Imagine you could exert an instant influence on a particle hundreds of kilometres away. Actually, little imagination is needed. Such entanglement – or “spooky action at a distance”, in Einstein’s derisive phrase – is a consistent effect in the quantum world.
So that’s not the anomaly. The anomaly comes in what this tells us about our perception of space and time.
This was brought into sharp relief through a series of experiments based on theoretical work done by the Irish physicist John Bell in the early 1960s. He worked out a mathematical way to tell if a measurement on one quantum particle (a photon of light, say) truly could change the result of a measurement carried out on another particle immediately afterwards, or whether some invisible, non-quantum influence was responsible.
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Implementing Bell’s test experimentally proved fiddly. The first attempt in 1981, with the result suggesting that only quantum effects could explain what was observed. But it was only last year that physicists at the University of Delft in the Netherlands devised a version of the test that conclusively ruled out standard information transfer, random particle fluctuations or detector snafus as the source of the effects. “This allows us to certify that the problem is not a feature of a particular setup or a consequence of the way in which an experiment was performed, but really points towards a fundamental feature of nature,” says of the Centre for Quantum Technologies at the National University of Singapore.
And the result is clear. In the quantum world, our normal understanding of space-time, and cause and effect within it, dictated by our intuitive sense of how the world works, does not apply. There is something we are just not getting.
What’s also clear is that since photons, electrons and other quantum particles can become entangled and follow these obscure rules, we can begin to devise experiments using them to explore the true nature of space-time. “These are particles we can manipulate,” Bancal says. “So we can consider using them to access [this layer of reality] indirectly.” How exactly to do that is a problem we’re still wrestling with, though – and until we find an answer, this is one anomaly our brains are hard-pressed to explain.
That’s odd: the accelerating universe
By any standards, this is an anomaly too large to ignore: distant supernova explosions are consistently dimmer than they should be, indicating they are further away than we expect. The standard explanation – that space’s expansion is speeding up thanks to an invisible “dark energy” that makes up two-thirds of the universe’s energy – replaces a big anomaly with an even bigger mystery.
Read more: “The 6 biggest glitches in physics”
This article appeared in print under the headline “That’s odd… Quantum entanglement”
