In the double-slit experiment, when performed with individual particles such as photons, electrons, or even larger molecules, it has been observed that these particles can exhibit interference patterns, as if they had traveled through both slits simultaneously. This phenomenon is often described as "wave-particle duality."
In the case of a single photon, it does not travel through both slits in a classical sense. Instead, the photon's behavior is described by a probability wave, represented by its wave function. The wave function evolves according to the laws of quantum mechanics, and it encodes the probability distribution for the photon's possible locations when measured.
Initially, the wave function encompasses both possible paths (slits) and interferes with itself, resulting in an interference pattern on the detection screen. However, upon measurement or interaction with the detection screen, the photon is detected at a specific location, appearing as a particle-like event.
The double-slit experiment demonstrates the wave-particle duality of quantum objects. While the wave function describes the probabilistic behavior of the photon, the actual measurement or interaction collapses the wave function to a definite outcome. The interpretation of this phenomenon has been a subject of debate among physicists, with various interpretations, such as the Copenhagen interpretation or the many-worlds interpretation, providing different perspectives on the underlying reality of quantum phenomena.