While we're getting the multiverse out of our system, let me point to this interview with Leonard Susskind by Amanda Gefter over at New Scientist (also noted at Not Even Wrong). I've talked with Amanda before, about testing general relativity among other things, and she was nice enough to forward the introduction to the interview, which appears in the print edition but was omitted online.
Ever since Albert Einstein wondered whether the world might have been different, physicists have been searching for a "theory of everything" to explain why the universe is exactly the way it is. But one of today's leading candidates, string theory, is in trouble. A growing number of physicists claim it is ill-defined, based on crude assumptions and hasn't got us any closer to a theory of everything. Something fundamental is missing, they say (see New Scientist, 10 December, p 5). The main complaint is that rather than describing one universe, the theory describes some 10^500, each with different kinds of particles, different constants of nature, even different laws of physics. But physicist Leonard Susskind, who invented string theory, sees this huge "landscape" of universes not as a problem, but as a solution. If all these universes actually exist, forming a huge "multiverse," then maybe physicists can explain the way things are after all. According to Susskind, the existence of a multiverse could answer the most perplexing question in physics: why the value of the cosmological constant, which describes how rapidly the expansion of the universe is accelerating, appears improbably fine-tuned to allow life to exist. A little bigger and the universe would have expanded too fast for galaxies to form; a little smaller and it would have collapsed into a black hole. With an infinite number of universes, says Susskind, there is bound to be one with a cosmological constant like ours. The idea is controversial, because it changes how physics is done, and it means that the basic features of our universe are just a random luck of the draw. He explains to Amanda Gefter why he's defending it, and why it's a possibility we simply can't ignore.
