One of the Doctor’s companions, Rose, is separated from him in a parallel universe, a motif that repeats itself throughout the show: there's the universe that the characters hope to get back to and another in which everything that could go wrong has gone wrong.
So, do parallel universes really exist? If Hugh Everett was right, then yes.
The world described by quantum mechanics is violently counterintuitive. At the subatomic level, particles can exist in mutually contradictory states at the same time. An electron, for instance, can simultaneously exist in multiple places. But the moment that a scientist precisely measures that electron's position, the smeared out collection of contradictory positions collapses into just one outcome.
But what happens to those other outcomes? Do they just go away? And if an electron can exist in more than one state, what about something bigger, like a cat?
Most physicists think that upon measurement those other could-have-been states are simply banished from existence. But Dr. Everett, who worked for the Defense Department building nuclear weapons systems, proposed that they continue to exist, in other universes.
According to Everett's many-worlds hypothesis, every time a Geiger counter detects a neutron decaying into a proton or a beam of light encounters an interferometer, every time you flip a coin or choose a salad dressing, every time any sort of measurement of the physical world is made, the universe splits in two. Reality is constantly branching, with each branch becoming an alternate universe, with its own future.
Everett’s many-worlds theory is not the only theory predicting multiple universes. Another proposal says that the universe is infinite but that the ways in which matter can arrange itself are still finite. That means that arrangements must repeat themselves, and that means parallel universes. Other theories have suggested that our universe is just one bubble in a collection of other bubble universes, or that ours is just one membrane in what can be imagined as layer after layer of membranes, like a cross section of an onion.
At the same time, not all scientists agree that there are multiple universes. Other theories of the universe (and that’s universe, singular), say that the idea of multiverses is pure fiction.
