is indeterministic, or that the world is deterministic and we have to explain Bell鈥檚 theorem,鈥� says at the University of Oxford, who studies superdeterminism and thinks the new work could help to do the latter. 鈥淲ithin my view, we have to explain Bell鈥檚 theorem within the deterministic world. I think it can be done by violating this measurement independence assumption.鈥�
Cabello and his colleagues are now developing experiments to take their new equations from theory to a real test of free will. Ramanathan says quantum computers may be helpful here because the question of relaxing some independence assumptions, or some parts of the computer influencing some of its other parts, can鈥檛 be avoided because of their compact design. 鈥淭he priority is to do the experiments,鈥� says Cabello.
Nature Communications
