According to the Copenhagen interpretation of quantum mechanics, the behavior of quantum entities, such as electrons or photons, is described by a wave function that evolves over time. In the double-slit experiment, when the which-slit measurement is not performed and there is no observation of the particle's path, the wave function of the quantum entity passes through both slits simultaneously.
In this interpretation, the quantum entity is considered to be in a superposition of states, existing as a wave that spreads out and interferes with itself. This wave nature of the particle is often represented mathematically using the concept of a probability wave, which describes the likelihood of finding the particle at different locations.
As the wave function passes through the double slits, it diffracts and creates an interference pattern on the screen or detector placed behind the slits. This pattern arises due to the constructive and destructive interference of the wave function with itself. The regions of constructive interference correspond to areas where the particle is more likely to be detected, while the regions of destructive interference correspond to areas of lower probability.
It's important to note that the Copenhagen interpretation doesn't provide a concrete physical picture of what the quantum entity is "doing" or "how it travels" through the slits. Instead, it focuses on the mathematical description of the wave function and the probabilities associated with different outcomes when measurements are made. The interpretation emphasizes the role of measurement in collapsing the wave function to a specific state, which determines the outcome observed by an experimenter.