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Double-Slit Interference

The Most Famous Experiment in Physics​

The double-slit experiment is often called "the most beautiful experiment in physics." It demonstrates the fundamental mystery of quantum mechanics: wave-particle duality.

"I think I can safely say that nobody understands quantum mechanics." — Richard Feynman

When particles are sent through two slits one at a time, they still form an interference pattern - as if each particle interferes with itself.

The Physics​

Two waves from the slits interfere:

  • Constructive interference (bright bands): Waves arrive in phase
  • Destructive interference (dark bands): Waves arrive out of phase

The Interference Condition​

Bright fringes appear where: d sin(θ) = nλ (n = 0, ±1, ±2, ...)

Where:

  • d is the slit separation
  • θ is the angle from center
  • λ is the wavelength
  • n is the fringe order

The central bright fringe (n=0) is the brightest, with diminishing intensity for higher orders.

What Makes It Quantum?​

The truly strange thing: even when particles pass through one at a time, the interference pattern still builds up. This means:

  1. Each particle somehow "knows about" both slits
  2. The particle doesn't have a definite path until measured
  3. The wavefunction passes through both slits simultaneously

The Simulation​

Grid: 512 × 512 points
Slit width: 6.0 units
Slit separation: 30.0 units
Wavepacket width: 25.0 units
Initial momentum: kx = 3.0

Animation​

The video above shows the wavepacket evolution in real-time, demonstrating the famous quantum interference pattern.

Anatomy of the Pattern​

                    ┌─────────────────────────────────────┐
                    │     Double-Slit Intensity Pattern    │
                    ├─────────────────────────────────────┤
      Intensity     │                 ██                   │
         ↑          │                ████                  │
         │          │               ██████                 │
         │          │        ██    ████████    ██         │
         │          │       ████  ██████████  ████        │
         │          │    ██████████████████████████████   │
         └──────────┴─────────────────────────────────────┘
                              ← Position →

                    Central    Secondary   Tertiary
                    Maximum     Maxima      Maxima

The pattern shows:

  • Central maximum: Brightest fringe at center
  • Side maxima: Regular spacing determined by d and λ
  • Envelope: Overall intensity modulated by single-slit diffraction

Run It Yourself​

claude -p "Simulate double-slit interference: Create a barrier at x=85 with two slits \
  (separation=20, height=10), use an elliptical wavepacket with width=[15,50] \
  (vertical long axis to span slits), show potential overlay, add sensor line at x=220, \
  and save to /tmp/double_slit.gif" --allowedTools "mcp__quantum-mcp__*"

The Fringe Spacing​

The distance between adjacent bright fringes on a screen at distance L: Δy = λL/d

This is how the wavelength of light (or matter waves) can be measured.

Historical Note​

  • 1801: Thomas Young first demonstrated double-slit interference with light
  • 1927: Davisson and Germer showed electron diffraction
  • 1961: Jönsson demonstrated double-slit interference with electrons
  • 2012: Demonstrated with molecules containing 800+ atoms