But that is almost quotidian compared to what quantum mechanics reveals. It is not just a new opening in the dome, but a new kind of opening. Physicists and philosophers have long argued over what quantum theory means, but, in some way or other, they agree that it reveals a vast realm lying beyond the range of our senses. Perhaps the purest incarnation of this principle—the most straightforward reading of the equations of quantum theory—is the many-worlds interpretation, put forward by Hugh Everett in the 1950s. In this view, everything that can happen does in fact happen, somewhere in a vast array of universes, and the probabilities of quantum theory represent the relative numbers of universes experiencing one outcome or another. As David Wallace, a philosopher of physics at the University of Southern California, put it in his 2012 book, The Emergent Multiverse, when we take quantum mechanics literally, “the world turns out to be rather larger than we had anticipated: Indeed, it turns out our classical ‘world’ is only a small part of a much larger reality.” [...]
MIT cosmologist Max Tegmark made this idea concrete during a talk he gave in 2002, which evolved into his 2014 book, Our Mathematical Universe. He describes several “levels” of multiverse. Level I simply refers to very, very distant regions of our own universe. Level III is his term for the quantum many worlds. (He also has levels II and IV, but we don’t need to worry about them here.) To see the similarity between levels I and III, you have to think about the nature of probability. If something can have two different outcomes, you only see one of them, but you can be sure that the other one has also happened—either in some other part of a giant universe or in a parallel world right here. If space is sufficiently large and filled with matter, events that occur here on Earth will also occur elsewhere, as will every possible variation of those events. [...]
The key, Bousso explains, is to think carefully about your point of view. Imagine taking a God’s-eye view of the multiverse in which you see all of the possibilities unfolding at once. There is no probability; everything happens with certainty, in some location. But from our own limited perspective, rooted here on planet Earth, various events unfold with various probabilities. “We trade a global picture, in which everything happens somewhere but nobody can see it all—for a local picture, where you have one patch, that could be in principle explored,” Bousso says.
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