The Double-Slit Experiment: A Quantum Enigma

Experiment in quantum mechanics that shows wave–particle duality.

The double-slit experiment is one of the most fascinating and perplexing demonstrations of quantum mechanics. It provides a clear and undeniable illustration of the wave-particle duality and the superposition principle, two of the most fundamental concepts in quantum theory.

The Experiment

The double-slit experiment involves shining light (or firing particles such as electrons) at a barrier with two slits. If light behaved strictly as particles, we would expect to see two bright lines on the screen behind the barrier, corresponding to the two slits. However, what we observe is an interference pattern of light and dark bands. This pattern indicates that light behaves as a wave, with the light waves interfering with each other to create the pattern.

When the experiment is conducted with particles like electrons, the results are even more surprising. Even when electrons are fired one at a time, an interference pattern still emerges over time. This suggests that each electron interferes with itself, as if it passes through both slits simultaneously and behaves like a wave.

Interpretation and Significance

The double-slit experiment challenges our classical understanding of reality. It suggests that particles can behave like waves and can exist in multiple places at once, a state known as superposition. This experiment is a clear demonstration of the wave-particle duality, the idea that all particles exhibit both wave and particle properties.

The results of the double-slit experiment also lead to the concept of quantum uncertainty. It appears that we cannot know both the path taken by the particle and the interference pattern. This is a manifestation of Heisenberg's uncertainty principle, which states that the position and momentum of a particle cannot both be precisely measured at the same time.

The double-slit experiment has profound implications for our understanding of the quantum world. It shows that at a fundamental level, reality does not behave in the deterministic way we might expect from our everyday experiences. Instead, quantum mechanics describes a world of probabilities and uncertainties, where particles can exist in multiple states at once and reality is created by the act of observation.