When it comes to Bell’s theorem, a cornerstone of modern quantum mechanics, there is one thing that everyone agrees on: it was published 50 years ago. Everything else is open to debate — especially its interpretation — and there is little prospect of these matters being settled soon. Indeed, Bell’s theorem has become synonymous with the most puzzling meeting of metaphysics and physics that science has to offer.
Nature prides itself on writing for the general reader, but explaining the idea published by Northern Irish physicist John Stewart Bell in 1964 poses a stiff challenge to that mantra of accessibility. But confused readers can be consoled by the fact that they are not alone: even the best quantum physicists are left bewildered by Bell’s theorem. Still, to unlock the secrets of the Universe, a little effort seems worthwhile.
In short, Bell predicted that measurements on entangled quantum particles will be incompatible with one of two common world views. The first is locality — the idea that a measurement on a London desk cannot be influenced by the setting of a measuring device in New York. The second is realism — that there is a reality that is independent of what we measure or observe.
Before Bell, both were common assumptions in science. For most people, they still are. But for physicists who step from the physical world into the quantum universe, Bell’s theorem poses a real challenge. They must accept either that entangled quantum particles can influence each other instantaneously, even if they are light years apart, or that in the quantum world there is no Moon if nobody looks. Bell’s predictions have withstood all experimental tests so far, so it looks like we have to give up at least one dearly held, intuitive concept.
“Even the best quantum physicists are bewildered by Bell’s theorem.”
The reluctance of physicists to choose either of the possible options is illustrated by the fact that they still disagree on what exactly to make of Bell’s theorem. For example, a conference in Vienna this week to celebrate the 50th anniversary of Bell’s big idea will not merely issue a few historic outlooks and then move on to the hot topics of today. Rather, the theorem itself remains hot. (Sample talk title in Vienna: ‘My struggle to face up to unreality’.)
It is not that quantum physics has gone nowhere over the past 50 years. On the contrary: in the 1990s, quantum physics experienced a boost that has been coined the ‘second quantum revolution’, when the theories developed in the first revolution were translated into practical quantum technologies such as unbreakable cryptography protocols and ultrafast computing concepts. After all, we can simply use the equations of quantum mechanics to invent new technology without understanding their deeper meaning.
Still, the second quantum revolution was at least partially triggered by contemplations about the meaning of it all. Quantum physicist Artur Ekert, for instance, devised one of the key ingredients for secure quantum communication while pondering the meaning of Bell’s theorem (A. K. Ekert Phys. Rev. Lett. 67, 661; 1991).
Today’s quantum-physics agenda holds great promise for such fruitful collaboration between fundamental research and practical applications. For example, the search for the biggest objects that can be subject to quantum superposition is not only motivating theorists to think about possible universal distinctions between the macroscopic classical and the microscopic quantum world, but also prompting the improvement of experimental tools that will probably become useful in other contexts.
See, that wasn’t too hard. Was it?
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