The Marvel Universe currently teeters on the edge of “Spider-Verse,” a reality-spanning epic that promises to feature “every Spider-Man ever,” as they try to avoid the predatory totem-eaters that cross between universes to strike at their prey.

Brothers from other realities. From Superior Spider-Man #32.

The Marvel multiverse is a many splendored thing with infinite possibilities, as opposed to the DC analogue that allows for only 52 alternates. Strangely enough, it’s a lot easier to explain infinite worlds than it is just a few. The willingness to accept such an idea continues to grow in the scientific community, but how realistic is it? And if there are other universes out there, how could we ever know?

The Many Concepts of Many Worlds

In 2003, cosmologist Max Tegmark classified the possibilities of what’s come to be called a “multiverse” into four different categories. Pop-science icon Brian Greene has since, in his 2011 book “The Hidden Reality,” upped the ante to nine overall ways to accomplish something similar, though some of those are ideas are from string theory, Greene’s highly-contested area of expertise, and more philosophical Matrix-like ideas. Let’s concentrate here on Tegmark’s more robust, mind-bending notions.

The most well-accepted ideas on the amount of matter and energy out there imply that the universe is likely flat in shape, extending outward potentially infinitely. Of course we can only see things that are less than 14 billion light years away, as the universe itself isn’t any older than that. No light, no observation.

Our best observations suggest there is just enough stuff in the universe to keep it flat, not curving in any direction. From

But if the cosmos is truly infinite, then inevitably every physically possible arrangement of matter occurs somewhere not only once, but over and over again. Correspondingly, just because of the sheer (never-ending) numbers, there should be exact copies of all us out there in the deep expanses of the universe, as well as other, slightly skewed versions. Tegmark calls this a Type I multiverse, and its existence is kind of inarguable if the universe really goes on without limit.

A Type II multiverse is born out of inflationary theory, a somewhat well-established mechanism that increased the size of the universe at an enormous rate within the first second of its existence, before a more moderate expansion set in. A special subset of this theory, called chaotic or eternal inflation, hypothesizes that the process didn’t just happen once, but is constantly happening in different parts of the universe, creating new “pockets” all the time, some maybe even with physical laws different than our own.

Does our universe resemble a fractal, with new “pockets” expanding geometrically all the time?

Neither of those really fit what we think of when we say “parallel universe,” though, do they? We want to imagine there’s something just to the side of us, somehow, not an inconceivable distance away. A Type III multiverse, as I’ve briefly discussed before, is more akin to the sci-fi idea of a world next door. In the 1950s, in response to the uncomfortable probabilistic nature of quantum mechanics, Hugh Everett suggested that instead of the wave function collapsing when a particle is measured, erasing all other possibilities provided by the Schrödinger equation, that those remaining outcomes do actually occur, just … somewhere else. Every measurement and every action would result in the birth of new universes, each treading a path not taken in this one.

The Type IV multiverse, devised by Tegmark himself, is a sort of Platonic idea where all mathematical structures imaginable give rise to different universes. He doesn’t really describe how these would come about, and anything significantly differing from our own universe wouldn’t be very hospitable for potentially Spidey’s, so we’ll just leave this one here and move on.

Put it to the Test

Those are some neat concepts, but how do they stack up against data and observation, and can they even be tested? Paul Steinhardt, one of the pioneers of inflationary theory, says the multiverse is useless as a scientific idea because it makes no specific predictions, replacing strict mathematics with an “anything goes” kind of attitude. Steinhardt argues that there’s no way to falsify the multiverse – to disprove it – so according to strict interpretations, it’s not even science.

Tegmark retorts that the various multiverse types could indeed be disproven if the theories that spawn them are falsified. Types I and II would be fairly demolished if inflation were cast away, which might still happen. Inflation is not a forgone conclusion, as Steinhardt himself now opposes it and recent confirmatory evidence has not held up to scrutiny. A Type III multiverse would be invalidated should the Schrödinger equation ever be found to falter, for example if quantum computers end up shown as impossible flights of fancy.

Quantum computing is still in its infancy, but NASA has been testing out a working model since the fall of 2013.

Tegmark insists that these multiverse ideas weren’t pulled out of a hat for no reason, but that they’re mathematical consequences of generally accepted theories, just as wormholes are predicted by general relativity. Some more daring cosmologists are searching for positive evidence of other universes, looking for ripples in the cosmic microwave background radiation that could represent other universes “bumping into” ours, but with no results so far.

The existence of wormholes, temporary bridges across space and time, are predicted by most interpretation of general relativity but have never been observed.

Don’t Wait on the Cavalry, Pete

“Bumping into” another universe might be the best we can do, even if they are real. There doesn’t seem to be any physics that would allow for travel between universes in a Type III world. Although a wormhole has never been observed, they could conceivably connect two disparate points of space in a Type I multiverse. Too bad theoretical wormholes are much different than those in science fiction, infinitesimally small points that collapse in a fraction of a second.

The existence of a multiverse, while controversial, could open endless possibilities if true. Sadly, there probably isn’t a way for those possibilities to intermingle. So even if they’re out there, don’t expect to run into your doppelganger anytime soon.