In the double-slit experiment, when no "which-slit" measurement is performed, there is empirical confirmation that quantum particles exhibit wave-like behavior and do not physically go through one definite slit.
The double-slit experiment is a classic demonstration of the wave-particle duality of quantum systems. When particles such as electrons or photons are sent through a barrier with two slits and allowed to interact with a screen on the other side, an interference pattern emerges, indicating wave-like behavior.
If the particles were to go through one slit or the other as classical particles, we would expect to see two distinct bands on the screen corresponding to the slits. However, what is observed is an interference pattern with alternating light and dark bands, which can only be explained if we consider the particles as waves interfering with each other.
This phenomenon has been extensively studied and confirmed experimentally with various particles, such as electrons, photons, and even large molecules like buckyballs. The interference pattern persists even when particles are sent through the slits one at a time, ruling out the possibility of particles interacting with each other.
Moreover, attempts to determine through which slit the particle passes, either by placing detectors or introducing any measurement apparatus, disrupt the interference pattern and result in a particle-like behavior, where the particles appear to go through one definite slit. This demonstrates the wave-particle duality and the influence of measurement on the behavior of quantum systems.
Therefore, the empirical evidence from the double-slit experiment strongly supports the idea that quantum particles do not physically go through one definite slit but instead exhibit wave-like behavior, leading to interference patterns when multiple paths are available.