The Multiverse Theory Completely Explained

The multiverse may seem like a modern idea, but it has roots growing far deeper in human history than you may realize. One of its earliest forms came from ancient Greece. As Scientific American explains, it was important in the philosophy of atomism, with philosophers like Democritus devising theories that required infinite worlds to work. As Professor of Religion Mary-Jane Rubenstein explains, their atomic theory meant that they no longer needed to rely on gods to explain the apparent perfection of their world. To the atomists, our universe was just one of an infinite number of others, each totally different. Democritus lived over two millennia ago, but his ideas were somewhat similar to some more modern ones.

The Greeks weren't the only ones to explore the idea of other universes though. An ancient Hindu philosophical text, Śrīmad-Bhāgavatam (Bhagavata Purana), talks extensively about cosmology. It mentions how "there are innumerable universes besides this one." This Persian polymath Fakhr al-Din al-Razi touched on a similar idea in his 13th-century book, "Matalib al-'Aliya (volume 4)." An article in the book "Islam and Science: Volume 2" explains how al-Razi analyzed a verse in the Qur'an which states "all praise belongs to God, Lord of the Worlds" and whether this may refer to other worlds within our own cosmos, or a multiverse beyond our own.

Erwin Schrödinger is a name that any physics student will recognize. He was so instrumental in devising the deeply vexing theory of quantum mechanics that his name is nearly synonymous with it. He's probably most famous for the Schrödinger's Cat thought experiment, as Discover Magazine explains. If a cat, trapped in a box, has a 50:50 chance of being alive or dead, there's no way of knowing for sure until you see the cat yourself. The bizarre conclusion is that, until you open the box to check, the cat is both dead and alive at the same time.

Similarly strange is the way quantum mechanics predicts parallel realities. The book, "The Simulated Multiverse," explains that during a lecture in 1952, Schrödinger explained an idea that he himself cautioned might sound crazy. Interpretations of his quantum equations suggested that, rather than there being a single course of history, there are actually several, each one as real as the one you're currently experiencing and each happening simultaneously.

This concept, with potentially infinite worlds all occupying the exact same space and time, is formally known as the Many-Worlds Interpretation of Quantum Mechanics. As the Stanford Encyclopedia of Philosophy explains, this gives reality a seemingly infinite complexity. For every quantum event with multiple possible outcomes, every outcome really happens. Every possibility is played out, somewhere in a different parallel reality. This, however, is just one of the many types of multiverse that have been hypothesized.

The multiverse concept can be a divisive one among scientists, but this varies wildly depending on the specific multiverse idea being considered. As Britannica discusses, the biggest criticisms are that some multiverse hypotheses are too speculative, being more philosophy than science. Another sticking point that many critics bring up is testability. Scientific method dictates that, for a hypothesis to be sound, it must be possible to test it to see if it's true. Therein lies the rub. Smithsonian Magazine explains that a problem with multiverse theories is that some of them may not be testable at all. Which is a fair point. It's impossible to see anything outside our own universe, by definition, making it seemingly impossible to ever see another one.

A principle that is often invoked in these arguments is known as Occam's Razor, which essentially states that, when there are two competing ideas to explain the same concept, the simplest idea is the better of the two. Multiverse critics would argue that, as people cannot observe any universe beyond our own, the best conclusion is that ours is the only one. Others counter this with the explanation that multiverse theories are well within known and understood physics. Caltech physicist and vocal multiverse proponent Sean Carroll, for example, emphasizes that the science behind multiverse theories is perfectly sound and even straightforward, based on staple scientific methodology like empiricism and reasoning. It's simply a matter of taking what is already known and extrapolating.

The very edge of the observable universe lies 46 billion years away, as Live Science explains, and that's where our vision ends, limited both by the speed of light and by the age of the universe itself. Sometimes referred to as a Hubble volume, it marks a spherical region of space around us. However, as noted by cosmologist Katie Mack, just because you can't see past the horizon does not mean it's the edge of the universe.

In the universe's first moments, it expanded explosively, ballooning by 100 trillion trillion times in less than the blink of an eye. This, as Space.com explains, was an event known as cosmic inflation. Fleetingly brief, inflation set the stage for our universe to grow into the way seen today, but it also carries the notion that there may be much more unobservable universe.

In "Science and Ultimate Reality: From Quantum to Cosmos," Max Tegmark discusses a prediction of this inflation, that the universe may potentially be infinite. In such an infinite universe, according to statistics, everything possible will happen. This means that, far enough away, there would be another reality like this one, with an identical copy of yourself. The distance you'd need to travel is so absurdly large that it's difficult to even write down. But you'd never be able to go there. The expansion of the universe itself would make the journey impossible.

As any fan of classic "Star Trek" will know, ye cannae change the laws of physics. While this may be true for humans, the universe itself may be under no such restrictions. According to Science Daily, there's some evidence that those laws of physics may not be the same in other parts of our universe. A mysterious piece of physics, known as the fine structure constant, is an essential component in understanding reality. Usually referred to by the Greek letter alpha, the fine structure constant relates to other physical constants like the speed of light, Planck's constant, and the elementary charge of electrons. But this constant may not be so constant.

A study published by Physical Review Letters reports evidence that the value of alpha may change in different parts of the universe. It's still a debated finding and hasn't been unambiguously confirmed to date but, if true, it lends support to the idea that other universes may operate under very different rules. As Max Tegmark explains in an article for Scientific American, there's good reason to believe that other universes may have different physics to our own. This could happen in the pocket universes formed by eternal inflation, or could even occur in a cycle of destruction and rebirth of universes.

In 2005, a group of theoretical cosmologists known as the Virgo Consortium ran a simulation of the universe. In what the Max Planck Institute called the largest cosmological simulation ever, it gave a picture of how galaxies form and how dark matter interacts at the largest scales. Of course, even our most advanced scientists are still unable to simulate a whole universe down to the level of individual atoms. Someone else out there, however, may have computing capabilities that could make our best supercomputer look like an abacus.

A disconcerting idea, the simulation hypothesis suggests our reality may not be as real as thought. Scientific American explains how philosopher Nick Bostrom originally formalized the idea in 2003 and, since then, it's been grappled with by physicists and fiction authors alike. Some suggest the odds may be 50:50 that everything is an elaborate simulation, and the chances may even increase with the creation an artificial consciousnesses -– although what a consciousness might even be is an altogether different philosophical question. 

If our universe is a simulation, it could be just one of many, and it's existentially troubling to realize that people have no way of knowing.

A lot of multiverse ideas are, ultimately, philosophical. This ties in to a specific school of thought known as modal realism, championed by philosopher David Lewis in his book, "On the Plurality of Worlds." Modal reality says, in a nutshell, that all possible worlds are real in the same way as the actual world. If something can exist, then it does exist, making our entire cosmos just one of a vast plurality of "cosmoi."

This may make any book you read feel a little bit like "The Neverending Story," but it's useful in physics too. As the Internet Encyclopedia of Philosophy explains, it underpins the idea that if something observable is theoretically useful, this is a good reason to believe that it exists -– a basic assumption that lies behind all scientific hypotheses. In standard scientific method, one must assert that something can exist before devising a way to prove it.

Lewis' ideas link back to a rather older concept that came from the 17th-century philosopher Gottfried Wilhelm Leibniz. Leibniz's "best of all possible worlds" idea, described by Britannica, is rather more religious than Lewis' work though, asserting that God would only create the best possible world for us. He intended to reconcile an old argument in theology, the question of why, if the universe was made by an omnipotent creator, evil still exists. The interesting thing is that Leibniz's argument is similar to a cosmological idea known as the anthropic principle.

Amid all the philosophy and abstraction, there are some rigidly scientific attempts to hunt for other universes. One good place to look is in the last echoes of the Big Bang itself, and this search began with the Wilkinson Microwave Anisotropy Probe (WMAP), launched into orbit in 2001.

WMAP's image of the Cosmic Microwave Background (above) shows the faint background radiation that pervades the entire universe around us. As NASA explains, it's the last remaining light from the Big Bang, stretched tenuously thin as the universe itself continues to expand and constantly bathed us with this invisible primordial light. Switch on an old CRT TV set, and you can see it in some of the fuzz on-screen.

As Scientific American explains, cosmologists have attempted to use this as a snapshot of the very first moments of the universe, when inflation was driving that initial burst of growth. Their idea was that, if the eternal inflation idea is correct, then our universe may have collided with others during its first moments of life. This would leave an imprint that could potentially be seen in the Big Bang's fading glow. As Jennifer Ouellette explains for Quanta Magazine, while WMAP couldn't give any conclusive proof, it did identify four spots in the sky that gave hints of possible universe bubble collisions. PBS reports a follow-up idea, that such collisions may have left behind gravitational waves that could help to confirm these footprints of other universes.