For two weeks every summer, my parents rented a holiday apartment by the beach in Vlora, an old coastal town along the Adriatic.
It was known as Aulona in Greek and Roman times, and was a special place to visit even during 1980s communist Albania. Aulona’s spirit, imprinted on the traditions, superstitions, and landscape of the place, floats outside of time. The town is guarded by a rugged terrain of high mountains, turquoise waters, and black rocks, which blend into silence at sunset. It is a place to dream absurd dreams.
My favorite evening activity was to sit on the deserted sand alone. I watched waves linger at the soundless horizon before breaking rhythmically onto the shore. As night fell, I waited until the line dividing sky and sea blurred away and all boundaries vanished. Of course everybody knew that the world beyond the horizon was strictly forbidden to those of us behind the Iron Curtain. But, sitting in the dark, I was free to imagine.
Were kids on the other side of the Adriatic equally enchanted by the edge of the sky we shared? Eventually my dad would come over and, without reprimand, sit on the sand next to me. Then it was the two of us in a hushed conversation with the sky. Before long, he would speak, telling me it was time to leave, and the gentle spell of the sea and the sky would break.
Twenty years later, in 2009, I sat with a few dozen other scientists in a room at the Kavli Institute for Cosmology, Cambridge at the University of Cambridge to watch the launch of the Planck satellite. A muffled buzz filled the room with cautious excitement. Casual conversation would be interrupted by concern over pauses in the live transmission. When the countdown began, the room fell eerily quiet, and with lift-off came deep cheers and loud applause.
Planck was on its way to measure the gentle glow of light left over from the fiery birth of our universe, called the Cosmic Microwave Background (CMB). The CMB is a detailed fingerprint that allows us to cast our gaze onto the first few moments of our universe’s existence, and to cast light onto some very ancient questions: Where are we from, and how did we get here? (See The Standard Model.)
This March, four years into its mission, the Planck collaboration released the most finely detailed map of the CMB ever measured.1 In its details was a bombshell: anomalies in the distribution of the CMB brightness that could not be the result of anything in our own universe. Here was an empirically observed hidden code pointing to a rich and vast cosmos, in which our own universe is but one humble member. The limits of our range of exploration had suddenly grown immensely. We were at the shore of the multiverse.
Contemplating the existence of other universes is not a new endeavour. From prehistoric times to the present day, this possibility has sparked the imagination of philosophers, writers, and scientists. But for most of history, it was not an idea that was taken seriously. Philosophically, it was an unnecessary complication, one that simply pushed the mystery of our origins to a new layer of reality that was unobservable in principle. And, since a theory needs to be falsifiable in order to be scientific, many scientists did not see the multiverse as “real” science. Aesthetically, too, the multiverse was not attractive.
Scientists believed Nature to be simple and economical. One universe was plenty, so why bother with more? As our scientific understanding developed, however, it became clear that the multiverse is an unavoidable prediction of our theories of nature, ones that we trust and cherish: quantum mechanics, inflation, and string theory. Today, in the face of the inertia and prejudice of the past, the multiverse is finally entering the realm of serious scientific research.
If you are thinking you might have heard this narrative before, you have. From Atomists to Stoics and third century Christians, from Lucretius to Descartes, from Ptolemy to Galileo and Kant, the battle between a single universe with special origins and a collection of universes each with random beginnings has been waged for centuries.
A similar battle was fought over planets and stars. In the 16th century, Copernicus took the Earth out of the center of our universe, in the face of stiff resistance from both the Church and other scientists. The multiverse might be the ultimate extension of the Copernican narrative: Even the entirety of the universe is not particularly important, but sits among infinitely many other entireties.
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