The Many-Worlds Interpretation (MWI) of quantum mechanics is a theory that suggests the existence of multiple parallel universes, or "worlds," to explain the behavior of quantum systems. In MWI, the double-slit experiment is understood in the following way:
In the double-slit experiment, when particles such as electrons or photons are sent through a barrier with two slits, they exhibit wave-like interference patterns on a screen behind the barrier. However, when a measurement is made to determine which slit the particle passes through, the interference pattern disappears, and a particle-like pattern emerges. This is known as the wave-particle duality.
According to the MWI, every possible outcome of a quantum measurement actually occurs in a separate universe or "branch" of reality. In the context of the double-slit experiment, when a particle is sent through the barrier, it simultaneously goes through both slits and creates an interference pattern on the screen. However, when a measurement is made to determine which slit the particle passes through, the observer becomes entangled with the particle's wavefunction, causing the system to split into multiple branches.
In one branch, the observer sees the particle passing through one specific slit, and in another branch, the observer sees the particle passing through the other slit. Each branch represents a different outcome of the measurement. The interference pattern disappears because the different branches of reality can no longer interfere with each other.
So, in the Many-Worlds Interpretation, the world splits happen at the point of measurement because that's when the observer becomes entangled with the quantum system, causing the system to branch into multiple possibilities. Before the measurement, the particle exists in a superposition of states, going through both slits simultaneously. But after the measurement, the different measurement outcomes are realized in separate branches of reality.
It's important to note that the Many-Worlds Interpretation is just one of the many interpretations of quantum mechanics, and it is still a subject of debate among physicists.