Originated by US physicist Hugh Everett in the late 1950s, this envisions our Universe as just one of numerous parallel worlds that branch off from each other, nanosecond by nanosecond, without intersecting or communicating.

Although the many-worlds idea is by no means universally accepted even today, his methods in devising the theory presaged the concept of quantum decoherence— a modern explanation of why the probabilistic weirdness of quantum mechanics resolves itself into the concrete world of our experience.

The first, set forth by a guy named Hugh Everett in the 1950s, is called the “Many-Worlds Interpretation of Quantum Mechanics” and essentially says that whenever there are multiple ways an event could turn out, the world “splits” into new worlds to accommodate each possible outcome.

The theory was scorned for decades until fellow Princetonian Max Tegmark created the quantum suicide experiment, which lends support to the interpretation [source: The Guardian]. According to the Many-Worlds theory, for each possible outcome to an action, the world splits into a copy of itself.