Parallel universes are not a theory, but deductions from the “Many Worlds” interpretation of quantum mechanics. And there is at least on other theory that predicts not alternate universes but parallel universes where the universes are just like ours, only slightly different, where different versions of us could exist. One is the idea of Infinite universes. The reason space appears to be only 13.7 billion light years in radius is because That limit is set by the distance that light has had the ability to travel since the instant of the Big Bang. The universe is likely larger, and perhaps infinitely larger. If space is infinite, then it must start repeating at some point, because there are a finite number of ways that any set of particles can be arranged. Physicist Brian Greene calculated this number to be 10^10^122 — this is a very very large number, but it is minuscule compared to infinity. So if you travel far enough in space, you should see another version of you living in completely different circumstances, perhaps as president of the United States, or as rich as Bill Gates.
But the most intriguing theory that leads to parallel universes comes from the many worlds interpretation was proposed by Hugh Everett for his phD thesis in 1957 at Princeton University. This theory says that the wave never collapses, that every time a wave seemingly collapses in our universe, there is a parallel universe where no such collapse happens. This interpretations implies that reality splits like a fork in the road whenever a wave collapse occurs. In effect, the entire universe is one gigantic wave function that contains all possible realities. Everett called it the “universal wave function,” in his thesis. So the universe is also in superposition of all possible states of its constituent particles. As it evolves, some of these superpositions break down, making certain realities or worlds distinct and isolated from each other.
The many worlds interpretation avoids the complication of wave function collapse, but it creates a near infinite number of parallel worlds. Just like in quantum mechanics, everything with a non-zero probability of occurring is a reality in some universe. In other words, there is a universe out there where everything happened just like it did in this one, except you made one or two decisions differently, and your life turned out completely differently – like you became as rich as Bill Gates, or married Kate Upton.
Bryce DeWitt who popularized the many worlds interpretation in the 1970’s says, “Every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies.” Quantum physics says that the splitting occurs whenever a wave collapses, meaning whenever a measurement of observation occurs. When you walk around your own house, there must be millions of splitting events because the molecules on your feet are interacting with the molecules of the floor. The numbers of splits and parallel universes has to be astronomical.
Oxford physicist David Deutsch says, many worlds is virtually a fact. It sounds fantastic, but the Many Worlds interpretation predicts outcomes that are completely consistent with physics experiments over the course of decades. MIT physicist Max Tegmark says, “The act of making a decision causes a person to split into multiple copies.”
But I have no awareness of the other copies. Columbia University Professor Brian Greene says we just need to broaden our minds of what “self” actually means. “Each copy believes that it is you. The real you is the sum total” of all the splittings
In the late 1990s a thought experiment was proposed by MIT physicist Max Tegmark — called quantum suicide. Let’s say someone makes a wager. You are put in a box with a gun pointed to your head and a button in your hand. The button controls a quantum splitter that detects the spin of an electron. If after you press the button the spin says up, the gun does not fire, you come out of the box and win a prize of $1 Billion. But if the spin shows down, then the gun fires and you die instantly. There is a 50% chance that the splitter would detect up, and 50% that it would detect down. Few people, I think, would accept 50/50 odds of life and death. But I might take this bet because If you truly believe in the concept of multiple worlds then you would take this bet every time. Because the only one of “you” that would ever come out of that box would be the one that remains conscious. In other words, in your world, for you to be conscious, you would always have to be in the reality that survived.
In another world, the other versions of you would be dead, but you would not know this. Your conscious self would always survive and become a billionaire.