The idea that our universe is just one amongst many – possibly even infinitely many – in a multiverse is now fairly widely accepted. Deriving from the “many worlds” interpretation of quantum physics, originating with Hugh Everett in the late 1950s, or from modern string theory, it provides answers to some fundamental questions about the nature of reality.
Science fiction has provided several approaches to how we might perceive a multiverse. Olaf Stapledon gave some remarkably prescient thoughts on the subject in his 1937 classic Star Maker, and there have been fascinating settings for alternative histories in stories which evoke parallel worlds as their locations. Philip K Dick’s 1962 novel The Man in the High Castle suggests that there are many realities, all equally real, and no one “true” reality in which we (or perhaps others) live.
But the over-riding problem is that there is no clear evidence that the multiverse actually exists, and it has by no means been an accepted feature of cosmology in the past. Back in my student days, a recommended book was Jagjit Singh’s Modern Cosmology, originally published in 1961 and updated in 1970. Early in his book, Dr Singh discusses the ideas of model universes and refers to the question raised by Herbert Dingle, President of the Royal Astronomical Society in the early 1950s, of whether, “since there is one and only one universe”, its uniqueness is essential or accidental.
Professor Dingle felt that “we have no assurance that the existence of any universe other than the one around us is possible.” Dr Singh made clear that “we depend for our knowledge of the universe on experience and observation, on the one hand, and thought and deductive reasoning on the other.” Mathematicians might construct model universes as a valid method of research, but our focus should be on the actual one in which we live, and which we can observe.
The way forward was therefore to concentrate on what we can test. Such a view continues in works such as Clifford M Will’s Was Einstein Right?: Putting General Relativity to the Test, published in 1986 and updated in 1993. The suggestion is that we should not focus on untestable matters such as quantum multiverses, but instead “on observable, operationally defined quantities”.
The view is taken considerably further in The Singular Universe and the Reality of Time, a 2015 book by the philosopher Roberto Mangabeira Unger and the physicist Lee Smolin. They avoid the need for an unobservable multiverse by arguing that the laws, symmetries and supposed constants of nature change over time, though Professor Smolin points out that they will change only at times of major transformations such as the Big Bang. Otherwise, they tend to give the appearance of being remarkably stable to us on a day-to-day basis. Which is just as well.
Rather than a multitude of other universes all existing at the same time, the authors present the conjecture of a succession of universes, one after another – in effect, only one at a time, and we are living in the only one that exists now. Professor Unger accepts that there might be disjoint parts of the one universe which have branched off in the course of its history but, if they exist, they are still parts of the one singular universe. There is “no sufficient reason to believe in the simultaneous existence of other universes with which we do not and cannot have, now or forever, causal contact.”
On this basis, we may have to give up the science fiction idea that a route to explore other realms is to tap into a multiverse. Such an array of universes may not exist. But the idea that the laws of the cosmos are subject to change, albeit only at times of major transitions, leaves a distinct uncertainty about our physical universe. Our reality may not be quite as stable as we have imagined it.
Richard Hayes, Assistant Editor (Odyssey)