Question

What is a parallel universe, and could there potentially be ones that contain doppelgängers of everyone on Earth?​

Answer 1

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A number of different “levels” of “parallel universes” have been suggested by reputable physicists, any one or more of which could exist in a semi-hierarchical structure. A multiverse is a collection of universes, or even of “lower level” multiverses, and the constituent universes in a multiverses could be considered as being “parallel”. For example, if the eternal inflation model of Big Bang cosmology were true then a (possibly infinite) number of “bubble” universes would spawned from the inflation field (forming the Inflationary Multiverse). If the number of these bubble universes was in fact infinite, then there would have to be an infinite number of universes identical in every way to each universe (such is the power of infinity). And by identical in every way I mean that they would have identical histories and contents, so yes they would have Earths and you’s and me’s etc.

Then there is the Quantum Multiverse, which arises from the many universes interpretation of quantum theory. This states that a universe splits into two universes whenever an observation is made that would otherwise cause the collapse of a particle’s wave function in the Copenhagen interpretation. These universes would be identical in every way except for the results of the quantum decision that forced the split in the first place. So you, I and everyone and everything else would split into two, one copy in each universe. Each of the you’s in the resultant parallel universes would have a common history (up to the point of the split), but separate histories thereafter.

There are other multiverses levels proposed, including those called Cosmological Multiverses and Mathematical Multiverses. If our universe is infinite then the Cosmological Multiverse does exist, because each universe in this multiverse is merely a distant volume of space in our continuum: in an infinite space, any organisation of matter must logically repeat an infinite number of times, so there would be an infinite number of regions of space identical to ours, including for example the position and velocity of every elementary particle and thus every material object.

All of the above proposals are, of course, speculative and contentious. That doesn’t mean that none of them are true!

Answer 2

Answer:

A parallel universe, also known as an alternate universe, or alternate reality, is a hypothetical self-contained plane of existence, co-existing with one's own. The sum of all potential parallel universes that constitute reality is often called a “multiverse".

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