The double-slit experiment is one of the most famous and important experiments in quantum physics. It demonstrates the fundamental mystery of quantum mechanics: wave-particle duality. The setup consists of an electron source, a barrier with two parallel slits, and a detection screen.
For classical particles like bullets or marbles, the behavior is straightforward. Each particle travels through either the upper slit or the lower slit, but never both. This creates two distinct bands on the detection screen, directly behind each slit. This is exactly what our everyday experience tells us should happen.
When classical waves like water waves or light waves pass through the double slits, they behave very differently from particles. The waves spread out after passing through each slit and interfere with each other. Where the wave peaks align, we get bright fringes. Where peaks meet troughs, they cancel out creating dark fringes. This creates the characteristic interference pattern.
Here comes the quantum mystery! When we fire electrons one at a time through the double slits, something incredible happens. Instead of creating two bands like classical particles, the electrons create an interference pattern just like waves! Each single electron somehow seems to go through both slits simultaneously and interfere with itself. This is the heart of wave-particle duality.
The most mind-bending aspect of the double-slit experiment is the observer effect. When we place a detector at one of the slits to determine which path the electron takes, something remarkable happens. The interference pattern completely disappears! The electron now behaves like a classical particle, creating two distinct bands. This shows that the very act of observation fundamentally changes quantum behavior, revealing the deep connection between measurement and reality in quantum mechanics.