If you’re a fan of science fiction, you’ve probably heard the word “multiverse” before. If you’re not a fan of science fiction, the multiverse is exactly what it sounds like, a collection of universes.

Just how many universes you may be wondering? An infinite number of them according to many popular multiverse theories. In a limitless multiverse, the possibilities would be quite literally endless.

Possibilities

Some say that for every possible eventuality or outcome, there exists a universe in which that reality exists. In one universe, the Nazis won World War 2. In another universe, the dinosaurs evolved into intelligent creatures and now facetime with each other on their computers. In yet another universe, you are the president of the United States of America, while in another, you killed Abraham Lincoln.

Some of these realities may have minimal or no changes from our own — maybe in one universe you are left handed instead of right, and that’s the only difference. In others, the differences are so large that the earth may not even exist at all. Perhaps the Milky Way galaxy doesn’t exist, or maybe the cosmological constants are completely different values which means the first stars never got around to forming in the first place, and the universe is just a barren soup of protons and electrons with no form or function… Such a universe would be quite a desolate place. Every possible reality you can think of would exist in this infinite multiverse, with some so strange that the very laws of physics in these wacky universes defy logic or comprehension.

The possibilities in this hypothetical multiverse would literally be limitless. The only limit would be your own imagination. Whatever you could conceive of would exist. The possibilities hypothesized above would exist, as an infinite number of variations of those possibilities. Maybe today you wore your blue shirt instead of your black shirt, or maybe you died today in a car accident. The size and scope of possibilities is so large that even our greatest minds have trouble wrapping their heads around these concepts and what it all potentially means.

Is There Any Scientific Evidence?

As of this writing, there is no hard scientific evidence to back up the premise of a multiverse. The idea itself isn’t even theoretical, it’s only hypothetical which is tantamount to a thought experiment. But if that’s the case, why are so many physicists and theoretical physicists so accepting of the notion? Why is it gaining in popularity?

It’s due in large part to the ‘soft’ evidence. Unlike many failed hypotheses throughout the human history, the multiverse does have a little science behind it by way of maths, probability and quantum mechanics. There are certain interpretations of quantum mechanics which give rise to a multiverse, as well as other intriguing maths in which a multiverse “pops right out of the equations” (like with string theory). Another thing going for a multiverse is that the concept isn’t forbidden or ruled out by any known physics or scientific laws. In fact, that’s actually why it’s gaining in popularity — it answers and explains many questions that have had us scratching our heads for years, decades, even centuries.

Today, we’ll go over a few of the most popular and common multiverse theories. Towards the bottom of this article, we discuss the problems with these multiverse hypotheses.

A Multiverse Within Our Own Universe

This particular version of a multiverse may very well be the most probable version as it requires very little to actually be true — it isn’t as big of a “reach”. This version of the theory takes advantage of, and expands on our belief that our own universe is infinite in size and scope.

Because there are only a limited or finite number of ways that atoms can organize and arrange themselves in a random, complex system, it means that the odds of another “mirror earth” existing somewhere out there are virtually guaranteed. That is to say, if you traveled far enough out in the universe, past billions, perhaps trillions of galaxies, eventually you’ll stumble across a solar system identical to ours, with an earth that perfectly mirrors our own. And not just one alternate earth, but an unlimited number of them. Each with a solar system and neighboring stars that perfectly mirrors our own. The odds of a galaxy or solar system forming that perfectly mirrors our own might be infinitesimally small, but in an infinite universe, such things would be an absolute certainty.

Unfortunately, don’t expect to travel to meet your doppelganger any time soon, however. The distances involved make any hope of traveling or even seeing such a planet impossible. Even if we discovered faster than light travel, you’re still talking billions if not trillions of years of travel time at superluminal speeds. Not even the Enterprise from Star Trek with its fantastical warp drive could traverse those distances.

Pocket or Bubble Universes

There’s an implication in the theory of inflation (which answers how our universe came to be right after the big bang) which says that because inflation doesn’t stop everywhere all at once, there ought to be areas in which inflation continues, and that where inflation does eventually stop, those areas become their own pocket universes. This multiverse idea has roots in a larger theory called “Eternal Inflation” and it too, allows for an infinite number of universes.

Imagine a loaf of baking bread. If you were to cut the bread open, you’d discover little pockets of air in which the bread did not fully expand into. That little pocket can be thought of as a universe. Now imagine a forever baking, eternally expanding loaf of bread; every time one of those little air bubbles forms inside this cosmic loaf of bread, another universe is born. That is how eternal inflation explains the expansion of our universe, and predicts the existence of a multiverse.

Quantum Mechanics – Many Worlds Interpretation (MWI)

The many-world interpretation of quantum mechanics is one of the most intriguing, and complex versions of a multiverse. First postulated by American physicist Hugh Everett III back in 1956, this is the version that many science fiction books, TV shows and movies use as their basis for their plots. Dr. Everett hypothesized that, at the quantum level, whenever the universe is confronted with a choice of paths, reality splits/branches and both choices or paths happen simultaneously. This is the quantum mechanics version of an infinite number of universes, and everything that could possibly have happened in our past, actually has occurred in other universes. For example, there’s a chance John Wilkes Booth could have missed his shot, and as a result Abraham Lincoln lived, or that the terrorists on 9/11 were caught before they had a chance to carry out their attacks. For every possible eventuality or outcome, there exists a universe in which that reality exists.

As our primary goal here at Zidbits is to explain complex ideas in layman’s terms, so keeping with that we’ll skip the technical details of this particular multiverse theory, but know it includes complex terminology like “quantum wavefunction collapse & decoherence” and “relativistic quantum field theory“. In layman’s terms, however, this interpretation postulates that thanks to quantum mechanics, reality is an infinitely branching tree. Every time an outcome or chance event can go one way or another, reality branches off into two separate, parallel realities running side by side. In one, the outcome went one way, and in the other branch, the outcome went the opposite way. So if you were driving down the road, lost, and you came to a fork in the road, which direction would you choose? Left or right? The many worlds theory claims that you actually go both ways — in one reality you went left, in another, you went right. And all the consequences that are the result of your decision occur in those realities. Every time you make a choice or decision, a new branch or reality is formed.

Time Travel Solution?

If time travel ends up being permitted by physics, the many-worlds interpretation could be the salvation time travelers have been looking for. It does this by resolving the majority of the paradoxes that arise from time travel, notably violating causality (killing your grandfather before you were born, etc) — since traveling into the past would itself be a quantum event significant enough to causing reality branching, time traveling would simply create a brand new timeline that exists parallel to the reality the traveler just came from, thus not violating causality.

Cyclic And Brane Multiverse

We’ve expanded on this one in a previous article titled, “What Happened Before The Big Bang?” This particular version postulates that our entire universe exists on a membrane (brane) which floats in a higher dimension or “bulk”. In this bulk, there are other membranes with their own universes. These universes can interact with one another, and when they collide, the violence and energy produced is more than enough to give rise to a big bang.

The branes float or drift near each other in the bulk, and every few trillion years, attracted by gravity or some other force we do not understand, collide and bang into each other. This repeated contact gives rise to multiple or “cyclic” big bangs. This particular hypothesis is falls under the string theory umbrella as it requires extra spacial dimensions.

Problems With The Multiverse

One of the biggest problems with the multiverse is that it’s currently an untestable theory. And unfortunately for the multiverse, a theory that can’t be tested is a death sentence, a dead end. Even taboo in some sciences. However, due to the compelling nature of the theory, and the sheer number of different disciplines of science it touches, it has persevered, even gaining popularity.

Not all hope is lost, however; one of the reasons it has hung on is because there may be indirect ways to gather evidence of a multiverse. One way that is currently underway as we speak is looking at an incredibly high resolution picture of the cosmic microwave background (the CMB). The CMB could show evidence of our universe interacting with another universe right next to ours, either by way of a cosmic umbilical cord, or simply bumping into each other via the cyclic brane scenario (such a collision would result in more than enough energy to cause a big bang). If we spot anomolies in the CMB, it could be indicative of a multiverse, or at least let us know we’re on the right track.

Sometime in the future, perhaps the distant future, we’ll find a way to test for the existence of other universe separate from our own, and then we’ll know once and for all.

Bonus Bit: Quantum Immortality

Ever wanted to be immortal? There is a tiny, incredibly small chance you may very well be immortal if the many worlds interpretation of quantum mechanics is correct.

Quantum Immortality is a fun thought experiment which, according to the many worlds interpretation of quantum mechanics, alleges we are immortal (at least on the quantum level). Imagine you were convicted of murder and on death row. The day of your execution arrives and you are now lying on a table waiting for your lethal injection which should come within minutes. However, MWI postulates that if there’s a chance, however small, that something happens which causes you to survive the situation — like the machine malfunctioning or an earthquake occurring which stops the procedure, that it will happen, and you will continue to live on. Reality will keep branching off, causing whatever kills you to fail, keeping you and your consciousness alive in a new branch of reality. In all the other realities you will pass on and die, and the people in those realities will experience your death, but your consciousness moves to the branch in which you somehow live.

If MWI is correct, you may notice that, despite all the odds, you somehow never seem to die. This is quantum immortality. Do you have your own theory of the multiverse or quantum immortality? Let us know in the comments!

References & Citations:

David Deutsch, The Beginning of Infinity, page 310.

Tegmark, Max (2014). “Our Mathematical Universe: My Quest for the Ultimate Nature of Reality.”

Greene, Brian (January 24, 2011). “A Physicist Explains Why Parallel Universes May Exist”

Everett, Hugh (1957). “Relative State Formulation of Quantum Mechanics”. Reviews of Modern Physics. 29: 454–462.

Pitowsky, I. (2005). “Quantum mechanics as a theory of probability”. arXiv:quant-ph/0510095

Everett, Hugh (1956, 1973). Theory of the Universal Wavefunction, Princeton University, pp 1–140

Scoles; Sarah (April 19, 2016), “Can Physics Ever Prove the Multiverse is Real”

Vilenkin, Alex (2007). Many Worlds in One: The Search for Other Universes

Tegmark, Max (November 1998). “Quantum immortality”

