The Double-Slit Experiment

The Most Beautiful Experiment in Physics

The double-slit experiment is often called "the only mystery" of quantum mechanics. First performed with light by Thomas Young in 1801, and later with electrons, it revealed something extraordinary: particles can behave like waves, and the act of observation changes reality.

The Classical Setup

Imagine shooting particles (like electrons or photons) one at a time through two narrow slits toward a detector screen. Classically, you'd expect each particle to go through one slit or the other, creating two bright bands on the screen corresponding to each slit.

But that's not what happens.

The Quantum Reality

Instead, an interference pattern emerges—alternating bright and dark bands, exactly like what you'd see if waves were passing through both slits and interfering with each other. This happens even when particles are sent one at a time, ruling out particle-particle interactions.

The mathematical description involves the wave function ψ (psi), which describes the probability amplitude of finding a particle at any location:

When both slits are open, the wave function is a superposition:

The Observer Effect

Here's where it gets truly bizarre. If you place a detector at the slits to determine which slit each particle passes through, the interference pattern disappears. The particles suddenly behave classically, going through one slit or the other and creating just two bands.

This isn't because the detector physically disturbs the particles. Even "gentle" measurements that don't transfer momentum still collapse the interference pattern. The mere act of gaining "which-path information" fundamentally changes the outcome.

What Does It Mean?

Several interpretations attempt to explain this:

• Copenhagen Interpretation: Particles exist in superposition (both paths simultaneously) until measurement collapses the wave function
• Many-Worlds: The particle takes both paths in different branches of reality
• Pilot Wave Theory: Particles have definite trajectories guided by a wave field

The Math: Interference Pattern

The intensity pattern on the screen follows:

The de Broglie wavelength for particles with momentum p is:

where h is Planck's constant. This means even massive objects have wavelengths, though they're typically too small to observe interference effects.

Why It Matters

This experiment demolishes classical intuition about reality:

• Reality is fundamentally probabilistic, not deterministic
• Observation plays a role in determining outcomes
• Particles don't have definite properties until measured
• Quantum systems can exist in superposition states

As Richard Feynman said, "Nobody understands quantum mechanics." But we can use it, predict with it, and even exploit it for technologies like quantum computers.