The Multiverse Theory and Parallel Universes Explained

The word 'multiverse' is 130 years old and was never meant to describe a universe at all.

Multiverse is a curious word. It isn't a portmanteau, but rather a compound word, combining multi with universe. It was actually first coined in 1895 by a man called William James, an American philosopher and psychologist, in an article in the International Journal of Ethics. He meant it to mean chaos, in the sense of there being no single unifying order or purpose in human existence.

The Word Resurfaces in Science Fiction

It wasn't a word that became widely used and it would be several decades before it would be seen again. Perhaps unsurprisingly, science-fiction was next to embrace the word.

British author Michael Moorcock, in his 1962 sci-fi novel The Sundered Worlds, used multiverse to describe a plot where his characters could move between different realities. This is certainly the popular meaning we have for multiverse today, the notion that there is more than one reality, literally many universes, perhaps connected in some way. In the Oxford English Dictionary, Moorcock is credited as the originator of the meaning, although interestingly they cite his follow-up novella The Blood Red Game in 1963 as the original source.

That largely explains the origin of the popular meaning of multiverse. In science circles, multiverse has a slightly different meaning and in order to explore that, we need to go back to the 1950s.

Hugh Everett and the Birth of "Many Worlds"

Hugh Everett III was an American physicist and while at Princeton he wrote his PhD thesis under supervisor John Wheeler (the man credited with popularising the term "black hole"). A shortened version of Everett's thesis was published in the Reviews of Modern Physics in 1957 under the title "'Relative State' Formulation of Quantum Mechanics".

Its central idea is that the wavefunction never collapses, with every possible quantum outcome continuing in a different branch of reality. This differs from the traditional Copenhagen interpretation of quantum mechanics, where a wavefunction collapses into one definite outcome when a measurement is made. It also differs from the original multiverse meaning in Moorcock's stories. Unlike the sci-fi plot, there is no travel between these branches. Each one is completely cut off from all the other possible realities.

Everett didn't actually call this a multiverse theory. That would come later via Bryce DeWitt in the 1970s after he wrote about Everett's theory in an article in Physics Today called "Quantum Mechanics and Reality". DeWitt explored the notion of these branches of reality across the universe, describing how every quantum transition splits our world into "myriads of copies of itself". He coined the phrase "many worlds" to explain what this would all mean.

The multiverse label was finally attached to this specific meaning decades later by David Deutsch, an Oxford physicist.

Is There Just One Meaning of Multiverse?

You may think we have now settled the meaning of the word multiverse. But, that is not the case. In fact, there are several different theories that use the term.

Eternal Inflation and Bubble Universes

One of those theories originates from Alan Guth, an American theoretical physicist and cosmologist. In 1980, he proposed cosmic inflation happened in the immediate moments after the Big Bang. This describes a rapid, exponential expansion of the universe and is now a mainstream part of cosmology. It solves two issues, the horizon problem (explaining how distant parts of the universe have the same temperature and density) and the flatness problem (explaining the universe's overall flatness).

Andrei Linde, a Russian-American theoretical physicist born in 1948, expanded on Guth's work. During his time at the Lebedev Physical Institute in Moscow, he postulated that inflation doesn't stop at the same time everywhere and continues in parts where the conditions for inflation still remain. This leads to the creation of bubble universes which may even have different laws of physics.

Linde called it 'eternal chaotic inflation' and it generally became known as 'eternal inflation'.

In later years, Linde started referring to his theory as a multiverse theory. In his paper "A Brief History of the Multiverse" he described the universe as "an eternally growing fractal consisting of exponentially many exponentially large parts."

The String Theory Landscape

The other theory that is associated with the multiverse is string theory. In simple terms, string theory is a candidate framework for an overarching law of physics, i.e. reconciling quantum mechanics and general relativity. It imagines that the universe's fundamental particles aren't points but rather tiny vibrating strings. It also leads to the notion that our reality has far more spatial dimensions than the three we experience. The reason we don't experience them is because they are "compactified" or specifically tightly curled up into incredibly small, imperceptible geometric shapes. Some estimates put the number of possible vacuum states in the string landscape at around 10⁵⁰⁰, potentially corresponding to different sets of physical constants. You can probably see where this is heading. It is the explanation of another multiverse. But in string theory it is only a mathematical calculation.

In conjunction with Linde's eternal inflation theory it starts to have legs as a multiverse theory, and Leonard Susskind in his 2003 paper "The Anthropic Landscape of String Theory" proposed just that. He reasoned that the eternal inflation theory gave the mathematical possibilities of string theory an actual mechanism to make the multiverse physically real.

What Branching Realities Mean for Us

We've explored the etymology and also the theories behind the multiverse hypothesis. Now I'd like to explore what this means for us as humans.

If we explore Everett's branching theory, this means at every given quantum event, myriad copies of ourselves are created. This leads to an unsettling question. Which one is the real you or me? There are differing opinions on this. Some say each one is equally you, a continuation of your singular identity. Others suggest that none of them are you, arguing that your identity doesn't survive the moment of branching. Another line of thinking attempts to sidestep the discussion entirely, saying that we don't have a single fixed identity anyway.

This is always the part of the theory that has interested me the most. In one sequence of myriad branches containing me, stretching out from the moment of my birth, I could have become Prime Minister or the richest person in the world. Of course, it would require a train of specific events and choices in my life, and the chances of it happening would be vanishingly small. Many-worlds doesn't imply that every imaginable outcome is inevitable. But such a radically different life could, in principle, occur in a branch.

Fine-Tuning and the Anthropic Principle

Another interesting concept that comes from the eternal inflation and string theory versions of the multiverse is how they explain why our universe is so finely tuned to our existence.

The thinking goes like this: the reason why we exist is because we had the fortune of living in one of the countless universes created that happened to have the correct laws of physics that allowed our evolution. In the other bubbles, the laws might just be very slightly different, meaning life as we know it could not have evolved.

While this is a perfectly reasonable hypothesis, some people bristle when this is used as a proof for the multiverse. They say that our existence guarantees we'd observe a universe that possesses the necessary constants for life, so it doesn't prove whether a multiverse exists or not.

So, Is the Multiverse Theory Real Science?

If Popper's falsifiability criterion is followed, the multiverse theory wouldn't be classed as scientific, because it can't be proven false by a conceivable observation. And when we look at the multiverse theories such as Everett's branching and Linde's bubble universes, it is apparent that we wouldn't ever be able to verify their existence by observation because they are closed off from us by their very nature. George Ellis and Paul Steinhardt are some of the scientists in this camp.

On the other side of the argument are Sean Carroll and David Deutsch, who feel that Popper's falsifiability criterion is too narrow and that if a theory has internal consistency it can still have merit.

Another point worth making here is that some scientists argue that it may actually be possible to find indirect evidence of a multiverse if it does exist, specifically the bubble universe version. They have even searched for it, hoping to find detectable imprints in the cosmic microwave background caused by collisions between the bubble universes. As yet, their searches have been fruitless.

And that, in essence, is what I have been able to find on the multiverse theory. Genuinely an interesting subject, if a little difficult to follow at times. I hope it has given you pause for thought, and if you are an expert in the field, let me know if I have missed anything!

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