What quantum physics tells us about reality

The writer is a science commentator

The four-page paper was so famous it became known by the initials of its authors. The EPR paradox, published by Albert Einstein, Boris Podolsky and Nathan Rosen in 1935, argued that quantum physics clashed with reality and was therefore nonsensical or incomplete.

The paper set in train, over many decades, a series of arguments and experiments that culminated in the 2022 Nobel Prize for Physics. It was awarded last week to a trio of experimenters who, against Einstein’s instincts, affirmed the bizarre implications of quantum mechanics, the laws that describe the subatomic world. While the work of laureates Alain Aspect, John Clauser and Anton Zeilinger is rightly celebrated for helping to lay the ground for quantum computing and cryptography, their insights also prompt philosophical musing on the peculiar nature of the universe. “What [this Nobel Prize] shows is that any serious philosopher who wants to talk about the nature of reality had better pay close attention to quantum physics,” says Vlatko Vedral, a professor of quantum information science at Oxford university.

Einstein and colleagues bristled at quantum theory because it seemingly broke the principle of “locality”, which states an event that happens in one place cannot instantaneously affect something very far away. Another way of saying this is that nothing, not even information, can travel faster than the speed of light.

To restate the paradox: picture a pair of linked (or “entangled”) particles, A and B, shooting out of a radioactive nucleus at the same moment, moving at the same speed but in opposite directions. Quantum theory dictates that each particle exists in multiple possible observable states simultaneously — until the instant at which it is observed, when it “collapses” into one state with definite properties (such as position). One such property is called spin; and it is possible to produce A and B in such a way that the spin of A is related to the spin of B. The crucial point on entangled particles is this: measuring A’s spin automatically reveals the spin of B, and neither number is pre. -determined.

But what if A and B ended up at opposite ends of the universe? Measuring A would instantaneously reveal the spin of B, perhaps trillions of light years away, violating locality. How could a measurement here affect a particle way over there? Einstein himself derided the scenario as “spooky action at a distance”, suggesting there may be non-quantum factors, or “hidden variables”, at play.

The laureates were able to prove, through a succession of intricate experiments, that this spooky alignment — now called quantum entanglement — between two specially produced particles does indeed exist across vast distances without any recourse to non-quantum factors. The original inspiration for the experiments goes back to a brilliant Northern Irish physicist called John Bell, who mostly worked on accelerator design at Cern but dabbled impressively in quantum theory in his downtime. (Bell, who died in 1990 aged 62, would surely have won a Nobel were he alive today.)

In the 1960s, Bell signposted a way of resolving the EPR paradox, by explaining how to sniff out non-quantum factors. Clauser was the first to pick up these ideas experimentally, demonstrating that linked particles showed a high degree of spooky alignment when observed by detectors; Aspect and Zeilinger went even further, with the latter going on to demonstrate a phenomenon called quantum teleportation. Between them, they proved quantum entanglement is real — and that Einstein’s interpretation was wrong.

Quantum physics bothered Einstein because it clashed with his intuitive grasp of physical reality. In the quantum realm, nothing can be said to exist until it is measured or observed. That jars with our belief that particles have intrinsic properties: surely a banana is curved and yellow even when we are not looking at it? Vedral explains: “There is no underlying reality of the kind that Einstein imagined. However, quantum physics does not say that there is no reality out there. It just happens to be quantum.”

Quantum entanglement takes that discombobulation to new heights. Philip Ball, author of Beyond Weird, a well-regarded explainer on quantum physics, offers this description: “Once two particles have become entangled, then . . . they are no longer different objects. They are the same entity that you can’t break down.”

It feels utterly maddening to live in a world displaying phenomena so at odds with our everyday experience. Still, on another level, quantum reality is neither here nor there.

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