In 1956 Hugh Everett III published his Ph.D. dissertation titled “The Theory of the Universal Wave Function.” In this paper Everett argued for the relative state formulation of quantum theory and a quantum philosophy, which denied wave collapse. Initially, this interpretation was highly criticized by the physics community and when Everett visited Niels Bohr in Copenhagen in 1959. Bohr was unimpressed with Everett’s most recent development.
In 1957 Everett coined his theory as the Many Worlds Interpretation of quantum mechanics. In an attempt to circumvent the problem of defining the mechanism for the state of collapse, Everett suggested that all orthogonal relative states are equally valid ontologically. What this means is that all possible states are true and exist simultaneously.
We have a problem of using secondary sources. I’ve provided a link below that takes you back to Everett’s original dissertation to read for ourselves.