Double-Slit Experiment
Double-Slit Experiment
The double-slit experiment is the central demonstration of quantum weirdness. Feynman called it the phenomenon that "contains the only mystery" — the one that "cannot be explained in any classical way, and which has in it the heart of quantum mechanics."
The setup: a source fires particles (electrons, photons, even entire atoms) at a barrier with two narrow slits. Behind the barrier is a detector screen that counts where particles arrive.
Three Versions
With bullets. Fire macroscopic bullets through two slits at a wooden backstop. Count where they land. The result is two piles, one behind each slit. Close slit 2 and you get pile P₁. Close slit 1 and you get pile P₂. Open both and you get P₁₂ = P₁ + P₂. Probabilities add. No interference.
With water waves. Create circular waves from a source through two slits in a barrier. The two emerging wave fronts overlap and interfere: where crests align, the waves add (constructive interference, large intensity); where a crest meets a trough, they cancel (destructive interference, near-zero intensity). The intensity at the screen I₁₂ ≠ I₁ + I₂ — instead it has a characteristic oscillating pattern. Intensities do not add; amplitudes do, and intensity goes as amplitude squared.
With electrons. Fire electrons from a gun at a barrier with two slits. Detect where individual electrons arrive — each one lands as a discrete click in the detector, like a particle. But gather statistics over many electrons and the pattern of arrival probabilities looks exactly like the water-wave interference pattern. The distribution is P₁₂ = |φ₁ + φ₂|², where φ₁ and φ₂ are complex probability amplitudes — not P₁ + P₂.
The electrons arrive in lumps (particle behavior), but their probability distribution shows interference (wave behavior). This is the mystery.
The Observation Problem
You might guess: maybe each electron splits and goes through both slits simultaneously, interfering with itself. This is plausible. So you add a light source behind the wall to "watch" which slit each electron actually goes through. You scatter a photon off each electron, catching a flash of light near one slit or the other. Result: every electron goes through one slit or the other — Proposition A (each electron goes through slit 1 or slit 2, not both) is experimentally confirmed.
But now the interference pattern is gone. The distribution is P'₁₂ = P'₁ + P'₂. Watching the electrons restores classical behavior.
Turn down the light to reduce disturbance. The photons become less energetic, less able to disturb the electrons — but also less able to locate them precisely. As the wavelength gets long enough that you can no longer tell which slit the electron went through, the interference pattern gradually reappears. There is a direct tradeoff: precision of which-path knowledge vs. visibility of the interference pattern. This tradeoff is the uncertainty-principle made concrete.
The Verdict
Feynman's conclusion: we cannot determine which hole an electron passes through without destroying the interference. If we watch, behavior is classical; if we don't, behavior is quantum. The act of observation is not passive — it necessarily involves physical interaction (minimum one photon) that changes the outcome.
This does not mean consciousness causes collapse, or that the observer matters as a person. It means that any physical interaction that would allow you to determine the path destroys the interference. The minimum disturbance required for knowledge is exactly enough disturbance to eliminate the effect.
Connections
- wave-particle-duality — the experiment that makes the duality concrete
- probability-amplitudes — the mathematical structure; P = |φ₁ + φ₂|² not P₁ + P₂
- observation-destroys-interference — watching which slit eliminates the pattern
- uncertainty-principle — which-path knowledge vs. interference visibility is the uncertainty principle in action