I HAVE a confession to make: I鈥檓 bored of quantum mechanics. This is an odd thing for a physicist to admit, but the most successful theory of modern physics has started to leave me cold. Perhaps I have just grown too used to its spooky predictions and its love of randomness. Or it might be the fact that, despite its many successes and the way it has captured popular imagination, there are hints that quantum mechanics isn鈥檛 as accurate a picture of reality as some would have you believe.
What really excites me is the idea that it might be no more than an approximation for some deeper, more intriguing theory lying just out of reach. The evidence? There are fundamental questions that quantum mechanics just can鈥檛 answer, and theoretical predictions that violate its premises. Coming to terms with such a theory, if one exists, would involve entering a world that makes the weirdness of quantum mechanics seem mundane, one where cause doesn鈥檛 have to precede effect and information can be lost forever. Quantum mechanics itself might hold the doorway to this world, if only we could push hard enough to break through.
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To move beyond quantum mechanics, we must first look to its birth. At the turn of the 20th century, before the term 鈥渜uantum鈥� had even been coined, there was a widespread view among those in the know that physics was more or less complete. In 1900, the physicist Lord Kelvin proclaimed that physics was virtually complete, with only minor mysteries left to be solved.
As it turned out, humble pie was on the menu. Within five years, quantum mechanics would appear on the scene, providing our most accurate description of things at the smallest scales. It brought with it a host of weird predictions that were impossible to reconcile with the classical picture. Knowing an object鈥檚 position fundamentally prevented you learning about its momentum, for example. And particles separated by vast distances could be so intimately linked that it seemed as if information was passing between them faster than the speed of light 鈥� even though no such information transfer occurs.
So why had nobody seen this revolution coming? Well, most quantum effects are invisible in our daily lives: once you zoom out from particles to the human level, quantum mechanics reduces to classical physics. We had to wait until laboratory experiments gained sufficient control over nature to reveal its slippery quantum character. If quantum mechanics is itself due a revolution, any future theory would similarly need to reduce to quantum physics at laboratory scales.
Our best hints that quantum mechanics might not be the full picture come, oddly enough, from the physics of the very large. To make sense of reality at the scale of galaxies, we need general relativity, a theory of gravity first laid out by Einstein in 1915. Although general relativity and quantum physics represent equally successful physical theories, the two are fundamentally incompatible.
One glaring problem concerns black holes, entities whose gravitational attraction is so strong that not even light can escape their pull. If you throw a quantum particle into one, information about its properties simply disappears. This is deeply weird: ordinarily, any missing information in quantum mechanics can always be found by expanding your viewpoint. Think of it as a missing set of keys: they are never really missing, only being searched for in the wrong places. Information about the physical properties of a particle lost to a black hole, however, can鈥檛 be found anywhere 鈥� even if you include the whole universe in your search. This black hole information paradox has bothered many great physicists, including Stephen Hawking.
The trouble doesn鈥檛 end there. Take causality, the cherished notion that cause always precedes effect. You may think this principle applies at all times and places throughout the universe. Instead, general relativity grants every observer their own notion of present and future, so it allows different people to disagree about the causal ordering of certain events.
In contrast, quantum mechanics says that events occur in a fixed causal order that different observers always agree on. To accommodate these conflicting ideas, whatever theory lies beyond quantum mechanics must allow for our notions of causality to get very funky indeed.
Unpredicted weirdness
Luckily for those of us pursuing such a theory, in the last decade or so researchers like Lucien Hardy at the Perimeter Institute for Theoretical Physics in Canada have been pushing the limits of quantum mechanics. By tweaking our standard notions, they have come up with a landscape of fresh theories that 鈥� at first sight 鈥� may not correspond to our physical reality, but do maintain some of the hallmarks of quantumness and allow a rigorous investigation of possible post-quantum theories.
My long-time collaborator John Selby at the Perimeter Institute and I used these to make . To find a theory that reduces to quantum mechanics 鈥� in the same way as quantum mechanics reduces to classical physics 鈥� we proved you must either abandon the concept of causality, or the notion that information is conserved, or both.
鈥淨uantum mechanics could hold the door to a new world 鈥� if we push hard enough鈥�
Although quantum mechanics is often regarded as weird, it had left untouched these two fundamental prejudices of ours about the natural world. Now it turns out that to find a better theory of reality, we need to look for the weirdness that quantum mechanics didn鈥檛 predict at all. The clouds on the horizon of quantum physics could be the harbingers of its eventual replacement.
Work done in the early 90s by Rafael Sorkin, a quantum gravity researcher now based at the Perimeter Institute, has helped us shed light on what such a replacement could look like. Sorkin considered one of the benchmark tests in quantum physics, the double-slit experiment, in which particles are fired one by one at a screen through a barrier with two slits. According to common sense, at the end of the experiment there should be only two slit-like marks on the screen, one for each opening the particles could have gone through. This doesn鈥檛 happen: quantum interference causes a pattern to appear, as though the stream of particles was behaving like a barrage of waves.
Sorkin calculated that when you add a third slit to the barrier, quantum mechanics would predict an interference pattern that looks like the patterns for each possible two-slit combination laid over one another.
In other words, no new mysterious effects emerge in the three-slit case that aren鈥檛 present in the two-slit one, although the reason for this is unclear. So far, experiments have put bounds on how big any hypothetical effects could be, but haven鈥檛 been able to rule them out 鈥� yet.
Along with our collaborators, Selby and I have shown that any such effects must violate information conservation. In other words, they could be a hallmark of any future post-quantum theory.
Of course, the jury is still out on the specific form such a theory would take. But one thing鈥檚 for sure: if you think quantum mechanics is weird, you ain鈥檛 seen nothing yet.
This article appeared in print under the headline 鈥淏eyond Quantum鈥�
