Universes: Parallel or Unique? (Part 2)

There's a second way in which the concept of parallel universes comes up in modern physics: Not so long ago our understanding of the cosmos was far more limited than today. It was literally a smaller universe. The more we learn, however, the bigger it all appears to be. In fact, some scientists pose the possibility that the universe is unimaginably huge; even infinite. There is currently no known reason why it couldn't be. If so, it opens up the possibility for something very weird: If the universe is actually infinite, there may be an infinite number universes contained within it. No limit.

Since there are only a finite number of elements that make up everything, an infinite universe offers the chance of an infinite number of ways that things can arrange themselves. Sooner or later, you and I will find ourselves being duplicated. It's like giving 100 monkeys 100 typewriters to play with (OK, today, it'd be 100 PCs). Given enough time (infinite), they would write all of Shakespeare's plays. It's a crazy thought... but it's theoretically possible.

And finally, there's a third way that parallel universes come up: via string theory. There are currently three contrasting ways that physicists model our universe: (1) classical mechanics (Newton's baby), (2) relativity (Einstein's baby), and (3) quantum mechanics. Each one uses a type of mathematics that works quite well within its domain—either large and slow, very fast, or at atomic scales. But when two of these domains intersect or overlap, they clash in their predictions of how things behave. These three approaches disagree with one another in that overlapping zone—something that causes the stomachs of physicists to wildly churn, or creates painful scientific headaches.

A few decades ago a novel theory came into existence to deal with this disagreement: string theory. It irons out a lot of overlapping wrinkles in the old theories—providing a seamless description of our world; big, small, fast, slow, etc. String theory has introduced a few wrinkles of its own, however. One is that science may never be able to come up with experiments to test its validity. That's a bummer in the eyes of physicists who know they'll eventually have to have experimental verification of their theories. So string theory, although attractive, awaits (maybe forever) some kind of proof. (Interestingly, Einstein's theory of relativity remained an abstract curiosity for a couple of decades until an experiment in 1919 proved it true, and Albert became an overnight sensation.)

A particular aspect of string theory also suggests the possibility of parallel universes. So science now has at least three ways to suggest the possibility of parallel universes: quantum mechanics, the realization that the universe may be infinite, and string theory. As a result, other universes are coming into vogue and several new terms have entered the discussion: parallel worlds, the multiverse, the metaverse, the megaverse, alternative universes, etc. The imaginations of science fiction writers have been stimulated by all these developments. The possibilities are endless.

As yet, however, the evidence for parallel universes is only conceptually possible. There is no proof. There may never be. That's disturbing for some physicists and has others toying with the many potential implications. Nobody yet knows. So expect ongoing speculation for some time yet... and maybe the dilemma will foster a bunch more great science fiction stories and movies.

Universes: Parallel or Unique? (Part 1)

People have enjoyed playing with the notion of parallel universes for many years. Science fiction stories have featured myriad possibilities of the existence of them, and often describe a hero accidentally being dumped into one of them, followed by harrowing and strange adventures. Our hero will discover the weird characteristics of the new universe, become trapped within it, finally escape, and return safely home, back to our unique universe. It's been a rich topic to explore.

One of the more modern fictional depictions of parallel universes is the notion that multiple, split-off copies of each of us exists in these countless other universes. How does this happen? At each moment, we are presented with multiple choices; the road branches again and again. We choose one path, but other copies of us (in other universes) make other choices, each time creating endless copies of ourselves that do their own thing in their own universe. These replicas of us carry on in their lives pretty much as we do, but pursue an infinite number of alternative possibilities.

After each moment's decision that we make, we sometimes wonder: What if I were to have chosen another path? (The fun part of the fiction is that another “me” did so, in another parallel universe.) What would my life be like today? If only I could peer into one of those alternative worlds. These fanciful questions often occur to us, and we can get into daydreams about another path that we might have taken. Another playground for science fiction.

Once the sole realm of science fiction, the possibility of the existence of parallel universes has entered mainstream science in recent years. Parallel universes pop up in physics and cosmology in several ways. One of the ways is via the field of quantum mechanics. Its predecessor, classical mechanics (the result of Isaac Newton's insights), described an exact, unique universe—in which the laws of physics allowed us to make precise predictions of the future behavior of things such as planets and billiard balls. Given enough information, one could describe precisely where that planet would be in a hundred million years, or in which pocket a billiard ball would eventually drop. Classical mechanics offered a certainty that we humans like to have.

But then quantum mechanics entered the picture and introduced the fact that chance, instead, may rule our beloved universe. Physics was transformed from an exact science to a probabilistic one. (This irked the hell out of some scientists... Einstein was one of them.) Quantum mechanics tells us that an event, rather than being unique, may have any number of outcomes—each with a given probability of happening. We won't know what outcome actually will occur, until we run the experiment. Afterwards, we may wonder why that particular result occurred. Another outcome theoretically could have happened. This probabilistic nature of the quantum world posits the possibility of multiply-different outcomes in multiply-different universes—each outcome equally likely to have occurred in its own universe. Weird!

More on alternative universes next time...