Quantum physics · Landmark · Young 1801 → understood
The Double-Slit Experiment
One thing at a time — and still it interferes.
Make a beam of light so faint that it comes out one tiny speck at a time — picture each as a little bullet of light. Fire these specks, one by one, at a wall with two narrow slits in it, and watch where each lands on a screen behind.
Every speck arrives at a single point, a bright dot — just like a thrown pebble. But here is the shock: as the dots pile up, hundreds and then thousands of them, they don't gather into two clumps behind the two slits. They slowly paint a striped pattern of bright and dark bands — as if each single speck somehow went through both slits at once and overlapped with itself.
And the moment you peek to see which slit a speck really goes through, the stripes vanish and you get just two plain bands. Looking at it changes what happens. Try the button under the picture.
The stripes are interference, and they come from treating light as a wave. By Huygens' idea, each slit acts as a fresh source of circular wavefronts; the two sets of ripples spread out and overlap. Where crest meets crest the waves reinforce (a bright fringe); where crest meets trough they cancel (a dark one).
Whether they arrive in step depends on the path difference — the extra distance from one slit versus the other. For slits a distance \(d\) apart, light heading at an angle \(\theta\) has a path difference \(d\sin\theta\), so the bright fringes sit where
\[ d\sin\theta = m\lambda, \qquad m = 0, \pm 1, \pm 2, \dots \]
The fringes are evenly spaced on a screen a distance \(L\) away by \(\Delta y \approx \lambda L / d\). So a longer wavelength or closer slits spread the fringes wider — drag the sliders and watch the spacing breathe. Thomas Young used exactly this, in 1801, to measure the wavelength of light.
Quantum mechanics assigns each particle a complex probability amplitude, and the probability of finding it somewhere is the squared modulus — the Born rule, \(P = |\psi|^2\). With both slits open the amplitude is a sum over paths, \(\psi = \psi_1 + \psi_2\), so
\[ P = |\psi_1 + \psi_2|^2 = |\psi_1|^2 + |\psi_2|^2 + \underbrace{2\,|\psi_1||\psi_2|\cos\Delta\varphi}_{\text{interference}} . \]
The first two terms are the dull single-slit blobs; the cross term is the fringes, oscillating with the relative phase \(\Delta\varphi = \tfrac{2\pi}{\lambda}\,(r_2 - r_1)\). Picture each amplitude as a little rotating arrow — a phasor. Where the two phasors point the same way they add (bright); where they oppose they cancel (dark).
Now introduce a which-path measurement. It entangles the particle with a detector, \(\psi_1|D_1\rangle + \psi_2|D_2\rangle\); ignoring the detector multiplies the cross term by the overlap \(\langle D_1|D_2\rangle\). As the path becomes knowable that overlap shrinks, the relative phase smears, and the interference term averages to zero — decoherence, recovering the classical sum \(|\psi_1|^2 + |\psi_2|^2\). The fringe visibility \(V\) and path distinguishability \(\mathcal{D}\) trade off as \(V^2 + \mathcal{D}^2 \le 1\). Dial the which-path slider and watch \(V\) fall.
Specks fired one at a time land as single dots — yet, dot by dot, a striped pattern appears. Press “watch which slit” to make the stripes collapse. Circular wavelets from the two slits overlap; dark channels are where they cancel, and the screen reads out the fringes. Slide \(\lambda\) and \(d\) to change the spacing. Each screen point sums two phasors: aligned ⇒ bright, opposed ⇒ dark. Add which-path info and watch the cross-term fade.
No one is watching — dot by dot, the stripes build up.
Reduced-motion is on: the figure shows a resolved still frame and the controls are paused.