‘Alternate worlds’ are such a staple of genre television, movies, and fiction and what better challenge to face the hero of a serialised story than to face down an evil doppelganger? How will they overcome a corrupted version of themselves, identical in every way barring their lack of moral fortitude… and sometimes with a beard? 

No platform has embraced the idea of the alternate world more than the superhero comic book. Since the Silver Age of comics during the 1950s and 60s, Marvel and DC have thrilled their readers with tales of alternate worlds and altered heroes and villains. DC’s ‘Infinite Earths’ and ‘Elseworlds’ grew so complicated and convoluted decades after the Flash took a trip to the rather dismissively named ‘Earth-2’ to meet his predecessor, the Golden Age Flash, that in 1986 they had to hold a ‘clearance event’ to get rid of some of this excess baggage… A situation the publisher has had to repeat several times since. 

Most of us won’t be surprised to learn that the idea of ‘alternative universes’ is a facet of science, particularly of physics. But actually, the idea of ‘many worlds’ and that of a ‘multiverse’ of alternative universes arise from very different and disparate concepts.

The former is an idea born of what is known as the wave-function collapse or measurement problem of quantum mechanics, whilst the latter is a proposition born from cosmology and the question of what existed ‘before’ our Universe began its process of rapid inflation and what exists outside of it now. Likewise, these parallel worlds are often referred to as ‘alternative dimensions’ — another phrase that can be found in the physics textbook, but with a radically different meaning than presented in sci-fi. 

These ideas, whilst suffering from some conflation in the minds of some science fiction writers and fans, could not be more different; one suggests an infinite number of almost identical Universes, whilst another suggests a finite set of Universes existing in their own bubbles. Some of which are anything but similar. And third, refers to hidden ‘directions’ curled up within the familiar 3 -D space that we inhabit. 

So, sit down with your evil twin, and whilst admiring their impressive goatee, take a journey with ZME Science through these hidden dimensions, many worlds and bubble-universes. And where better to begin our journey than at the beginning. 

Meeting the Multiverse

“For a start, how is the existence of the other universes to be tested? To be sure, all cosmologists accept that there are some regions of the universe that lie beyond the reach of our telescopes, but somewhere on the slippery slope between that and the idea that there is an infinite number of universes, credibility reaches a limit.” 

Paul Davies, A Brief History of the Multiverse.

There was a time when the word ‘universe’ referred to everything is existence, but modern cosmology has changed this concept irrevokably. There is now the possibility of being ‘outside’ the Universe. In fact, our Universe maybe just a small part of of a much larger patchwork.

As Paul Davies states above, one of the most dangerous things about the concept of a ‘multiverse’ — a stack of Universes placed alongside each other, is how close it veers towards mysticism. This becomes even more of an issue when considering that even many proponents of this hypothetical idea doubt that it could ever really be experimentally tested.

For others, however; the question is fundamental to science, and the closer we come to a complete picture of our Universe we come to, the more tempting it is to consider others.

Fred Adams, an American astrophysicist and Ta-You Wu Collegiate Professor of Physics at the University of Michigan, sees the need for a series of alternative or parallel universes as a necessary extension of the fact that our’s is just too convenient. Why is the Universe ‘fine-tuned’ for life? “The laws of physics are described by a collection of fundamental constants that could, in principle, take on different values,” Adams explains. “Determining the range of constants that allows for a working universe helps quantify the degree to which our Universe is special — or not.”

Adams suggests that our Universe has just the right parameters to support the formation of structure, stars, planets, and even biological systems, but there may be a multitude of ‘empty’ Universes where the conditions were not quite so favourable. And, on the other hand, Adams suggests that there could be universes alongside ours even more favourable to the development of such objects. Universes that are, therefore, even more, favourable to life. That is as much as one could expect to a hypothesised set of over 10⁵⁰⁰ universes. 

But, with even such a large set of ‘alternate Universes’ the chances of finding another ‘you’ is still pretty slim. Especially as the laws of physics in these worlds are likely to be radically different, some even precluding the clustering of fundamental particles and the formation of large scale bodies like stars and planets.

One of the more popular ideas for how a series of Universes could grow and co-exist is the inflationary multiverse theory. Introduced by Paul Steinhardt, Albert Einstein Professor in Science at Princeton University, in 1983 and adapted and advanced by such luminaries in physics as Alan Guth, this theory suggests the idea that inflation doesn’t end with our Universe. It could be eternal with the totality of space broken up into bubbles or patches. Each of these bubbles could possess different physical laws, just as Adams puts forward. 

This idea of eternal inflation does run into the problem that it may well be untestable and thus, unfalsifiable, a key aspect of a scientific theory according to one of history’s most important philosophers of science, Karl Popper. However, this doesn’t deter supporters of the theory, with Alan Guth, in particular arguing that a multiverse is simply a logical extension of the fact we have found our own Universe to be undergoing inflation. 

“It’s hard to build models of inflation that don’t lead to a multiverse. It’s not impossible, so I think there’s still certainly research that needs to be done,” Guth remarked during a news conference in 2014. “But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking the idea of a multiverse seriously.”

Another interesting concept for the structure and arrangement of this multiverse is American theoretical physicist, mathematician, and string theorist, Brian Greene’s ‘Brane theory.’ This posits that our Universe and all others sit on a vast membrane located in a higher dimension. Alongside it reside all other universes.

As these universes move around this ‘brane’ they occasionally collide, with each other. These bumps release vast amounts of energy causing ‘big bangs’ to occur and lead to the birth of further universes. 

Greene’s theory is classified as a superstring theory, a hypothetical concept that underlies all physics and unites quantum physics and general relativity — putting forward a theory of quantum gravity. But, superstring theories are in need of an added element, with this need dictating where our trip must head next — in search of hidden dimensions. 

‘I Need Some Space.’ Exploring Hidden Dimensions

“If string theory is right, the microscopic fabric of our universe is a richly intertwined multidimensional labyrinth within which the strings of the universe endlessly twist and vibrate, rhythmically beating out the laws of the cosmos.”

Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory

The statement that string theory–which suggests that fundamental particles are string-like loops vibrating in space–is in need of ‘hidden dimensions’ may initially summon images of alternate universes occupied by all manner of strange creatures, maybe even an alternative version of you, but, your evil beard wearing doppelganger may find the kind of ‘alternate dimensions’ discussed in string theory a bit of a tight squeeze.

One of the exciting things about superstring theories is that, unlike other theories in physics, this class of explanations are able to predict the number of dimensions that the spacetime platform in which they play out possesses. To explain this a little better; general relativity — the geometrical theory of gravity developed by Einstein — plays out in four dimensions, x, y, and z — the spatial dimensions, and time. 

But, this dimensional prediction alludes that string theory requires ten — possibly eleven, maybe even 26 — dimensions to be consistent. This fact leaves a very pressing and pertinent question; where the heck are these other six or seven (or 22!) dimensions? Why do we only perceive the world in four dimensions?

The most simple and straightforward way of answering these questions is to suggest that these added dimensions are ‘curled up’ hidden within the three spatial dimensions of which we are aware. Physicists define this as these dimensions being ‘compacted on an internal manifold’ but for our purposes, it’s just as easy as thinking of them as being very small.

This idea referred to as ‘compactification’ actually predates string theory. It was first put forward by Theodor Kaluza and Oskar Klein in the 1920s, the eponymous theory which introduced compactification was a suggestion to unify gravity and electromagnetism. It’s perhaps ironic it now finds itself employed to unite gravity and quantum mechanics.

The idea of dimensions being hidden due to size isn’t as counter-intuitive and extraordinary as it may initially appear. Think about a cylinder. When held close the object appears 3 dimensional, but take that cylinder to a sufficient distance and it will appear two-dimensional.

Analogously, at low energies and the scale at which we view the Universe, space appears 3-dimensional with us aware of that four dimension — time. At sufficiently high energies, however, these hidden dimensions may become observable. Thus, the search for such hidden dimensions is now focused on particle accelerators such as the Large Hadron Collider (LHC). 

That’s now two disciplines within physics scoured and no evil-doppelgangers to be found. Can quantum physics rescue this much-loved sci-fi trope? 

Many Worlds, Many Yous? 

“I am about to say something that might sound lunatic…” 

Erwin Schrodinger about to discuss in public the idea of many worlds existing simultaneously for the first time, 1952, (possibly apocryphal).

It’s a well-established rule of quantum physics that things are always found in the last place you look*; so it is fitting that the last realm of physics we search for our counterparts is the quantum realm. 

The ‘Many Worlds’ interpretation of quantum physics, first suggested by Hugh Everett III in the mid-1950s, suggests a solution to the problem of wave-function collapse in quantum mechanics. It was decades, however, before physcists began to take it seriously.

Here’s the problem it attempts to answer; whilst conducting the famous double-slit experiment researchers find that electrons propagate as waves yet interact with other systems as particles — appearing as a single spot on a fluorescent detector. Likewise, when given a binary choice between two slits an electron will pass through as a wave unless a detector is placed on the side of the slit. The attempt to detect which slit an electron passed through causes it to ‘choose’ either slit A or slit B.

The Copenhagen interpretation of quantum mechanics suggests this choice arises from the collapse of the wavefunction–wave-like behaviour being destroyed and giving way to particle-like action. The only problem; there is no solid answer to what causes this collapse.

The Many Worlds interpretation suggests another way around the collapse issue; maybe there is no collapse. Everett suggested that instead of collapsing, the wave function grows exponentially, quickly engulfing the researchers, their lab, planet, galaxy, and then their entire Universe.

Therefore, whereas in the Copenhagen interpretation the electron goes through either slit A or slit B, the Many Worlds interpretation says the electron goes through both and when the researchers examine which slit the electron passed through, what they are actually discovering is if they are in a universe in which the electron went through slit A, or if they are in a universe in which it went through slit B. 

So, how does this reflect on the chances of finding your doppelganger? Well, it makes it a certainty. In fact, one of the problems that many physicists have with the the ‘Many Worlds’ interpretation is the fact that it creates the need for infinite worlds. If you consider just the act of turning on a lightbulb, the photons streaming everywhere, there should be a world for every outcome of every interaction. 

And if that hasn’t boggled your mind, consider it in light of the multiverse and hidden dimensions. Every one of these worlds that branches out has its own hidden dimensions curled up within it AND carries with it its own version of the multiverse starting with one difference: slit A not slit B. 

So, how does this reflect on the chances of finding your doppelganger? Well, it makes it a certainty. In fact, one of the problems that many physicists have with the the ‘Many Worlds’ interpretation is the fact that it creates the need for infinite worlds.

If you consider just the act of turning on a lightbulb, the photons streaming everywhere, there should be a world for every outcome of every interaction. 

And rather than starting from the bottom-up as a universe inflating in a bubble would, this new world has a head-start, everything that already exists is there present and correct. The physical laws are identical, large-scale structure exists and so do you.

And if that hasn’t boggled your mind, consider it in light of the multiverse and hidden dimensions. Every one of these worlds that branches out has its own hidden dimensions curled up within it AND carries with it its own version of the multiverse starting with one difference: slit A not slit B. 

Me, is that you?

“Penny, while I subscribe to the many-worlds theory which posits the existence of an infinite number of Sheldons in an infinite number of universes, I assure you in none of them am I dancing.”

Sheldon Cooper, The Big Bang Theory

Even with all this in mind and us determining that if the Many Worlds interpretation of quantum physics is true there almost infinite versions of ‘you’ out there, what are the chances of finding one that is *cues ominous music* PURE EVIL… possibly, with a beard…

Just like the electron faced with the ‘choice’ of which slit to pass through, every time you are faced with a choice, no matter how minute, neurons fire in your brain corresponding to the decision you make. Thus, it’s quite possible that there is a version of you out there who always made the wrong choice. In fact, if there are infinite worlds, it’s a certainty. 

The rotter.

The main issue with the Many Worlds interpretation is the idea of its testability. One of the rules of the Many Worlds interpretation is the inability of these worlds to interact. 

Suggestions have been made regards falsifying Many Worlds but they all require placing a macroscopic object into a quantum ‘superposition.’ This is something that is currently beyond experimental limits, though researchers are constantly finding quantum effects in increasingly larger collections of atoms.

Likewise, the idea of the Multiverse is currently untestable. Doing so would probably require viewing the edge of our Universal bubble, and as this is accelerating away from us, possibly faster than light, as the Universe expands that isn’t likely to happen.

At the moment, the most likely of the ideas discussed above to be evidenced is that of hidden dimensions. These could ‘unfurl’ from the 3 spatial dimensions of our visible Universe at high energies. Energy levels that were present in the early universe and could conceivably be reached at the LHC after it’s high luminosity upgrades. 

Just don’t expect to be faced with your bearded, evil, but otherwise exact duplicate from another world any time soon… Probably. 

*There’s probably a universe where this is true, anyway.