Six experiments that broke classical intuition.
From Young's 1804 slits to Wheeler's delayed-choice, the double-slit is the single experiment that refuses to fit. Six sections, each interactive. No mysticism — the resolution is Feynman's path integral.
Light is a wave.
Two coherent sources produce bright and dark fringes where wave crests add or cancel. Slide the wavelength, slit separation and screen distance. The fringe spacing follows x = λL/d.
One particle is enough.
Even with a single quantum in the apparatus at any time, the interference pattern builds up dot by dot. The pattern is not a collective wave phenomenon — it exists per particle. Pick a mass: from a single photon to a 25 kDa oligoporphyrin (Fein-Geyer-Arndt 2019, still showing 90 % visibility).
Watching destroys the pattern.
The instant the apparatus could in principle reveal which slit the particle went through, the fringes wash out into a classical sum. Englert formalised this with the inequality V² + K² ≤ 1: full which-way information K = 1 forces visibility V = 0, and vice versa. No consciousness needed — a sliver of paper behind a slit suffices.
Every path contributes.
Feynman's path integral: the particle does not 'choose' a slit. Every conceivable trajectory contributes an amplitude e^(iS/ℏ), the sum gives the probability. Classical trajectories emerge as the stationary-phase pile-up. Move the screen target — watch the rotating phasor sum bend into constructive or destructive interference live.
Erase the knowledge — the fringes return.
A pair of entangled photons born together via SPDC. One photon (Signal) goes through the slits with quarter-wave plates that mark its polarisation per slit. The fringes are gone. But measure the partner photon (Idler) in the complementary basis and you erase the which-way information — fringes reappear in coincidence. Critical: the marginal distribution of Signal never changes. Information does not flow backwards.
Decide after the photon entered.
Wheeler's gedanken: only after a photon has passed the first beam splitter, decide whether to put a second one in place (interferometer → wave behaviour) or leave it open (which-path → particle behaviour). Jacques et al. 2007 realised this with a 48 m Mach-Zehnder, an electro-optic modulator switching in under 40 ns, and space-like separation. The photon shows the matching behaviour either way.
Section 1 renders the 2D wave field with 1/√r damping — a cylindrical-wave idealisation; the real free-space wave falls as 1/r. Section 2 samples from P(y) ∝ cos²(πdy/(λL)) with a Gaussian envelope. The on-screen wavelengths are visual scalings; the de-Broglie readout shows the real values (electron 50 keV → 5.4 pm relativistic, C₆₀ from Arndt-Zeilinger 1999 at ~220 m/s, 25 kDa oligoporphyrin from Fein-Geyer-Arndt 2019 at 175 m/s). Section 4 uses the optical action approximation S = p·L; the ℏ slider is a visualisation parameter, not the actual constant. Section 5 plots the clean Walborn theory V = |cos 2θ|, not sampled data. Section 6 simplifies BS2-in to always D₁ (phase difference 0); a real Mach-Zehnder shows an interference pattern as a function of arm-length difference.
- Young, T. (1804) — The Bakerian Lecture: Experiments and Calculations Relative to Physical Optics. Phil. Trans. R. Soc. 94.
- Davisson, C. & Germer, L. H. (1927) — The Diffraction of Electrons by a Crystal of Nickel. Phys. Rev. 30.
- Tonomura, A. et al. (1989) — Demonstration of single-electron buildup of an interference pattern. Am. J. Phys. 57.
- Arndt, M. et al. (1999) — Wave-particle duality of C60 molecules. Nature 401.
- Bach, R. et al. (2013) — Controlled double-slit electron diffraction. New J. Phys. 15.
- Fein, Y. Y. et al. (2019) — Quantum superposition of molecules beyond 25 kDa. Nat. Phys. 15.
- Feynman, R. P. (1948) — Space-Time Approach to Non-Relativistic Quantum Mechanics. Rev. Mod. Phys. 20.
- Englert, B.-G. (1996) — Fringe Visibility and Which-Way Information: An Inequality. Phys. Rev. Lett. 77.
- Dürr, S., Nonn, T. & Rempe, G. (1998) — Origin of quantum-mechanical complementarity probed by a 'which-way' experiment in an atom interferometer. Nature 395.
- Scully, M. O. & Drühl, K. (1982) — Quantum eraser: A proposed photon correlation experiment concerning observation and 'delayed choice' in quantum mechanics. Phys. Rev. A 25.
- Walborn, S. P. et al. (2002) — Double-slit quantum eraser. Phys. Rev. A 65, 033818.
- Kim, Y.-H. et al. (2000) — A Delayed Choice Quantum Eraser. Phys. Rev. Lett. 84.
- Jacques, V. et al. (2007) — Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment. Science 315.
- Manning, A. G. et al. (2015) — Wheeler's delayed-choice gedanken experiment with a single atom. Nat. Phys. 11.