Many-Worlds Interpretation
A serious physics reading of quantum mechanics in which every possible outcome happens, each in its own branch of reality.
In 1957 Hugh Everett III, then a Princeton graduate student, published a paper arguing that quantum mechanics needs no mysterious "collapse" when something is measured. Instead, the equations keep describing every possible outcome, and each outcome becomes part of a branching, ever-growing set of histories. This is now called the many-worlds interpretation.
It is not a fringe claim. Surveys of physicists show it is one of several mainstream readings of quantum theory, alongside others such as the Copenhagen interpretation and pilot-wave theory. David Deutsch and Sean Carroll are among the physicists who defend it at length in books and papers.
What is missing is a test. Every interpretation of quantum mechanics currently makes the same predictions in the laboratory, so no experiment has told many-worlds apart from its rivals. That is why we place it at Almost credible: real physics, real defenders, no decisive evidence.
Do not confuse it with the idea that you could visit another branch. The theory itself says branches cannot interact once they have separated.
In many-worlds, a quantum measurement does not pick one result. Every result happens, and you split along with it.
Critics such as Adrian Kent argue that many-worlds has trouble explaining why outcomes have the probabilities they do, and many physicists object that it cannot be tested.
