In a double-slit experiment, if the path of the particle is not observed or measured, an interference pattern will be observed on the screen or detector placed behind the slits. This pattern arises due to the wave-like nature of particles, such as electrons or photons, which can exhibit interference effects.
When particles pass through the double slits, they behave as waves that can diffract and interfere with each other. This interference occurs when the waves from the two slits overlap and either reinforce each other (constructive interference) or cancel each other out (destructive interference). The result is an alternating pattern of bright and dark fringes on the detector.
Importantly, the interference pattern suggests that the particle behaves as if it has gone through both slits simultaneously and interfered with itself. This phenomenon is often explained using wave-particle duality, which states that particles can exhibit both particle-like and wave-like properties depending on the experimental setup.
It's worth noting that once the path of the particle is observed or measured, for example by placing a detector at one of the slits, the interference pattern disappears, and the particles behave more like individual particles, showing a pattern consistent with the slits' positions. This is known as the "collapse of the wavefunction" or "wavefunction collapse," where the act of measurement disturbs the system and determines which path the particle takes, destroying the interference pattern.