Anyone who says they understand' Quantum Theory', doesn't understand it,

 Meanwhile,  an agnostic overview of one of the more colorful of the hypotheses, the many-worlds, or multiple universes, theory. For overviews of the other five leading interpretations, 

one may find I think such speculatons crazy, compared with common sense,  But in the quantum world , crazy does not necessarily mean wrong, and being more crazy does not necessarily mean more wrong.

That was something that Bohr bolted on to the theory to “explain” why we only see one outcome of an experiment — a dead cat or a live cat — not a mixture, a superposition of states. But because we only detect one outcome — one solution to the wave function — that need not mean that the alternative solutions do not exist

In a paper he published in 1952, Schrödinger pointed out the ridiculousness of expecting a quantum superposition to collapse just because we look at it

Updating his terminology, there are two parallel universes, or worlds, in one of which the cat lives, and in one of which it dies. When the box is opened in one universe, a dead cat is revealed. In the other universe, there is a live cat. But there always were two worlds that had been identical to one another until the moment when the diabolical device determined the fate of the cat(s). 

Nearly every result [the quantum theorist] pronounces is about the probability of this or that … happening — with usually a great many alternatives. The idea that they may not be alternatives but all really happen simultaneously seems lunatic t just impossible. One  thinks that if the laws of nature took this form for, let me say, a quarter of an hour, we should find our surroundings rapidly turning into a quagmire, or sort of a featureless jelly or plasma, all contours becoming blurred, we ourselves probably becoming jelly fish, nature is prevented from rapid jellification only by our perceiving or observing it