In string theory, the multiverse is a theory in which our universe is not the only one; many universes exist parallel to each other. These distinct universes within the multiverse theory are called parallel universes. A variety of different theories lend themselves to a multiverse viewpoint.
In some theories, there are copies of you sitting right here right now reading this in other universes and other copies of you that are doing other things in other universes.
Other theories contain parallel universes that are so radically different from our own that they follow entirely different fundamental laws of physics (or at least the same laws manifest in fundamentally different ways), likely collapsing or expanding so quickly that life never develops.
Not all physicists really believe that these universes exist. Even fewer believe that it would ever be possible to contact these parallel universes, likely not even in the entire span of our universe’s history. Others believe the quantum physics adage that if it’s possible, it’s bound to happen somewhere and sometime, meaning it may be inevitable that quantum effects allow contact between parallel universes.
The idea of a physical multiverse came later to physics than it did to religion and philosophy. The Hindu religion has ancient concepts that are similar. The term itself was, apparently, first applied by a psychologist, rather than a physicist.
Concepts of a multiverse are evident in the cyclical infinite worlds of ancient Hindu cosmology. In this viewpoint, our world is one of an infinite number of distinct worlds, each governed by its own gods on their own cycles of creation and destruction.
The word multiverse was originated by American psychologist William James in 1895 (the word “moral” is excluded from some citations of this passage):
“Visible nature is all plasticity and indifference, a [moral] multiverse, as one might call it, and not a [moral] universe.”
The phrase rose in prominence throughout the 20th century, when it was used regularly in science fiction and fantasy, notably in the work of author Michael Moorcock (though some sources attribute the word to the earlier work of author and philosopher John Cowper Powys in the 1950s). It is now a common phrase within these genres.
According to MIT cosmologist Max Tegmark, there are four levels of parallel universes:
Level 1: An infinite universe that, by the laws of probability, must contain another copy of Earth somewhere
Level 2: Other distant regions of space with different physical parameters, but the same basic laws
Level 3: Other universes where each possibility that can exist does exist, as described by the many worlds interpretation (MWI) of quantum physics
Level 4: Entirely distinct universes that may not even be connected to ours in any meaningful way and very likely have entirely different fundamental physical laws
Tegmark’s approach is one of the few attempts to comprehensively categorize the concepts of parallel universes in a scientific (or, as some see it, pseudoscientific) context. The full text of Tegmark’s 2003 paper on this topic is available at his MIT website, for those who don’t believe that these concepts are scientific. (They may not be scientific, but at least they’re unscientific musings by a scientist.)