Alternate vs Parallel
My all-time favorite quote is from the opening line of Carl Sagan’s book Cosmos, which states that: “The Cosmos is all that is or ever was or ever will be.” The thing is that, in the days of old, this would have simply meant the universe and nothing more. Nowadays though, contemporary cosmologists have come to believe that our universe is just one spacetime continuum among many others. These separate universes are all thought to exist in what is called the “multiverse”. According to the basic hypothesis, these universes collectively comprise everything that exists: the entirety of all space, time, mass, and energy, as well as the physical laws and constants that describe them. This idea has been around for a while but has only become popular in modern times. The American philosopher William James used the term “multiverse” back in 1895, but he was referring to something different. It wasn’t until 1952 that the famous physicist Erwin Schrodinger stated that his equations seemed to describe several different histories of the universe, which were “not alternatives, but all really happen simultaneously”. The term “multiverse” then found its way into science fiction in 1963 in Michael Moorcock’s novella The Sundered Worlds. Since then the multiverse has become increasingly popular in mainstream media, so the term is well on its way to becoming a household word.
Throughout the years, physicists have come up with several ideas about things like extra dimensions and different kinds of universes. As part of this, what various different scientists have called hyperspace, the meta-universe, and bulk space is actually all just the same thing, in many ways. The point is that these terms are essentially synonymous with the word multiverse. For instance, one way to think of this is that the multiverse is what the universe is expanding into. I thoroughly described that process in the essay Levity Theory: Dark Energy Hypotheses. Regardless, according to certain theoretical models, there are countless other universes expanding into the multiverse, along with ours. Of course, it’s important to understand that this is just philosophical speculation, and has not been verified by scientific observation or experimentation. Still, the idea is very compelling. Just ask Michio Kaku or Neil deGrasse Tyson. They know that we live in a multiverse of numerous other universes. As part of that ensemble, The Illustrated Atlas of the Universe explains the “multitude of universes” like this:
Some theorists speculate that we live in a “multiverse” or “metauniverse” — ours being just one cosmos among an infinite number. While seemingly within the realms of science fiction, these theories are based on actual mathematical modeling and existing physical laws. A number of hypothesized multiverses are possible. For instance, bubbles from the “foam” of a parent universe might form brief small universes; wormholes in space-time; or more prolonged universes like our own. Various interpretations of quantum mechanics permit other “parallel universes,” either identical to ours or with different sets of physical laws.
The important thing to note here is the use of the term “parallel universes” and not “alternate universes”. This is a very important distinction and the point that I want to get across to you the most. As I mentioned earlier Schrodinger was the first person to introduce the idea of the multiverse into physics, but his conceptualization and the justification behind it are flawed. Similarly, the “many-worlds ” model is based on an untenable hypothesis. I explained all of this in The So-Called Measurement Problem, so I won’t get into it here. The point is that the “Everett interpretation” implies that all possible alternate histories and futures are real, giving rise to universes that are nearly identical to one another. This couldn’t be further from the truth. There isn’t a universe out there where I have children or didn’t drop out of college. That’s preposterous. There’s no such thing as an alternate universe. The “Copenhagen interpretation” is the correct understanding of quantum mechanics, meaning there aren’t a bunch of different versions of you in alternate universes somewhere and somewhen else. The thing is that this doesn’t prevent the existence of parallel universes in any way though.
A world-famous theoretical physicist and science popularizer named Brian Greene has suggested that there are nine different possible forms that the multiverse could take — namely quantum, inflationary, quilted, holographic, simulated, landscape, brane, cyclic, and ultimate. Of these, the “quantum multiverse” creates a new universe when a diversion in events occurs, as in the Everett interpretation of quantum mechanics. The “inflationary multiverse” is composed of various pockets in which inflation fields collapse and form new universes. The “quilted multiverse” requires the cosmos to be infinite because, with an endless amount of space, every possible event will occur an infinite number of times. The “holographic multiverse” is derived from the theory that the surface area of any given extent of space can encode the contents of the volume of the region. The “simulated multiverse” exists on complex computer systems that simulate entire universes. The “landscape multiverse” relies on string theory’s Calabi-Yau spaces, such that quantum fluctuations drop the manifold shapes to a lower energy level, creating a pocket with a set of laws different from that of the surrounding space. The “brane multiverse” postulates that our entire universe exists on a multi-dimensional surface which floats in the higher dimensions of bulk space. Then, the “cyclic multiverse” requires branes to collide with each other to produce Big Bangs. Finally, the “ultimate multiverse” contains every mathematically possible universe under different laws of physics.
As you might have noticed, a few of these different scenarios are derived from mathematical models based on string theory and its higher-dimensional extension, M-theory. The latter was developed by Edward Witten who is a theoretical physicist and professor of mathematical physics at the Institute for Advanced Study in Princeton, New Jersey. Either way, these theories require the presence of 10 or 11 spacetime dimensions respectively. The extra 6 or 7 dimensions may either be compactified on a very small scale, or our universe may be localized on a dynamical object, known as a brane. This is an extended structure with any given number of dimensions, of which strings in string theory are examples with one dimension. Thus, the local universe is a 3-brane. This opens up the possibility that there are other branes which could support parallel universes. As part of this, in The Grand Design, the world-renowned physicists Stephen Hawking and Leonard Mlodinow stated that:
According to M-theory, ours is not the only universe. Instead, M-theory predicts that a great many universes were created out of nothing. Their creation does not require the intervention of some supernatural being or god. Rather, these multiple universes arise naturally from physical law. They are a prediction of science.
Later in the book, Hawking and Mlodinow claimed that:
Quantum fluctuations lead to the creation of tiny universes out of nothing. A few of these reach a critical size, then expand in an inflationary manner, forming galaxies, stars, and, in at least one case, beings like us.
Along with this, Alan Guth, the person who actually developed the theory of cosmic inflation, famously said:
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. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking it seriously.
The emeritus distinguished professor of complex systems, George Ellis once stated that:
Many physicists who talk about the multiverse, especially advocates of the string landscape, do not care much about parallel universes per se. For them, objections to the multiverse as a concept are unimportant. Their theories live or die based on internal consistency and, one hopes, eventual laboratory testing.
Brian Greene put it best while hosting The Fabric of the Cosmos. He pointed out that: “However unfamiliar and strange the multiverse might seem a growing number of scientists think it may be the final step in a long line of radical revisions to our picture of the cosmos.” What I’m getting at is that the cosmological community has been debating about the various multiverse theories for quite some time now, but we seem to be on the verge of a paradigm shift. Still, prominent physicists are very much divided about whether or not any other universes exist outside of our own, to this very day. This is due to the fact that there is a great deal more that needs to be learned about the multiverse, which has yet to be observed and possibly never will, but we must not be dismissive of the idea all the same. For all we know, the multiverse might have just as many anthropological implications as it does cosmological ones. Unless we investigate further then we will never know for certain. Personally, I’m convinced of the existence of the multiverse, and I think it’s an important part of the “Big Picture”. Ultimately, the multiverse is a vital part of our origin story. The idea of parallel universes doesn’t take anything away from our existence, it actually adds to it. Knowing that there are parallel, but not alternate universes means that there are lots of other worlds out there, possibly even with intelligent life in them, but there is only one world with you in it. Thus, an appropriate understanding of the accurate multiverse theory can give greater meaning to life, and there’s nothing more important than that. When you get right down to it, the multiverse offers an elegant solution to so very much, regarding life, the universe, and everything. The bottom line is that if a genuine “Theory of Everything” is ever to emerge, it will undoubtedly have to be based on the mathematics of the multiverse.