What is the parallel universe theory and is it true? originally appeared on Quora - the knowledge sharing network where compelling questions are answered by people with unique insights.
Answer by Richard Muller, professor of physics at UC Berkeley, author of Now: The Physics of Time, on Quora.
The parallel universe theory is basically scientists engaging in a pleasant form of science fiction. Sometimes they will pretend that they are discussing real science, but they aren't. Similarly, some magicians will claim they are doing true magic, rather than just tricks.
In science, a theory is not a valid theory unless it shows what kind of observation would falsify it. That was a criterion developed by the great theorist of science, Karl Popper. It was the standard that I was taught when I was a graduate student. I consider it to be the fundamental foundation of science; the criterion that separates science from non-science. The parallel universe "theory" doesn't qualify.
When I came up with my new theory of time (see my book, Now: The Physics of Time) I considered it essential to devise an experiment that could falsify the theory, or confirm it--but it had to be able to do both. It was hard, but as I was finishing the book (actually, when the book was in proof!) LIGO discovered its incredible gravitational waves, and I immediately realized that such observations provided the key to testing my theory. I made some additions in the proof that show how to falsify my theory.
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