Enlightenment Now: The Case for Reason, Science, Humanism, and Progress

The other explanation is that our universe is just one region in a vast, possibly infinite landscape of universes—a multiverse—each with different values of the fundamental constants.³⁶ We find ourselves in a universe compatible with life not because it was tuned to allow us to exist but because the very fact that we exist implies that it is that kind of universe, and not one of the vastly more numerous inhospitable ones, that we find ourselves in. Fine-tuning is a fallacy of post hoc reasoning, like the Megabucks winner who wonders what made him win against all odds. Someone had to win, and it’s only because it happened to be him that he’s wondering in the first place. It’s not the first time that a selection artifact has fooled thinkers into searching for a nonexistent deep explanation for a physical constant. Johannes Kepler agonized over why the Earth was 93 million miles away from the sun, just right for water to fill our lakes and rivers without freezing solid or boiling away. Today we know that the Earth is just one of many planets, each at a different distance from our sun or another star, and we are unsurprised to learn that we find ourselves on that planet rather than on Mars.

The theory of the multiverse would itself be a post hoc excuse for an explanation if it were not consistent with other theories in physics—in particular, that the vacuum of space can spawn big bangs which grow into new universes, and that the baby universes can be born with different fundamental constants.³⁷ Still, the very idea repels many people (not least some physicists) because of its mind-boggling profligacy. An infinity of universes (or at least a number large enough to include all possible arrangements of matter) implies that somewhere there are universes with exact doppelgangers of you except that they married someone else, were killed by a car last night, are named Evelyn, have one hair out of place, put the book down a moment ago and are not reading this sentence, and so on.

Yet however unsettling these implications are, the history of ideas tells us that cognitive queasiness is a poor guide to reality. Our best science has repeatedly insulted our ancestors’ common sense with unsettling discoveries that turned out to be true, including a round Earth, a slowdown of time at high speeds, quantum superposition, curved space-time, and of course evolution. Indeed, once we get over the initial shock, we find that a multiverse is not so exotic after all. This is not even the first time that physicists have had a reason to posit multiple universes. Another version of the multiverse is a straightforward implication of the discoveries that space appears to be infinite and that matter appears to be evenly dispersed through it: there must be an infinity of universes dotting 3-D space beyond our cosmic horizon. Still another is the many-worlds interpretation of quantum mechanics, in which the multiple outcomes of a probabilistic quantum process (such as the trajectory of a photon) are all realized in superimposed parallel universes (a possibility that could lead to quantum computers, in which all possible values of the variables in a computation are represented simultaneously). Indeed, in one sense the multiverse is the simpler theory of reality, since if our universe is the only one in existence, we would need to complicate the elegant laws of physics with an arbitrary stipulation of our universe’s parochial initial conditions and its parochial physical constants. As the physicist Max Tegmark (an advocate of four kinds of multiverse) put it, “Our judgment therefore comes down to which we find more wasteful and inelegant: many worlds or many words.”

If the multiverse turns out to be the best explanation of the fundamental physical constants, it would not be the first time we have been flabbergasted by worlds beyond our noses. Our ancestors had to swallow the discovery of the Western Hemisphere, eight other planets, a hundred billion stars in our galaxy (many with planets), and a hundred billion galaxies in the observable universe. If reason contradicts intuition once again, so much the worse for intuition. Another advocate of the multiverse, Brian Greene, reminds us:

From a quaint, small, earth-centered universe to one filled with billions of galaxies, the journey has been both thrilling and humbling. We’ve been compelled to relinquish sacred belief in our own centrality, but with such cosmic demotion we’ve demonstrated the capacity of the human intellect to reach far beyond the confines of ordinary experience to reveal extraordinary truth.³⁸