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193 Cosmos after cosmos issued from his fervent imagination, each one with a distinctive spirit infinitely diversified, each in its fullest attainment more awakened than the last; but each one less comprehensible to me. —Olaf Stapledon, Star Maker, 1937” We face a further radical enlargement of the cosmos: There may be parallel universes. Some scientists see them as a direct implication of cos- mological observations. “The simplest and arguably most elegant theory involves parallel universes by default,” declared cosmologist Max Tegmark. Steven Dick found this to be a remarkable revival of the Greek concept of a plurality of kosmoi—isolated ordered systems." Consider “brane” theory, one of several proposed concepts. Multiple universes may exist as membranes through a multidimensional hyperspace. Our observable universe may be a three-dimensional surface or mem- brane—brane for short—in a higher-dimensional world. Another brane—a parallel universe—might reside a subatomic distance away. Additional dimensions could be the size of atoms, or infinitely large. We would never be able to enter them, yet they could have profound effects on the physics of our own universe."” The cosmos might be a multifoliate object, made of sheets that are con- stantly splitting and occasionally fusing. Unlike previous versions of paral- lel universes, these “world sheets” can interact with one another. However, an observer embedded on one sheet—one “universe”—would find his or her view limited to that sheet.”° There may be many universes, but only a tiny fraction of them is actually cognizable. This, commented Koerner and LeVay, is surely the ultimate in Copernican thinking.” These ideas pose a fundamental problem for scientists. As physicist Roger Newton pointed out, their contact with any conceivable experimen- tal test in the near future—or ever—appears to be minimal. If the concept is pushed too far, warned Davies, the rationally ordered and apparently real world we perceive gets gobbled up in an infinitely complex charade, with the truth lying forever beyond our ken.” The widely accepted Cosmological Principle tells us that our universe is much the same everywhere. Most scientists engaged in the debate about finding extraterrestrial intelligence also accept the Copernican Principle: Humankind does not enjoy a preferred position in either space or time. Most have been deeply suspicious of any theory that makes our location or epoch in any way special. By implication, many sites (and times) in the The Anthropic Challenge The Multiverse The Anthropic Challenge