Physics
Bell's Theorem
Quick fact
When experiments tested Bell's theorem in the 1970s and 1980s, they consistently violated the Bell inequality, confirming that the quantum world is inherently nonlocal—a result so surprising that Einstein called it 'spooky action at a distance'.
Why this is interesting
You probably assume that objects have definite properties even when you're not looking—and that nothing can travel faster than light. But what if these two commonsense ideas can't both be true at the same time?
Read the full explanation
Understanding Bell's Theorem
Imagine you have two magic coins: whenever one lands heads, the other always lands tails, no matter how far apart they are. That's like quantum entanglement. In the 1930s, Einstein, Podolsky, and Rosen (EPR) argued that such perfect correlations could only be explained if both coins actually had predetermined outcomes from the start—like carrying hidden instructions. This idea is called 'local realism': objects have real properties (realism) that are not influenced by distant events (locality). But in 1964, John Bell devised a clever mathematical test. He showed that if local realism holds, the correlations between measurements on entangled particles must obey a strict inequality. Quantum mechanics, however, predicts that this inequality can be violated. When experiments finally performed these measurements, the inequality was indeed violated—meaning local realism cannot be true. Something about the world is either nonlocal or not real (or both).
A deeper explanation
Bell's theorem works by considering measurements on pairs of entangled particles, such as photons with correlated polarizations. Each observer can choose between different measurement angles. If local realism were true, the outcomes would be predetermined by some hidden variable λ, and each particle's result would depend only on its local measurement setting and λ, not on the distant setting. Bell derived an inequality—the CHSH inequality, for example—that any local realistic theory must satisfy. Quantum mechanics violates this inequality for certain measurement angles (e.g., at 22.5° and 67.5°). Experimental tests, like those by Alain Aspect in 1982, closed key loopholes (locality and detection) and confirmed the violation. This means that either distant events can instantly influence each other (nonlocality), or objects have no definite properties until measured (anti-realism). Most physicists accept nonlocality because it aligns with quantum field theory and relativity's constraint that information cannot travel faster than light (though the nonlocality in Bell tests does not allow signaling). Bell's theorem thus laid the foundation for quantum information science, where entanglement is a resource for secure communication and computing.