The many-worlds interpretation (MWI) is a controversial interpretation of quantum mechanics that proposes the existence of multiple parallel universes. It was initially proposed by physicist Hugh Everett III in 1957. Whether the MWI is considered as physics or philosophy can be a matter of debate and perspective.
From a physics standpoint, the many-worlds interpretation is an attempt to explain the mathematical formalism and the probabilistic nature of quantum mechanics. It is considered a valid interpretation within the framework of quantum theory, even though it is not the only interpretation. The MWI provides a way to interpret quantum phenomena by positing that all possible outcomes of a quantum measurement occur in different branches or parallel universes, each with a different outcome.
On the other hand, the many-worlds interpretation also raises philosophical questions and implications. It challenges our notions of reality, causality, and the nature of observation. The idea of multiple universes branching off with each quantum event has implications for our understanding of consciousness, free will, and the nature of existence itself. These philosophical aspects are often debated and explored by philosophers, scientists, and scholars in addition to physicists.
So, while the many-worlds interpretation has its foundations in physics and attempts to explain quantum phenomena, it also has profound philosophical implications. The categorization of MWI as physics or philosophy can vary depending on the context and the perspective of the individuals discussing it.