![]() Suppose these entangled electrons are separated and transported to distant laboratories, and that teams of scientists in these labs can rotate the magnets of their respective detectors any way they like when performing spin measurements. What’s remarkable about this entangled state is that, although the total spin has this definite value along all axes, each electron’s individual spin is indefinite. Consider a specific example of an entangled state: a pair of electrons whose total spin is zero, meaning measurements of their spins along any given axis will always yield opposite results. Local Hidden VariablesĪrmed with this understanding of spin, we can devise a thought experiment that we can use to prove Bell’s theorem. Quantum theory asserts that this property of spin detectors is actually a property of spin itself: If an electron has a definite spin along one axis, its spin along any other axis is undefined. It turns out it’s not possible to build any detector that can measure a particle’s spin along multiple axes at the same time. ![]() You’ll always measure a binary spin value - either up or down - along any axis. Again, the electron will always deflect by the same amount toward one of the poles. Now rotate the axis between the magnet poles away from vertical, and measure deflection along this new axis. Measuring its deflection will reveal whether the electron’s spin is “up” or “down” along the vertical axis. Imagine an electron passing through a region with the north pole directly above it and the south pole directly below. This shows that the electron’s spin is a quantity that can have only one of two values: “up” for an electron deflected toward the north pole, and “down” for an electron deflected toward the south pole. ![]() When, for instance, an electron passes through a magnetic field created by a pair of north and south magnetic poles, it gets deflected by a fixed amount toward one pole or the other. Particles with spin behave somewhat like tiny magnets. To understand entanglement more precisely, consider a property of electrons and most other quantum particles called spin. The unpredictable outcome of one measurement appears to instantly affect the outcome of the other, regardless of the distance between them - a gross violation of locality. Yet measuring the properties of entangled particles yields results that are strongly correlated, even when the particles are far apart and measured nearly simultaneously. Famously, in quantum mechanics a particle’s location, polarization and other properties can be indefinite until the moment they are measured. The “spooky action” that bothered Einstein involves a quantum phenomenon known as entanglement, in which two particles that we would normally think of as distinct entities lose their independence. Here’s how Bell’s theorem showed that “spooky action at a distance” is real. “The quantum revolution that’s happening now, and all these quantum technologies - that’s 100% thanks to Bell’s theorem,” says Krister Shalm, a quantum physicist at the National Institute of Standards and Technology. In the years since, experiments have vindicated quantum mechanics again and again.īell’s theorem upended one of our most deeply held intuitions about physics, and prompted physicists to explore how quantum mechanics might enable tasks unimaginable in a classical world. Bell proved that quantum mechanics predicted stronger statistical correlations in the outcomes of certain far-apart measurements than any local theory possibly could. Then in 1964, with the stroke of a pen, the Northern Irish physicist John Stewart Bell demoted locality from a cherished principle to a testable hypothesis. ![]() Physicists wondered whether quantum mechanics was missing something. So when Albert Einstein and two colleagues showed in 1935 that quantum mechanics permits “spooky action at a distance,” as Einstein put it, this feature of the theory seemed highly suspect. ![]() This principle, which physicists call locality, was long regarded as a bedrock assumption about the laws of physics. We take for granted that an event in one part of the world cannot instantly affect what happens far away. ![]()
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