The double-slit experiment revealed that particles such as electrons and photons behave like waves until they are observed, at which point they act like particles. This observation led to one of the most profound ideas in quantum mechanics — that physical systems are described by a wavefunction, a mathematical expression representing the probability of finding a particle in a particular state or location. Unlike classical objects, which have definite properties, quantum particles exist only as probabilities until measured.
When no attempt is made to determine which slit a particle passes through, its wavefunction spreads out and interferes with itself, producing a characteristic interference pattern. This wave represents all possible paths the particle could take, and the pattern on the screen reflects the probability distribution of those possibilities. In this sense, quantum physics does not describe what is happening, but what might happen, assigning a likelihood to every outcome.
However, when a detector is placed at one of the slits to observe the particle’s path, the interference pattern disappears. The wavefunction, which once described multiple potential states, “collapses” into a single definite state — the particle is detected at one slit or the other. The act of measurement therefore changes the outcome, forcing nature to choose one possibility from many. This process, known as wavefunction collapse, sits at the heart of the quantum mystery.
Physicists still debate what exactly causes this collapse. Some interpret it as a real physical process triggered by measurement; others view it as a change in our knowledge about the system rather than in the system itself. Regardless of interpretation, the concept of wave probability and collapse reshaped how science understands reality. It implies that the quantum world is fundamentally indeterminate and that observation itself plays an essential role in bringing potential outcomes into existence. This marks the point where physics begins to merge with philosophy, questioning the boundary between what is possible and what is real.