In this article in the New Scientist, the question is asked, Does the idea of parallel universes really describe reality?
inflationary cosmologists have opened the speculative throttle so fully that physicists now talk routinely of such things as an infinitude of parallel universes, or a “multiverse”. In the multiverse, every conceivable world exists, and individuals identical to you and I live out parallel lives in places we cannot have access to.
The article talks about a book by Max Tegmark, ‘In Our Mathematical Universe, which talks about seemingly outlandish theories of multiverses as ‘almost obvious and unavoidable’.
He sets out four types of parallel universe:
His first set, the Level I Multiverse, refers to an idea that many cosmologists already accept. Rapid early inflation would have created what Tegmark describes as “universe-sized parts of space so far away from us that light from them hasn’t had time to reach us”. These other domains – or “universes” – could well exist, although we currently have no observational evidence for them.
Tegmark’s Level II Multiverse refers to a bolder idea, championed by physicist Alexander Vilenkin and others. There may be other domains of space also created by inflation that are too far away to see. These will forever remain out of our reach because continuing inflation drives them from us faster than the speed of light. This idea refers to real, distinct, physical universes that cannot ever be observed.
At this point in the taxonomy, however, Tegmark leaves cosmology behind. In reading, I began to feel that his aim is to see parallel universes in as many places as he can. Enter the Level III Multiverse. This turns out to be a language for talking about the mathematics of quantum theory using the many worlds interpretation of that theory, first proposed by physicist Hugh Everett in the 1950s.
This interpretation describes all physical processes as part of an ongoing, perpetual branching of the universe into many other universes. It is indeed possible to interpret quantum theory this way, but readers should know that many other interpretations, equally in tune with observations, don’t invoke the idea of parallel universes at all.
Then there is the Level IV Multiverse. Again, this has nothing to do with cosmology, but is an ambitious thought about mathematics. Tegmark argues that reality isn’t simply described by mathematics, as most physicists readily accept, but that it is, in fact, mathematical.
Of course, the parallel universe glimpsed in Split Symmetry is pure fantasy…