Imagine dropping two rocks into a pond. The waves spread out, overlap, and create a pattern of peaks and gaps. Now imagine two lines of billiard balls rolling toward two openings. Each ball goes through one slit or the other and lands in a predictable scatter. Waves blend. Particles take paths.
The double-slit experiment takes that simple contrast and turns it into one of the great mysteries of physics.
In the classic version, light is sent through two narrow slits. Instead of forming two simple bands behind the slits, it creates an interference pattern, just like overlapping waves in water. That showed light behaves like a wave.
But later versions made the mystery deeper. When physicists sent electrons, atoms, or even larger particles through the slits one at a time, the same kind of interference pattern slowly appeared. Each particle landed in one place, like a particle. But the pattern built up over time, like a wave.
That is the strange part.
If the experiment is set up so we can tell which slit the particle went through, the interference pattern disappears. The result becomes particle-like. But when the path is not measured, the wave-like pattern returns.
The double-slit experiment does not prove that particles are magic, conscious, or literally choosing. It shows that our everyday categories break down at the quantum level. Quantum behavior is empirical. The wavefunction is rational. The deeper meaning is metaphysical.
Wave or particle?
The answer seems to be: it depends how nature is tested.