The double-slit experiment is one of the most famous experiments in physics, and it dramatically reveals the wave-like behavior of particles, such as electrons, photons, and even larger atoms and molecules.
In the classic version, when light or particles are fired at a barrier with two narrow slits and then strike a screen behind it, something surprising happens:
- If both slits are open and no measurement is made to determine which slit the particle goes through, the pattern on the screen is not just two bands (as you’d expect if the particles behaved like tiny balls). Instead, an interference pattern appears—alternating bright and dark fringes—which is a hallmark of wave behavior.
- However, if a detector is placed to observe which slit the particle passes through, the interference pattern disappears, and the particles behave like classical particles, producing just two bands.
This experiment demonstrates the principle of wave-particle duality, a core concept in quantum mechanics. It shows that particles such as electrons and photons can exhibit both wave-like and particle-like properties, depending on how they are observed.
Key implications include:
- The act of measurement affects the system—an observer influences the outcome.
- Particles exist in a superposition of paths until measured.
- Quantum systems do not behave like classical objects; instead, they follow probabilistic laws.
The double-slit experiment challenges our intuition and reveals the fundamentally strange and non-deterministic nature of the quantum world.