Alice and Bob, the stars of so many thought experiments cook dinner when accidents happen. Alice accidentally drops a plate; the sound startles Bob, who burns himself on the stove and screams. In another version of events, Bob burns himself and screams, knocking down a plate for Alice.
Over the past decade, quantum physicists have explored the implications of a bizarre realization: In principle, both sides of the story can happen at the same time. In other words, events can occur in an undefined causal order, where “A cause B” and “B cause A” are simultaneously true.
“It sounds outrageous,” admitted Časlav Brukner, a physicist at the University of Vienna.
The possibility stems from the quantum phenomenon known as superposition, where particles simultaneously maintain all possible realities until the moment they are measured. In laboratories in Austria, China, Australia and elsewhere, physicists observe an indefinite causal order by placing a particle of light (called a photon) in a superposition of two states. They then submit one branch of the superposition to process A followed by process B, and submit the other branch to B followed by A. In this procedure, known as a quantum switch, the outcome of A influences what happens in B, and vice versa; the photon simultaneously undergoes the two causal orders.
Over the past five years, a growing community of quantum physicists have implemented the quantum switch in tabletop experiments and explored the benefits of indefinite causal order for quantum computing and communication. It is “really something that could be useful in everyday life,” said Giulia Rubino, a researcher at the University of Bristol who led the first experimental demonstration of the quantum switch in 2017.
But the practical uses of the phenomenon only make the deeper implications more acute.
Physicists have long felt that the usual picture of events unfolding as a sequence of cause and effect fails to capture the fundamental nature of things. They say this causal perspective probably has to go away if we are ever to determine the quantum origin of gravity, space, and time. But until recently, there weren’t a lot of ideas for how post-causal physics might work. “A lot of people think that causation is so fundamental in our understanding of the world that if we weaken this notion, we wouldn’t be able to make coherent and meaningful theories,” said Brukner, who is one of the leaders in the study of indefinite causality.
This changes as physicists contemplate the new quantum switching experiments, as well as related thought experiments in which Alice and Bob face the causal indefiniteness created by the quantum nature of gravity. Taking these scenarios into account has forced researchers to develop new formalisms and mathematical ways of thinking. With emerging frameworks, “we can make predictions without having well-defined causation,” Brukner said.
Correlation, not causation
Progress has accelerated recently, but many practitioners trace the origin of this line of attack on the problem of quantum gravity at work 16 years ago by Lucien Hardy, Anglo-Canadian theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. “In my case,” said Brukner, “it all started with the article by Lucien Hardy.”
Hardy was best known at the time for taking a conceptual approach made famous by Albert Einstein and applying it to quantum mechanics.
Einstein revolutionized physics not by thinking about what exists in the world, but by considering what individuals can possibly measure. He notably imagined people on moving trains making measurements with rulers and clocks. Using this “operational” approach, he was able to conclude that space and time must be relative.
In 2001, Hardy applied this same approach to quantum mechanics. He reconstructs all quantum theory from five operational axioms.
He then set out to apply it to an even bigger problem: the 80-year-old problem of reconciling quantum mechanics and general relativity, Einstein’s epic theory of gravity. “I’m motivated by the idea that maybe the operational way of thinking quantum theory can be applied to quantum gravity,” Hardy told me of Zoom this winter.
The operational question is: in quantum gravity, what can we, in principle, observe? Hardy thought that quantum mechanics and general relativity each have a radical characteristic. Quantum mechanics is notoriously indeterminist; its overlays allow simultaneous possibilities. General relativity, on the other hand, suggests that space and time are malleable. In Einstein’s theory, massive objects like the Earth stretch the “metric” of space-time – essentially the distance between hash marks on a ruler and the time between ticks of clocks. The closer you are to a massive object, for example, the slower your clock will run. The metric then determines the “cone of light” of a nearby event – the region of space-time that the event can causally influence.