Bell's theorem

Week 2: reality ยท quantum

Bell's theorem

Walk through the inequality, what experiments measure, and which assumption gives way.

In this lesson

  1. Read the core idea (3 min).
  2. Walk through one worked example.
  3. Take the 4-question quiz; pass with 4/4 or better.

Hook

EPR forced a choice: keep locality and conclude quantum mechanics is incomplete, or accept that the wave function describes everything and that distant measurements somehow influence each other. For thirty years the choice looked like a matter of taste. Then in 1964 John Bell showed it was a matter of arithmetic.

Bell proved that any theory which is local AND assigns definite values to measurements before they happen must satisfy a specific inequality. Quantum mechanics predicts, and experiments confirm, that this inequality is violated. One of the two assumptions has to go.