Hardy’s paradox, proposed by physicist Lucien Hardy in 1992, reveals a deep and surprising conflict between quantum mechanics and classical logic, even in the absence of Bell-type inequalities or strong entanglement.
The paradox involves a thought experiment with two particles—typically an electron and a positron—traveling through overlapping interferometers. Under certain conditions:
- Quantum mechanics predicts that there’s a nonzero probability the particles will both be detected in locations that should be mutually exclusive according to classical reasoning.
- Classically, if the particles take specific paths, they should interact and annihilate—yet quantum theory allows for both to appear in separate detectors without annihilation.
This leads to a logical contradiction:
- If we assume the particles have well-defined paths (a classical idea), the detection outcomes shouldn’t be possible.
- But quantum theory predicts—and experiments confirm—that such outcomes do occur with measurable probability.
What makes Hardy’s paradox especially intriguing is that:
- No Bell inequality is violated,
- And no maximal entanglement is required,
- Yet it still challenges the core assumptions of classical realism and locality.
In essence, Hardy’s paradox highlights how quantum systems can defy classical expectations in subtle yet profound ways, reinforcing the non-intuitive nature of quantum logic—even in seemingly simple setups.