The Double-Slit Experiment

The double-slit experiment is one of the most famous experiments in physics, demonstrating the fundamentally probabilistic nature of quantum mechanical phenomena and highlighting the wave-particle duality of matter and energy. First performed with light by Thomas Young in 1801, it has since been performed with electrons, atoms, and even large molecules.

Interactive Visualization

This interactive simulation demonstrates the double-slit experiment. Particles are emitted from a source on the left and pass through two slits in the barrier. The resulting pattern on the detector screen shows the characteristic interference pattern of waves, even though we're sending individual particles through the apparatus one at a time.

How to use: Watch as particles travel from the source (left) through the slits and create an interference pattern on the detector screen (right). Toggle between particle and wave views, and reset the experiment to start over.

Experimental Setup

The classic double-slit experiment consists of:

  • A source that emits particles or waves (e.g., electrons, photons)
  • A barrier with two parallel slits that the particles pass through
  • A detection screen that records where the particles land

When particles like electrons are fired one at a time, logic would suggest they should create two bands on the screen (one corresponding to each slit). Instead, they gradually build up an interference pattern characteristic of waves, suggesting each particle somehow interferes with itself.

The Paradox

The double-slit experiment reveals the paradoxical nature of quantum mechanics:

  • Wave behavior: When we don't observe which slit the particle passes through, an interference pattern forms on the screen, suggesting wave-like behavior.
  • Particle behavior: When we observe which slit the particle passes through (by placing detectors at the slits), the interference pattern disappears and we see two distinct bands, suggesting particle-like behavior.

This demonstrates that the mere act of observation affects the outcome of the experiment, a cornerstone of quantum mechanical interpretation.

Mathematical Description

In quantum mechanics, the probability of finding a particle at a point on the screen is proportional to the square of the absolute value of the wave function:

P(x)=∣ψ(x)∣2P(x) = |\psi(x)|^2

For the double-slit experiment, the wave function is the sum of the wave functions from each slit:

ψ(x)=ψ1(x)+ψ2(x)\psi(x) = \psi_1(x) + \psi_2(x)

The interference pattern arises because:

∣ψ(x)∣2=∣ψ1(x)+ψ2(x)∣2=∣ψ1(x)∣2+∣ψ2(x)∣2+2∣ψ1(x)∣∣ψ2(x)∣cos⁡(Δϕ)|\psi(x)|^2 = |\psi_1(x) + \psi_2(x)|^2 = |\psi_1(x)|^2 + |\psi_2(x)|^2 + 2|\psi_1(x)||\psi_2(x)|\cos(\Delta\phi)

where Δϕ\Delta\phi is the phase difference between the two paths. This interference term is what creates the distinctive bands of the interference pattern.

Historical Significance

The double-slit experiment has played a pivotal role in the development of quantum mechanics:

  • 1801: Thomas Young first performed the experiment with light, demonstrating its wave nature.
  • 1920s: The experiment was conceptualized for electrons as part of the early development of quantum mechanics.
  • 1961: Claus Jönsson performed the experiment with electrons, confirming wave-like behavior of particles.
  • 1974: The experiment was performed with single electrons by Pier Giorgio Merli, Gianfranco Missiroli, and Giulio Pozzi.
  • 1999-2013: The experiment was extended to larger and larger particles, including molecules with up to 810 atoms (phthalocyanine molecules and their derivatives).

Interpretations

The double-slit experiment has been interpreted in various ways:

  • Copenhagen Interpretation: Particles exist as probabilities (wave functions) until measured, at which point they "collapse" to definite states.
  • Many-Worlds Interpretation: All possible outcomes occur in different "branches" of the universe, with our consciousness following just one path.
  • Pilot Wave Theory: Particles always have definite positions, but are guided by a real physical wave.
  • Quantum Decoherence: Interaction with the environment (including measurement devices) causes the quantum system to lose its wave-like properties.

Richard Feynman noted that this experiment "contains the only mystery" of quantum mechanics, encapsulating the core non-intuitive nature of quantum reality.

Related Concepts

Wave-Particle Duality →
Heisenberg's Uncertainty Principle →