Entanglement and Bell's Test
Two particles are entangled when their shared state cannot be split into a separate state for each one. Their results are then linked more strongly than any pre-set plan could explain, which John Bell turned into a testable limit in 1964. Experiments by Clauser, Aspect, Zeilinger and others broke that limit, earning the 2022 Nobel Prize, yet entanglement never lets anyone send a message faster than light.
Why did Einstein think entanglement meant something was missing?
In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen wrote a famous paper, now called EPR. They pointed out that quantum physics allows two particles to be linked. Measure one, and you know right away what the other will give, even if it is far away.
Einstein thought this showed that quantum physics was incomplete. Surely, he argued, each particle carries hidden instructions from the start. Then there is nothing strange. For almost 30 years, nobody knew how to test this.
Why should you care? Entanglement is a core resource in quantum computing. Most useful quantum algorithms create it. Quantum networks are built to share it.
- Isn't it just like a pair of gloves in two boxes?
- Run it: make an entangled pairINTERACTIVE
- How does Bell's test work? Work the numbers.
- Try this: Bob tilts his setting by 45 degreesINTERACTIVE
- Try this: the one setting pair that disagreesINTERACTIVE
- Who did the real experiments?
- Can entanglement send a message faster than light?
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