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In 1954, a young Princeton University doctoral candidate named Hugh Everett III came up with a radical idea: That there exist parallel universes, exactly like our universe. These universes are all related to ours; indeed, they branch off from ours, and our universe is branched off of others. Within these parallel universes, our wars have had different outcomes than the ones we know. Species that are extinct in our universe have evolved and adapted in others. In other universes, we humans may have become extinct.


This thought boggles the mind and yet, it is still comprehensible. Notions of parallel universes or dimensions that resemble our own have appeared in works of science fiction and have been used as explanations for metaphysics. But why would a young up-and-coming physicist possibly risk his future career by posing a theory about parallel universes?

With his Many-Worlds theory, Everett was attempting to answer a rather sticky question related to quantum physics: Why does quantum matter behave erratically? The quantum level is the smallest one science has detected so far. The study of quantum physics began in 1900, when the physicist Max Planck first introduced the concept to the scientific world. Planck's study of radiation yielded some unusual findings that contradicted classical physical laws. These findings suggested that there are other laws at work in the universe, operating on a deeper level than the one we know.

In fairly short order, physicists studying the quantum level noticed some peculiar things about this tiny world. For one, the particles that exist on this level have a way of taking different forms arbitrarily. For example, scientists have observed photons -- tiny packets of light -- acting as particles and waves. Even a single photon exhibits this shape-shifting [source: Brown University]. Imagine if you looked and acted like a solid human being when a friend glanced at you, but when he looked back again, you'd taken a gaseous form.

This has come to be known as the Heisenberg Uncertainty Principle. The physicist Werner Heisenberg suggested that just by observing quantum matter, we affect the behavior of that matter. Thus, we can never be fully certain of the nature of a quantum object or its attributes, like velocity and location.

This idea is supported by the Copenhagen interpretation of quantum mechanics. Posed by the Danish physicist Niels Bohr, this interpretation says that all quantum particles don't exist in one state or the other, but in all of its possible states at once. The sum total of possible states of a quantum object is called its wave function. The state of an object existing in all of its possible states at once is called its superposition.

According to Bohr, when we observe a quantum object, we affect its behavior. Observation breaks an object's superposition and essentially forces the object to choose one state from its wave function. This theory accounts for why physicists have taken opposite measurements from the same quantum object: The object "chose" different states during different measurements.

Bohr's interpretation was widely accepted, and still is by much of the quantum community. But lately, Everett's Many-Worlds theory has been getting some serious attention. Read the next page to find out how the Many-Worlds interpretation works.

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