If the truth be told, few physicists have ever really felt comfortable with quantum theory. Having lived with it now for more than a century, they have managed to forge a good working relationship; physicists now routinely use the mathematics of quantum behavior to make stunningly accurate calculations about molecular structure, high-energy particle collisions, semiconductor behavior, spectral emissions and much more.
But the interactions tend to be strictly formal. As soon as researchers try to get behind the mask and ask what the mathematics mean, they run straight into a seemingly impenetrable wall of paradoxes. Can something really be a particle and a wave at the same time? Is Schrödinger's cat really both alive and dead? Is it true that even the gentlest conceivable measurement can somehow have an effect on particles halfway across the Universe?
Many physicists respond to this inner weirdness by retreating into the 'Copenhagen interpretation' articulated by Niels Bohr, Werner Heisenberg and their colleagues as they were putting quantum theory into its modern form in the 1920s. The interpretation says that the weirdness reflects fundamental limits on what can be known about the world, and just has to be accepted as the way things are — or, as famously phrased by physicist David Mermin of Cornell University in Ithaca, New York, “shut up and calculate!”
But there have always been some who are not content to shut up — who are determined to get behind the mask and fathom quantum theory's meaning. “What is it about this world that forces us to navigate it with the help of such an abstract entity?” wonders physicist Maximilian Schlosshauer of the University of Portland in Oregon, referring to the uncertainty principle; the wave function that describes the probability of finding a system in various states; and all the other mathematical paraphernalia found in textbooks on quantum theory.
Over the past decade or so, a small community of these questioners have begun to argue that the only way forward is to demolish the abstract entity and start again. They are a diverse bunch, each with a different idea of how such a 'quantum reconstruction' should proceed. But they share a conviction that physicists have spent the past century looking at quantum theory from the wrong angle, making its shadow odd, spiky and hard to decode. If they could only find the right perspective, they believe, all would become clear, and long-standing mysteries such as the quantum nature of gravity might resolve themselves in some natural, obvious way — perhaps as an aspect of some generalized theory of probability.
“The very best quantum-foundational effort,” says Christopher Fuchs of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, “will be the one that can write a story — literally a story, all in plain words — so compelling and so masterful in its imagery that the mathematics of quantum mechanics in all its exact technical detail will fall out as a matter of course”.