Reality Is Complicated

When the Plot Twists Once More

We’ve all seen classic heist movies, right? In heist movies, there’s almost always a scene where the thieves apparently trigger the alarm during the break-in. Ultimately, we viewers learn that the thieves wanted to get caught. We discover that it was all part of their plan. Imagine how it must feel to be a security guard in such a movie – it must be frustrating! You’d think you’ve managed to catch the culprits, but the real plan contains a twist within a twist. The truth turns out to be even more complicated than you initially imagined.

Sometimes physics research feels like the plot of a heist movie. Scientists here are like the security guards while Nature is conducting the elaborate heist. It is the scientists‘ job to keep tabs on what Nature is doing. Are the principles governing the Universe satisfying the rules we’ve come to expect, or is Reality actually more complicated than it appears? And sometimes we scientists think we’ve ‘caught Nature red-handed’, that we have finally grasped the plans of Nature. However, it turns out that we are still missing some part of the story. 

Physicist John Stewart Bell came up with a mathematical argument, or theorem, in 1964 that ‘caught Nature red-handed’. It forced a major transformation of our understanding of correlations (when things repeatedly and consistently happen at the same time). We recently proved an additional twist, showing that Bell’s theorem is actually not the whole story.

Time and time again throughout the history of science, major discoveries changed our way of thinking and led to further discoveries. For example, it was already a big deal when we realized that some life forms could be microscopic. Then, our idea of life radically changed when we discovered that all life comes from prior living matter, as opposed to spontaneous generation (the theory that life pops out without explanation). Another example is when there was a massive cultural upset from the realization that the Sun does not orbit the Earth. Then, our idea of the universe took on yet more complexity when we further realized that the Earth does not move around the Sun in a perfect circle.

This pattern of repeated plot twists holds as well when it comes to understanding Quantum Theory, the fundamental theory that provides a description of Physics at the scale of atoms (the basic elements that make up all things).

Bell’s Theorem: A New Kind of Common Cause

A fundamental idea of science is that observations require explanation. Specifically, if two events are correlated (which is established by some observations), there must be a reason. In other words, there must be an explanation for the correlation between two observations. For instance, two observations can be directly related. In this case, one event causes the other to occur. It can also be that two observations are correlated because they are both caused by a third event. Or, it might even be a combination of these two possibilities! For example, we tend to see an opposing connection between personal daily water consumption and blood pressure. This is called a negative correlation. In this case, the negative correlation is that people who drink more water have lower blood pressure on average. This negative correlation suggests that increasing the amount of water you drink can lower your blood pressure, but that is not really the case. The correlation is mostly explained by the fact that low blood pressure and high-water intake are both indicators of regular exercise. Some people live an active lifestyle, some are more sedentary. People who are very active tend to have better heart health, and also tend to drink more water, since they sweat more on average. In this case, the increased amount of water you drink and the lower blood pressure are caused by a higher level of exercise. The difference of exercise in the population is essentially a “hidden variable” that might be missing from data and not originally observed. This variable thus serves as a common cause of both blood pressure and water consumption.

With the instance of water consumption and blood pressure, we are sure that water consumption does not influence blood pressure and vice versa. If we are sure that one quantity does not influence another quantity (and vice versa), then the correlation can only be attributed to some common cause, which might be hidden. Again, exercise is the hidden common cause of blood pressure and water consumption. The name in our field, the field of Quantum Theory, for this intuitive model of common cause is “shared randomness”, and for a long time we thought it was the only model possible…

It turns out that the rules of Quantum Theory appear quite different from the shared randomness model. Actually, the situation is far worse. Recent quantum experiments have observed some correlations which cannot be explained by any theory compatible with the old shared randomness model. In other words, all physical theories relying on shared randomness are wrong, and will stay forever doomed.

The story behind this discovery starts in 1935, when physicists Albert Einstein, Boris Podolski and Nathan Rosen remarked that Quantum Theory was not based on the shared randomness model of common causes. Since at the time there was no alternative model, Einstein, Podolski, and Rosen claimed that the mathematical description of Quantum Theory was problematic. They advocated for a replacement for Quantum Theory, although they did not have any specific candidate in mind. A search thus began. Many wished for a new theory that would be able to explain at least every observable event which Quantum Theory could explain, all while obeying the shared randomness logic that was followed by all physical theories before it.

In 1964, defying all expectations, physicist John S. Bell showed that no such theory can exist. To prove his extraordinary claim, he proposed an experiment in which the behavior of two particles of light cannot be explained by shared randomness. Bell never performed the experiment himself, but later some experimental physicists did. They reliably confirmed that two events can have a common cause that is not shared randomness — we will call it nonclassical, or more precisely, two-event nonclassical. These physicists were awarded the 2022 Nobel Prize in physics, one of the highest achievements in science.

A New Twist About Common Causes

The experiments of Bell proved that Quantum Theory was not simply complicated for the sake of being complicated, but that the World was in fact complicated. There exist at least two different types of common causes: shared randomness and two-event nonclassical. But Quantum Theory also describes weird correlations between three or more events. Could these be explained in simpler terms? Mirroring the valid skepticism of Einstein, Podolsky, and Rosen, our research team looked at this problem: Can we devise an experiment with three events that cannot be explained without involving a third type of common cause?

While the answer turned out to be yes, reaching that answer was far from straightforward because in general it is hard to tell whether the result of an experiment merely looks complicated, or whether it is truly complicated. We thus came up with a series of experiments involving three events. First, we tested that the three events could not be explained by common causes that were only shared randomness. Second, we tested that the three events could not be explained by common causes that were only two-event nonclassical. At last, and this was the tricky part, we combined this all together to test that the three events could not be explained by any combination of the two previous types of common causes. For this last experiment, the only explanation left was, therefore, the complicated one: a three-event nonclassical common cause.

The Complexity of Quantum Theory & Nature

Just like Bell’s experiments, we proved that the World is again more complicated than we might think. The existence predicted by Quantum Theory of the two-event nonclassical common causes now has been expanded to cover at least three events. But does this conclusion stop at the number three? No! We also designed experiments that could rule out — once realized —  any cap on the number of events involved in a nonclassical common cause. In other words, it is well known in our field that describing large groups of events using Quantum Theory is complicated. We showed that it could not have been otherwise… Reality is indeed complicated!

Now, a skeptical reader might wonder if what we claim can be verified. After sharing our work with the physics field, three different experimental teams independently verified our claim. Does it mean that our conclusions are definitive? That they will remain true even if we one day find out that Quantum Theory is not completely right? Actually, the story is more complicated. These three experiments, even if at the edge of what is technologically feasible today, still do not exactly match the design of the quantum experiment we proposed. They rely on hypotheses that make the experiments easier to conduct. Perhaps, these experiments are not the end of the story and further twists are awaiting us.

Written By: Xavier Coiteux-Roy, Elie Wolfe, and Marc-Olivier Renou

Academic Editor: Neuroscientists & Physicist

Non-Academic Editor: Operations Manager

Original paper

• Title: No Bipartite-Nonlocal Causal Theory Can Explain Nature’s Correlations

• Authors: Xavier Coiteux-Roy, Elie Wolfe, and Marc-Olivier Renou

• Journal: Physical Review Letters

• Date Published: 10 November 2021

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