Why John Stewart Bell has been haunting quantum mechanics for decades

Some people think they have a poltergeist in their attic, some say they saw ghosts in dark nights – I have John Stewart Bell. The research of the physicist and his formidable inheritance haunt me for years.

I guess I shouldn’t be surprised. Have you ever thought about the quantity of what we are experiencing as reality is actually, objectively, unambiguous? I have to do it, or I could not write about the nature of space and time, and complex activities in the quantum field. Bell also liked to think about these things, and his work has forever changed the way we understand them.

He was born in Belfast in 1928 and was, by all accounts, an exceptionally curious and brilliant child. He hung on to early physics, winning his first concert as a laboratory technician at the age of 16. He has been trained in theoretical and experimental physics and has built a large part of his career in the world of particle accelerators, where he has worked on complex calculations that we now relegate them to the Superordinators. But what really kept the bell at night was the cracks he could see in the foundations of quantum theory.

Today, this is an established field of physics and many of its practitioners have been presented in the pages of New scientist – Contemporary physics is not hostile to those who ask questions that are on the border of physics, mathematics and philosophy. However, when Bell came as a researcher, physicists have always been taken by the debates between the first wave of great theory – people like Niels Bohr and Albert Einstein – and considered them established or thought that what remained was a question of philosophy rather than physics.

So Bell only worked after the hours, almost like a hobby. This changed in 1963 when he and his wife, also an accomplished physicist, took sabbatical leave from their accelerator work and Bell used this time to transform his hobby in a pair of founding papers. Although they were received without fanfare and have been largely neglected for years, their importance cannot be overestimated.

Bell took a line of this philosophical questioning and transformed it into something that could be studied in a laboratory. It was focused on the idea of ​​”hidden variables” in quantum mechanics.

As developed by Bohr and his colleagues in the 1920s and 30s, quantum mechanics is not a friend of certainty or determinism. Sadly famous, you can say very little that is definitely on a quantum object until you interact with it. You can predict which properties it could have when measuring, but only in a probabilistic manner. For example, you may know that an electron has a 98% chance of having a certain amount of energy when you measure it, and 2% chance of having another energy, but the one it really is completely random.

How does nature decide what energy to serve you at random? An explanation is that it is not really chance in play here, but that some properties – certain variables – are hidden from researchers. If they could simply pin what these hidden variables are, physicists could bring absolute predictability to quantum theory.

Bell has designed a test that would eliminate a wide strip of theories hidden from competition to replace, or at least modify quantum theory. This test provides two experimenters, generally nicknamed Alice and Bob. Pairs of tangled particles are produced several times, then a particle in each pair is sent to Alice, while its partner particle goes to Bob in a distant laboratory. By receiving their particles, Alice and Bob each choose to measure a special property. For example, Alice could measure the rotation of her particle.

At the same time, Bob also makes measures and chooses how to do them, but Alice and Bob do not communicate during the experience. At the end, they connect their respective data to an equation that Bell derived in 1964. This “inequality” equation tests the data for the correlations between the measures of Alice and Bob. Even without quantum effects, certain correlations can occur by chance. But Bell has determined a level of correlation which demonstrates that something else happens: the particles are correlated in a way that exists only in quantum physics and can only exist if there are local hidden variables.

In this way, Bell’s test is more than diagnosing quantum theory as a better description of our reality than these deterministic and hidden theories – it also zero on the strange property of “non -locality” as something that seems to be a bizarre characteristic of our reality. Non-locality means that quantum objects can maintain a connection and that their behaviors can remain inextricably correlated, regardless of their distance. Einstein was a huge criticism of this, in part because it was uncomfortably close to instant communication between objects, which is strictly prohibited by his theory of special relativity.

Bell was in a way an acolyte of Einstein, but the whims of physical reality led him to ultimately prove his idol. His test has pointed out a firm finger towards our quantum world, something with which researchers are still struggling today, especially with regard to the apparently insurmountable chasm between quantum theory and our best understanding of the gravity developed by Einstein.

I did not find any mention of Bell actually working on the experimental implementations of his test himself, and this has long proven technologically difficult. Although the first experience of this type was completed in 1972, it took until 2015 for a testless test – as rigorous as possible – to finally put the last nail in the coffin of hidden -variable local theories. In 2022, the physicists Alain Appearance, John F. Clauser and Anton Zeilinger jointly received the Nobel Prize in Physics for their decades of work on these experiences.

So why do I still see John Stewart Bell wherever I turn? Have I been subjected to a quantum curse?

The short answer is that his work, and all the experiences that have tested it, have opened almost as many questions about quantum physics and the nature of the physical reality they have decided to answer. For example, while many physicists agree that our world is simply not local, some always try to determine exactly which physical mechanism underpins non-logging. Others work to develop new hidden theories that cannot be blocked by the Bell test. Others still carefully unravel all the mathematical hypotheses that Bell made in his papers from the 1960s. All seem to believe that finding a new angle on Bell’s work, or a neglected complexity in him, could be a skeleton key to pushing the interpretations of quantum theory beyond his current state and perhaps even build an elusive theory of everything.

Bell’s work training effects are everywhere in quantum physics. In fact, we have improved the tangled particles simply by trying to do bell tests in the past 50 years. But it’s just the beginning. A few weeks ago, I spent a lot of time speaking with physicists who found a way to take advantage of Bell’s work to design quantum tests to know if free will can be partial, that is to say if our freedom of choice can be constrained in a cosmic manner in some cases but not others. Then, I went out on the phone with another team of researchers, probably to discuss the gravity and nature of space and time, but I ended up talking about Bell again. These physicists were inspired by his approach and wanted to conceive a test similar to his but for the gravitational properties of reality, rather than quantum.

That too, I think, explains why I cannot escape Bell – his ability to transform philosophical problems into tangible tests of reality reflects the attraction at the heart of physics. The promise of physics is that it can help us to ward off the most confused mysteries in the world through experiences, and Bell’s test is an incredibly elegant incarnation of this promise.

If I have to be haunted by something, I could not honestly ask for a better ghost.