The double slit experiment is one of the fundamental experiments in quantum mechanics. The Englishman Thomas Young originally did it in 1802 to prove that light consists of waves. Since the 1920s, it has also been used to investigate whether particles of matter have wave or particle character. It quickly became clear that both are the case: matter and light exhibit something known as wave-particle duality. If quantum objects such as electrons, photons or neutrons fly through a double slit onto a detection screen, an interference pattern is observed that can be explained by their wave nature. Mathematically, this is expressed by its wave function. A secondary wave is created at each slit, and mutual amplification and cancellation produce fringes of an interference pattern on the screen. But as soon as you detect the waves, they appear as particles. Each of these creates only one dot on the screen. The interference pattern only gradually becomes visible through its statistical distribution. Interestingly, this also works if particles are allowed to arrive individually and there are never several in the experimental setup at the same time.
This “single-particle interference” is already a first indication that each object goes both ways at the same time. However, because the interference pattern only becomes visible in the collective, interpretations of quantum mechanics are also conceivable that assign a clearly defined trajectory to the individual particles. the Bohmian mechanics interprets the wave function as a “pilot wave” that explores, so to speak, which trajectories are possible. Each particle then moves on an apparently randomly chosen path, which, however, is clearly localized. According to the Many Worlds Theory, all possibilities are lived out in separate parallel worlds.
In order to examine such relationships more closely, one would have to measure which path an individual particle actually takes. But as soon as the experiment is designed in such a way that the particles can be detected as they fly past the slit, the interference disappears. Their visibility V and the ability P to determine the path are complementary, expressed mathematically by P² + V² ≤ 1.