- Entangled particles show correlations that classical physics cannot explain.
- Bell's inequality sets a measurable limit that quantum experiments consistently violate.
- Entanglement cannot transmit information — the no-communication theorem proves this mathematically.
John Clauser spent the early 1970s trying to prove Einstein right. The experiment he and graduate student Stuart Freedman ran at UC Berkeley in 1972 was supposed to show that quantum mechanics was incomplete - that the "spooky action at a distance" Einstein mocked couldn't really exist.
The particles disagreed.
Their photons, separated by just three meters, showed correlations that classical physics couldn't explain. More experiments followed. In 1982, Alain Aspect at the University of Paris improved the test, making measurements so fast that no signal could pass between detectors. The correlations held.
Key figure
1.3 km
Distance between detectors in the 2015 loophole-free Bell test at Delft University
What Bell's Inequality Actually Tests
In 1964, physicist John Bell devised this test for a deceptively simple question: do particles have fixed properties before we measure them, or does measurement somehow create those properties?
Every experiment since Freedman and Clauser's has confirmed quantum violations.
What is Bell's inequality?
A mathematical limit on how strongly two particles can be correlated if the world works classically. Think of it like a ceiling: if particles have fixed properties before measurement and can't communicate faster than light, their correlations can't exceed a certain value. Quantum mechanics predicts violations of this ceiling - and experiments consistently confirm those violations.
The 2015 test led by Ronald Hanson at Delft University separated detectors by 1.3 kilometers, closing the last technical loopholes. The particles still showed impossible correlations.
The Catch That Protects Causality
Here's what entanglement doesn't do: it doesn't let you send information.
When you measure one entangled photon, you instantly know something about its partner - even across a galaxy. But that knowledge is useless until you compare notes with whoever measured the other photon. And comparing notes requires ordinary communication, limited by the speed of light.
The no-communication theorem, proven mathematically, explains why. You can't encode a message by manipulating your entangled particle. Any attempt to force it into a particular state breaks the entanglement entirely. The correlation exists, but it carries no controllable signal.
A Nobel Prize for Proving Einstein Wrong
In 2022, the Nobel Committee awarded the physics prize to Clauser, Aspect, and Anton Zeilinger. The citation honored not just the demonstration that entanglement is real, but the foundation their work built for quantum cryptography and quantum computing.
The irony would not have been lost on the laureates. Clauser started his career trying to vindicate Einstein's skepticism about quantum mechanics.
Instead, he helped establish that nature violates local realism - the intuition that objects have definite properties independent of measurement.
Entanglement remains genuinely strange. Two particles can share a connection that transcends distance, their fates correlated in ways no classical explanation can match. But the universe has rules. Information still travels at light speed or slower.
Einstein was wrong about entanglement being impossible. He was right that nothing outruns light.
Sources
- Primary Research: Nobel Prize in Physics 2022 awarded to Alain Aspect, John Clauser, and Anton Zeilinger for experiments with entangled photons
- Additional Context:
- Proving that quantum entanglement is real (Caltech)
- Bell test experiments (Wikipedia)
- No-communication theorem (Wikipedia)
Fact Check: Claim-by-Claim Verification Verified
The recap accurately reflects the history, purpose, and implications of Bell-test experiments and correctly explains why entanglement cannot be used for faster-than-light communication.
Commentary
- The description that the 2015 Delft test “closed the last technical loopholes” reflects the common characterization of “loophole-free” Bell tests, though specialists note that exotic possibilities like superdeterminism remain logically open; this subtlety is beyond typical popular-science scope but not misleading for general readers.
- The statement that “any attempt to force [an entangled particle] into a particular state breaks the entanglement entirely” is broadly accurate as an intuitive explanation, though in full quantum information theory one distinguishes between measurements, general local operations, and protocols that may partially preserve or redistribute entanglement.
Sources used for verification
Academic/Peer-reviewed:
- Experimental Test of Local Hidden-Variable Theories - aps.org
- Experimental Test of Bell's Inequalities Using Time-Varying Analyzers - unicamp.br (Phys. Rev. Lett.)
- Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km - arxiv.org
- Bell’s Theorem - stanford.edu
- Colloquium: Bell’s theorem and the experiments - aps.org
Other reliable sources:
- Proving that Quantum Entanglement is Real - caltech.edu
- The Nobel Prize in Physics 2022 – Press release - nobelprize.org
- Loophole-free Bell test at TU Delft crowns 80-year-old debate - tudelft.nl
- Physics Nobel honors foundational quantum entanglement experiments - physicstoday.org
- Pioneering Quantum Physicists Win Nobel Prize in Physics - quantamagazine.org
Fact-checked by Perplexity Sonar Pro on 2026-01-10
