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Can entanglement send messages faster than light?

Myth · The ansible of Ursula K. Le Guin's fiction, and every "quantum radio" since

Real seed. One true idea sits underneath. Everything built on it is invented.

What the scene or the claim says

Twist one particle of an entangled pair and its partner reacts at once, however far away, so you can signal instantly.

What is real

Entangled particles give measurement results that are correlated more strongly than any pre-agreed classical rule allows. That is tested: three loophole-free Bell experiments in 2015 (Delft, Vienna, NIST), and the 2022 Nobel Prize in Physics went to Aspect, Clauser and Zeilinger for this line of work.

What stops it working that way

• On its own, each side sees a stream of random coin flips. Nothing you do at one end changes what the other end sees.
• The correlation appears only when the two records are compared, and comparing needs an ordinary message, at light speed or slower.
• This is the no-signalling theorem. It holds in standard quantum mechanics at any distance.

If you wanted it to work

A faster-than-light message would need either a break in no-signalling, which by the usual relativity argument also lets you message the past, or a theory that replaces quantum mechanics. Every test so far, including Bell tests with distant stations, matches quantum mechanics. A departure from it would be a huge result, and a check worth running hard. The real payoff of entanglement is secure keys and linked computers.

Learn the physics

• The CHSH game: beat the classical limit

Sources

• Hensen et al. 2015, loophole-free Bell test (Nature)
• 2022 Nobel Prize in Physics

See it in the swipe deck, with the other cards.