String theory has a strange and remarkable history in which the conventional wisdom of the field has sometimes changed chaotically. After the mathematical consistency of superstrings (strings that accommodate a “supersymmetry” relating bosons and fermions) was demonstrated in 1984, a consensus arose that string theory would offer a unique solution that describes our universe. The belief in a unique vacuum is, to me, a Ptolemaic view—akin to the ancient belief in a unique place for Earth. As I wrote in 1989, a Copernican view, in which our universe is only one of an infinity of possibilities, is my preference, but there were very few Copernicans in the 1980s. Today, the string-theory community is engaged in a lively debate about a “landscape” with many solutions. 1 That debate represents a shift away from the idea of uniqueness and toward the possibility of multiple universes, a multiverse. Another idea from string theory...

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