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"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 the way it is. Now string theory, one of today's leading candidates, is in trouble. A growing number of physicists claim it is ill-defined and based on crude assumptions. Something fundamental is missing, they say. The main complaint is that rather than describing one universe, the theory describes 10⁵⁰⁰, each with different constants of nature, even different laws of physics.

But the inventor of string theory, physicist Leonard Susskind, sees this “landscape” of universes as a solution rather than a problem. He says it could answer the most perplexing question in physics: why the value of the cosmological constant, which describes the expansion rate of the universe, appears improbably fine-tuned for life. A little bigger or smaller and life could not exist. 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 thinks it's a possibility we cannot ignore."