Double-Slit Experiment

The double-slit experiment sends waves or quantum particles through two openings and observes interference at a detection screen when the alternatives remain coherent and indistinguishable. In the quantum version, single detections can be localized while the accumulated distribution shows interference.

Formula Hook

If the two paths contribute amplitudes $\mathcal A_1$ and $\mathcal A_2$ to the same detection event, then

$$\mathcal A = \mathcal A_1+\mathcal A_2, \qquad P = \lvert\mathcal A\rvert^2.$$

Expanding,

$$P = \lvert\mathcal A_1\rvert^2 + \lvert\mathcal A_2\rvert^2 + 2\operatorname{Re} \left( \mathcal A_1^*\mathcal A_2 \right).$$

The final term is the interference term. If which-path information becomes available in principle, the interference term is reduced or absent in the corresponding measurement context.

Canonical Home

This glossary entry is the current named-experiment home. For routing, see From Experiments to Formalism, Probability Amplitudes, Born Rule, and Decoherence Preview.

Common Confusions

• The experiment does not say that a particle is simply a tiny classical wave.
• Localized detection events and an interference distribution are compatible in the quantum description.
• Adding probabilities is correct only when alternatives are distinguishable or incoherent in the relevant context.
• “Observed” should not mean human consciousness; physical which-path records are enough to change the interference pattern.

Related Entries

• Amplitude
• Born Rule
• Decoherence

References

• C. Jönsson, “Elektroneninterferenzen an mehreren kuenstlich hergestellten Feinspalten,” Zeitschrift fuer Physik 161, 454-474, 1961.
• A. Tonomura et al., “Demonstration of single-electron buildup of an interference pattern,” American Journal of Physics 57, 117-120, 1989.
• R. P. Feynman, R. B. Leighton, and M. Sands, The Feynman Lectures on Physics, Volume III, Addison-Wesley, 1965.