Many-worlds interpretation (MWI) is a particular interpretation of quantum mechanics that suggests that all possible outcomes of a quantum measurement actually occur in separate branches of reality. In MWI, the superposition of microscopic states leads to the idea of multiple parallel universes, where each possible outcome of a quantum event exists in its own branch.
To understand how quantum superpositions of microscopic states can lead to the concept of many worlds, let's consider a simplified example using the famous Schrödinger's cat thought experiment. In this scenario, a cat is placed in a box along with a quantum system, such as a radioactive atom. The system is prepared in a superposition of two states: one in which the atom has decayed (cat is dead), and the other in which it hasn't decayed (cat is alive). According to quantum mechanics, until the box is opened and the system is observed, the cat exists in a superposition of being both dead and alive.
In the framework of MWI, the superposition of the cat's state leads to the branching of reality into two separate universes: one in which the observer finds a dead cat and another in which the observer finds a live cat. Both outcomes are considered equally real but exist in separate branches of reality.
MWI extends this idea to all quantum events, not just macroscopic examples like Schrödinger's cat. It suggests that any measurement or interaction that leads to the collapse of a quantum superposition actually results in the branching of the universe into multiple worlds, each corresponding to one of the possible outcomes.
The reason MWI can apply to macroscopic scales is that quantum superpositions, in principle, can involve systems of any size. While it is more challenging to maintain quantum coherence and superposition in macroscopic objects due to interactions with the environment (decoherence), MWI suggests that if quantum coherence is preserved, multiple parallel branches of reality can arise.
It's important to note that the many-worlds interpretation is just one of several interpretations of quantum mechanics. While MWI provides a consistent mathematical framework to describe quantum phenomena, it is a controversial interpretation and not universally accepted by all physicists. Alternative interpretations, such as the Copenhagen interpretation, offer different perspectives on the nature of quantum superpositions and the collapse of the wave function. The choice of interpretation ultimately depends on philosophical and conceptual preferences, as the mathematical predictions of quantum mechanics remain the same regardless of the interpretation chosen.