Parallel universes often grace the covers of comic books and fantasy films. However, recent explorations in quantum mechanics may prove that parallel universes are far more real than we have ever imagined.

The proposal of the idea of parallel universes was initially posed by Hugh Everett III in 1954, as a way to determine why quantum matter erratically takes different forms: for example, why do photons act as both waves and particles? Physicist Werner Heisenberg suggested that just observing quantum matter has the capability to change it, making its traits impossible to definitively ascertain. Danish physicist Neils Bohr suggests that by observing quantum matter, one forces the quantum object to choose a state, causing arbitrary states: the quantum object “chose” a different state at differing measurements.

Hugh Everett III disagreed with this principle, arguing for the Many-Worlds theory, which states that observing a quantum object doesn’t force it to choose a state, but causes a split in the universe. The possible forms simultaneously exist, but in non-interacting worlds. This take relies on the Schrödinger theory to explain quantum mechanics and is one of the simpler explanations.

The Schrödinger equation is used in quantum mechanics to explain how electrons behave. 

While it looks incredibly complicated, the Schrödinger equation simply follows the principle of conservation of energy which states that the total energy of an isolated system remains constant. The psi (Ψ) symbol represents the wave function, a mathematical description the quantum space of an electron in relation to momentum, time, position, and spin. This wave function is a combination of all the possible places an electron could be—a superposition.

However, while the Schrödinger equation calculates the wave function, it does not reveal what the wave function specifically is. Max Born, a German physicist and mathematician, proposes that rather than providing specific positions, the wave function is a probability amplitude where Ψ^2 provides researchers with the probability of an electron existing in a certain position. 

As the Schrödinger equation helps predict the state of an electron, it is fundamental to understanding the Many-Worlds theory of quantum mechanics. Before a person interacts with an electron, the electron has all of its possible positions as predicted by the wave function. Then, after a person interacts with that electron in all of its possible positions, that person evolves into a superposition, and there are now many worlds–one for each of the possible positions in which the person interacted with the electron, giving rise to Hugh Everett’s theory.

A superposition is what occurs when a given property is not measured (and, therefore, not determined yet) and is, instead, considered a “superposition” or amalgamation of different outcomes that are possible. Unlike the multiverses populating comic books, however, these worlds can not interact and are completely separate from each other. 

How does this branching happen? That’s where decoherence comes in. Decoherence refers to the process by which a quantum system loses its “quantum-ness” and becomes more classical (i.e., behaves like the objects we see in our everyday world). This happens when a quantum system interacts with its environment, causing the different possibilities in superposition to become entangled with their surroundings. As decoherence occurs, the various states of the system stop interfering with one another, making the world “split” into distinct, non-interacting branches.

The decoherence concept is what helps explain why we don’t see superposition or branching in our everyday experiences. Although particles are constantly in superposition at the quantum level, decoherence ensures that, on macroscopic scales, we only observe one outcome. This process is why objects around us don’t appear to split into multiple realities, despite the underlying quantum mechanics.

Issues:

While the many Many-Worlds theory has certainly gained traction, some scientists assert that its popularity is simply due to how attractive it sounds.

In contrast, French physicist Roland Omnès points out most minor quantum measurements do not result in drastic changes. This finding decreases the probability that quantum measurements are important enough to spawn a whole different world. Others point out the logical inconsistencies. For example, if someone were to make a bet where they either become a billionaire or lose all their savings, the Many-Worlds theory assumes that they will become rich beyond their wildest dreams with 100% probability. They can also land in poverty with 100% probability. Yet, how can two mutually exclusive outcomes both happen for certain?

Conclusion:

Nonetheless, the Many-Worlds theory is certainly widely believed and relevant to the study of quantum mechanics today. Like our favorite sci-fi movies come to life, there might just be hundreds of thousands of versions of each of us somewhere in the multiverse.