You drop a cup and it smashes. I flick a light switch and the bulb glows. Effect follows cause – it is a hard-and-fast rule of the universe. Except, perhaps, at a fundamental level. Because when we are dealing with the electrons behind the working of the light switch and the atoms in the bulb that convert electrical energy to light, causality appears to be a lot fuzzier.
In 2017, a team at the University of Vienna in Austria described an experiment demonstrating that, in the quantum realm of atoms and particles, it is impossible to say which observations were the effect and which were the cause. It was, in the words of the researchers who did the experiment, “the first decisive demonstration of a process with an indefinite causal order”.
And yet the wider research community didn’t drop their coffee mugs. On the contrary, it was welcome news for at least some of those seeking to figure out where space-time comes from. For them, a quantum theory of gravity, in which space-time would be an emergent property of more fundamental constituents of the universe, might necessarily lack the definite one-way causality of everyday life.
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Space-time, as described by Albert Einstein’s theories of relativity, already has some fuzziness when it comes to defining the order of events. People moving through space and time in different ways have different “reference frames”, and those moving in different ways won’t always agree on whether event A happened before event B.
This doesn’t allow breaches of causality, though. The blurring of before and after happens only over distances so large that those regions of space can’t affect each other because of the limit on the speed of light. “If one event could send a light signal to the other, then there is no reference frame in which you could confuse their order,” says , now at the University of Bristol, UK, who led the 2017 work.
However, “indefinite causality” is possible in the minuscule quantum world because the rules that govern the behaviour of atoms, electrons and photons of light permit a phenomenon called “superposition”. This is where a system of such entities can exist in two or more states simultaneously – even if common sense would say that it is impossible for those states to co-exist. Thus the 2017 experiment involves creating a superposition of “A causes B” and “B causes A” when dealing with photons, or particles of light, that are themselves in superposition.
That isn’t discombobulating for physicists because the fundamental laws of quantum physics don’t specify a direction for time, says at the University of Cambridge: “Physics doesn’t care about the difference between past and future.”
Reverse causality
Hence, there is scope for “time-reversal symmetry”, where particles behave in the same way if you make time run in the opposite direction. Nor do physicists rule out the possibility of backwards causation, or °ů±đłŮ°ů´Çł¦˛ąłÜ˛ő˛ą±ôľ±łŮ˛â”, in which a light bulb’s glow could cause its switch to turn on.
The reason why some theorists embrace indefinite causality is that if space-time is fundamentally quantum mechanical – as many think it must be – then gravity must somehow be quantum mechanical too. To figure out what this quantum gravity looks like, at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, has suggested combining the characteristic features of general relativity and quantum mechanics: the malleability of space and time and superpositions of simultaneous possibilities, respectively. If you do that, he reasons, conventional notions of fixed time and causality must go.
“It seems that some situations, such as indefinite causal order, should be natural in quantum gravity,” says at the Institute for Quantum Optics and Quantum Information in Vienna.
