If you define the universe as all that ever possibly could be, then there is certainly only one universe. But if you get a little more specific, you run into an interesting idea. What if we define the universe as all that could ever possibly be observed with infinitely advanced technology? Or what if you define the universe as pockets of spacetime expanding with one another? Then you are left with the possibility of a multiverse.
Many theories in physics independently predict the existence of a multiverse in which our universe is but one of infinite universes. This may seem bizarre or improbable. And yet, one of the most compelling explanations of the multiverse comes directly from interpretation of the fundamental mathematics behind quantum mechanics. This is called the many-worlds interpretation.
Hugh Everett III, born in DC in 1930, was a mathematician and quantum physicist. He proposed the many-worlds interpretation. The idea was initially met with scorn which caused him to live with the resignation that his interpretation was not respected. But his idea was very logical. He simply interpreted the consequences of quantum mechanics very literally, and thus, came to a conclusion that is hard to believe.
Elementary particles exist in a state of superposition, meaning they exist in all possible states simultaneously. This is a perplexing concept, because on the macroscopic level, this is unrealistic. The classical interpretation of the wave function of particles, described elegantly through Schrodinger’s equation, is that particles exist in a state of superposition until they are observed. The act of observation limits them to a single possibility.
That’s not how Everett interprets it. In the many-worlds idea, rather than separating the observer from the system and assuming that observation interrupts the state of subatomic particles, Everett envisioned the possibility of the particles continuing to exist in every other state with a separate observer and universe to go along with each possible state. The universe would branch into each possibility, and once reality assumed a branch, the branches would have no interaction with each other. Each reality would also have its own copy of the observer that detects each reality separately. Each would have an entirely separate future. Every quantum possibility spurs another distinct universe. This means anything that could ever possibly be true is true simultaneously.
Everett was met with academic scorn for his ideas, and he lived the life of a hermit. He died at age 51, in the ‘80s. He remains a hero of the sci-fi community, where the multiverse is often a subject of writing. His ideas are still divisive, but they have a place in a modern cosmologists’ idea bank. Imagine a parallel universe in which Everett could witness the serious consideration of his ideas in the scientific community amongst string theorists.
So why is the multiverse idea not considered a theory? Mainly because it is untestable and therefore unfalsifiable. Any truly scientific claim must be able to be proved wrong. You cannot set up an experiment to see if there is another universe. However, with the confirmation of gravitational waves last week, the dialogue is coming to a head – gravitational waves are a main postulate of inflation theory. Inflation is the leading theory for what was happening in the universe shortly after the big bang. It involves rapid expansion early on in the big bang timeline. Eternal inflation is the idea that different pockets of space stop inflating at different times, thus creating distinct “bubbles” of fundamentally different universes with different properties. As our technological capabilities increase, bizarre postulations such as the existence of a multiverse come closer to being scientific claims.
For more on the multiverse and quantum physics, check out Michio Kaku’s “Parallel Worlds,” and Brian Greene’s “The Elegant Universe.” Stay curious, and ingest heavy doses of sci-fi.