The Double-Slit Experiment
Richard Feynman called the double-slit experiment 'a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics.' When particles pass through two narrow slits, they create an interference pattern — even when sent through one at a time. This simulator lets you watch this iconic experiment unfold.
Wave-Particle Duality in Action
Each photon in this simulation arrives at a definite point on the detection screen — that's the particle aspect. But the probability of where it lands is determined by the wave equation. With just a few photons, the pattern looks random. As hundreds and thousands accumulate, the familiar bright and dark interference fringes emerge from the statistical distribution. This is wave-particle duality made visible.
The Physics of Interference
The interference pattern combines two effects. Double-slit interference (the cos^2 term) creates regularly spaced bright fringes, with spacing determined by the ratio of wavelength to slit separation. Single-slit diffraction (the sinc^2 envelope) modulates these fringes, creating a broader pattern governed by the individual slit width. Together, I(theta) = I_0 cos^2(pi d sin theta / lambda) sinc^2(pi a sin theta / lambda).
What the Parameters Control
Wavelength affects fringe spacing — longer wavelengths produce wider fringes and different colors on the detection screen. Slit separation controls the interference pattern spacing inversely — wider separation means closer fringes. Slit width affects the diffraction envelope — narrower slits spread light more widely. Photon count controls how many particles are detected — start with just a few to see the randomness of individual detections, then increase to watch the pattern emerge.
A Deep Mystery
The double-slit experiment raises the deepest question in quantum mechanics: how does each individual particle 'know' about both slits? If you block one slit, the interference pattern vanishes. If you try to determine which slit the particle went through, the pattern also vanishes. The particle seems to explore all possible paths simultaneously, only 'choosing' a definite outcome when detected. Different interpretations of quantum mechanics (Copenhagen, many-worlds, pilot wave) offer different answers to this profound puzzle.