Ultimate ensemble
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The ultimate ensemble theory, also known as the mathematical universe hypothesis, and radical platonism, is a philosophical theory proposed by the cosmologist Max Tegmark. It claims that all existence is mathematical existence — that all objects are ultimately mathematical objects. Physics is ultimately mathematics, and our universe is a mathematical object. According to Tegmark, all mathematically possible universes are actual.
He presents it as a physics "theory of everything" (unification of all the known forces of physics), but really is more a philosophy, since it is so high-level that it is essentially impossible to express in the form of equations. It is a theory which makes no predictions — all possible observations are consistent with it. As such, it is not a scientific theory, but a piece of philosophy.
His theory is very similar to the modal realism of David Lewis — the claim that all logically possible universes are actual, and that the "actual universe" is simply an indexical, a way of saying "our universe" — just as "our house" has no special ontological status, it is only special to us in the sense that it is ours. Although Tegmark speaks of mathematically possible universes, and Lewis of logically possible universes, it seems that that mathematical and logical possibility are the same thing.