Eventually the hard questions snuck back in. When Linde came up with chaotic eternal inflation in that fateful half hour, he immediately realized the implications. In an infinite multiverse where physical constants can vary from one universe to the next, everything that can happen will happen—an infinite number of times. Every possible world, every incarnation of reality, every possible version of you living every possible version of your life. What then does it mean to want something, to do something, to be something? It was a vertiginous thought, but Linde didn’t let it get to him. So what if there were infinite Andrei Lindes? If I killed myself, he figured, it’s not like I’d survive as a copy—my death would simply become the moment that I was no longer identical to my copy, because I, unlike him, would be dead.
In any case, it wasn’t clear that the copies existed in any meaningful way. That was the thing about quantum mechanics—the very nature of things seem to be determined by what an observer can measure. In the world of classical physics, you could have two baseballs that were identical in every way, and yet it’s fair to say that there are two of them. In the quantum realm, if you have two indistinguishable particles, you only have one particle. Wheeler and Feynman had emphasized that—in a sense, they said, there’s only one electron in the universe. Linde could never quite shake that.
Even those quantum fluctuations—the very fluctuations that gave rise to the stars, polarized the microwave light, and created universe after universe—they are determined directly by what an observer can measure. Position and momentum, time and energy—these partners bound by uncertainty are so bound because the accurate measurement of one precludes the accurate measurement of the other. A particle doesn’t have a simultaneous position and momentum because an observer can’t measure a simultaneous position and momentum. Gravity waves are waves of uncertainty—uncertainty not only of existence but of observation. It was a fact that seemed to suggest that observers play some deep role in the nature of reality, a fact that Linde kept tucked away in the back of his mind. What is it to feel, to think, to live, to exist? If there was no observer who could simultaneously observe more than one Andrei Linde, then on some level you might say there’s still only one.
Despite this, Linde was convinced that the existence of all those parallel universes held great explanatory power. While the multiverse was ultimately governed by the same laws of physics—by quantum mechanics and relativity, by inflation itself—each universe would be born with its own local sub-laws, a set of accidents that would determine its geometry, its physical constants, its particles, its forces, its own unique history. Inflation meant diversity. And diversity, Linde realized, was its own kind of explanation.
So many features of our universe appear inexplicably fine-tuned for the existence of biological life. Change the strength of a force here or the mass of a particle there and poof!—no stars, no carbon, no life. Such coincidences demand explanation, and inflation had one: the strengths and masses vary from universe to universe, and we just happen to find ourselves in the one in which we can live. The inflationary multiverse may not have been predictive or observable, but it was explanatory. It could explain the illusion of design, the comprehensibility of the cosmos, the unreasonable effectiveness of mathematics. It could explain why the cosmological constant is so small and why the universe is so big. It could explain why we are here, why anything is here, because at the end of the day, Linde knew, physics isn’t really about the universe. It’s about us.
