I decided to sit down at my computer and start writing this article on parallel universes. Of course, picking this topic was entirely my own choice, and I could just as easily have chosen an entirely different topic instead.
For example, I could be writing now about how the new findings at the Göbekli Tepe site have radically changed the things we know about our history. Maybe I could be writing about how the human ageing process can be slowed down through genetic manipulation or about how our civilization will suffer if we fail to develop alternative energy sources. If any of these had been the case, you would not be reading an article on parallel universes but another one instead. Maybe you wouldn’t care and just flip through the pages without reading it.
All of these examples describe the successive series of possibilities that are connected to our choices at any given moment of our daily lives. So, could there be a parallel reality where I chose to write about another topic? Is there another one depending on whether you chose to read it or not? Is there a copy of you in another parallel world who, rather than reading this article, decides to go for a walk or see a new movie instead?
When we put it this way, the question seems odd to us, but most of us use similar phrases all the time:

  • “If I’d accepted that job offer, I would have been in a better position now and earning a better salary. I made the wrong choice.”
  • “If my son had not insisted on going to that concert, he wouldn’t have been in that bus accident. He would still be alive.”
  • “If my favorite sports team had done things a little differently, they could have been champions now, and I would’ve made a stack of cash on that bet. They didn’t, though, dashing the dream of them being champions and flushing my bet down the drain.”
  • “If I’d listened to my friend’s warnings and sold that company’s stock, my losses would’ve been much less. That mistake cost me dearly.”

In the flow of the life, we frequently face situations where we need to make choices. These choices sometimes seem insignificant and ordinary, and other times, the choice we make creates a breaking point in our lives that we did not foresee. The reality we live in right now is formed from the direct consequences of our choices and decisions. Before we make a choice, we see a composite image of the multiple possibilities, but usually we cannot clearly see what the direct consequences of our choices will be. The reality only becomes solid when we make a choice and encounter its consequences. Take for example the years before you graduate from high school. If you’re wondering whether to study medicine or engineering, you will only see what one of these will bring when you actually choose, because the situation then becomes your reality. Before you choose, only possibilities and uncertainties show themselves.
At this point, a question surely pops up: At the point of a decision, is it possible there is a different reality for each possible choice where its consequences are encountered? To put it another way, in the earlier example of someone who regretted turning down a job, did he accept it in a different reality and live a very different life? Could the son who wanted to go to a concert still be alive in another reality, because he decided not to go and never boarded that doomed bus? Maybe the bus never crashed at all because he didn’t get on it. Is there another reality where your favorite sports team made better choices and won the championship? Could there be an alternative reality where the person who invested badly chose a different stock instead, ending up rich instead of bankrupt?
These questions reside in the area of parallel universes, a tempting and exciting topic that has been dear to sci-fi writers for some time. The main principal of this attractive theme is simple:  The situation in which a choice is made is just one branch of the tree of multiple possibilities. In other words, the resulting reality excludes all the other possibilities, yet all of the choices facing you become realized independently of each other. These other choices become realized in other planes of an unending multiverse system.

Parallel universes in pop culture

In popular culture, such as sci-fi or fantasy movies and books, choices and decisions sometimes determine splits that happen in someone’s personal life, just like in the above examples. The movie Sliding Doors, starring Gwyneth Paltrow, is one such movie. Paltrow’s character, Helen, arrives at the PR firm she works for and learns she’s fired. She journeys home, feeling down, and we watch the interesting split that emerges when Helen either catches the subway train or misses it. If she catches it, she arrives home early and catches her boyfriend cheating on her with another woman. In this case, Helen dumps the boyfriend, finds a new job, and moves on with her life. If she missed the train, however, she never catches her boyfriend “in the act,” so she continues with this pointless relationship with a cheating lover and lives an unpleasant life. Before our eyes, the movie unravels two different parallel universes where each of these possibilities becomes realized from the time of the event.
Some other works based on this theme focus not on the small details in individual lives but rather on parallel outcomes that affect nations and even the whole world. For example, The Man in the High Castle, by the well-known sci-fi writer Philip K. Dick, takes the determining event of World War II to a parallel reality. In Dick’s book, changes in pre-war events lead to the war being won by the Axis Powers (Nazi Germany, Imperial Japan, and Fascist Italy) rather than the Allied Forces, and the face of the world is considerably different. In this story, Japan rules over Australia and New Zealand and most of the Far East, while Germany rules western Asia and much of Europe,  with Italy ruling over many of the Mediterranean regions. The USA has been torn apart by the victors and turned into a puppet federation. In his book, Philip K. Dick tries to question what our planet would have turned into by pointing out the alternative consequences of World War 2, something that strongly affected the current reality of our world.
We see a similar approach in a 1992 novel by Harry Turtledove, The Guns of the South. A group of white, racist South Africans travel back in time to the fierce days of the American Civil War. They bring aid to the commander of the Confederate Army, General Robert E. Lee, in the form of AK-47 assault rifles and nitroglycerin tablets to treat General Lee’s heart condition. Thanks to this decisive aid, reality as we know it is changed, and the war is now won by the Confederacy. Of course, the consequences of this split change not only the visage of America but also of the whole world. In Turtledove’s fiction, a parallel universe is constructed where the Confederate Army has won the American Civil War, and the subsequent political developments follow in accordance with this critical difference.
For those eager to wander through the labyrinths of history and ponder on other alternatives that could have changed the current reality of the world, is it possible not to consider what kind of a world would result? For example, take the situation of 4th century Rome: Emperor Julian, who became sole emperor after the death of Constantius II, decided to restore the ancient belief system and denounce Christianity, which had recently become the “official religion.” He was such a powerful and influential leader that he could enforce this transformation through determination. As he was about to successfully achieve this, however, he died unexpectedly during the Persian campaign. (According to some sources, this was an assassination planned by his opponents.) This changed everything. His opponents, who then ruled in Rome, not only reinstated Christianity as the official religion, but they also banned all other ancient beliefs, temples, and rituals. This transformation marked the beginning of the Dark Ages, which probably would have not happened at all if Julian had not died during the Persian campaign. Rome, and much of the known world, would have progressed in a completely different direction. Rather than churches and cathedrals, imagine European cities with magnificent temples dedicated to Mithra, Juno, or Cybele. These changes would not have been limited to architectural buildings and belief systems—they would have affected our sense of philosophy, which would be very different to what it is today.
Speculation and mental gymnastics begin when we say things like, “What if that event had ended with a different outcome.” It’s one thing when we talk about the existence of multiple universes—the multiverse is another thing entirely. The multiverse is an infinite number of universes where every possible option becomes realized. We live different lives in some of them, but we were never born at all in most of them. Taking this further, assume that in many of these universes, the principles and laws are different from in our current reality. Now, do our “natural laws” prevail? This rather challenges the limits of our minds, doesn’t it? This way of thinking is not just the fantasy of imaginative sci-fi writers— it’s also is fed by theories in modern physics.
Looking at the books and movies with sci-fi themes, we may mistakenly assume this thinking belongs to recent history only. We might also think that it started during the nineteenth century, when the momentum of science and technology started to gather and there were favorable environments where fantastic ideas (think of Jules Verne and H.G. Wells) were able to germinate in our cultures. However, this is not the case. Surprisingly, the history of concepts like multiple worlds and parallel universes goes back ages, even millennia.

The idea of parallel universes: A brief history

Roughly 2,500 years ago, philosophers (known then as “atomists”) in ancient Greece proposed the hypothesis of multiple universes. They theorized that the system we live in couldn’t be the only universe possible. According to them, each universe stands apart from ours and has its own closed structure with its own stars and so on. They believed a great many universes went through their own processes at distances we couldn’t imagine, so there was no way to validate their existence. These independent universes, which have no contact and connection with each other, are parts of a multiversal system with an infinite size. If you think that time was too early for such a radical idea, let’s continue by saying that at least 3,500 years ago, a similar thought structure emerged in Hindu culture: Vedic cosmology. It’s likely that it actually originated much earlier. Its rich mythologies, which are interwoven with colorful stories, talk about many different universes existing separately. Some of these universes are similar, while others display completely different features. Even if there is no proof of its similarity with the parallel universes idea, the fact that the concept of multiple universes appeared in ancient thinking is certainly interesting enough.
Well, when did this brave and radical thinking change occur? And when did the idea of “there is only one universe” emerge? The general consensus is that this approach was proposed by Aristotle in Ancient Greece. The universe we are in, according to Aristotle, is the one and only and has no other alternative. This “unique universe” model, which was embraced by the theologians of the following centuries, is in harmony with religious dogma, because it places the world and its special inhabitants, us, in a privileged position.
We know the belief that God created just this special, one-of-a-kind universe has been eagerly embraced and promoted by religious authorities, so it ruled in Western philosophies for some time. Even so, it could not completely prevent discussions about multiple universes among the theologians of the middle ages. It’s possible that a paradigm shift started in the 18th century with the Age of Enlightenment.
The old model of the universe, which is geocentric, assumes that all celestial objects revolve around the Earth. It started to be discredited with the developments in astronomy in the 16th century, yet the concept of a single universe prevailed for a few more centuries.
Thomas Lepetier, a historian and philosopher of science, says in his book Univers Paralleles that the first thinker who elaborated upon the concept of the multiverse, as we understand it now, was not a physicist or astronomer but rather Auguste Blanqui, a well-known French socialist. As one of the leaders of The Paris Commune of 1871, he had been arrested just before the uprising and imprisoned at Taureau Castle. This famous socialist spent his time in prison by pondering astronomy, which he believed “was central to everything.” He wrote his work Eternity by the Stars, in which we encounter the first striking elaboration on the idea of parallel universes. As he worked in his cell, he considered there are an infinite number of Blanquis, existing simultaneously in separate universes and making a different decision at each different option. He theorized that these Blanquis advance on completely different life paths within the countless branches of the tree of possibility, and he tries to formulate this model. The book, which he completed in prison, presents the idea that “there is an endless number of universes, distinct from each other, where at each moment, each possibility is actualized.” Lepeltier says in his book, “It is possible to observe that this thesis became a current issue again in the late 20th century… and it is emerging not from the depths of a dungeon anymore but from the most prestigious universities in the world. Its strongest advocates now come from Cambridge, Oxford and Princeton.”

Science, not fantasy

When we touch upon the topic of parallel universes, we’re no longer talking about the fantasies of sci-fi but rather the theories and studies of the leading physicists of our time. Especially after the dizzying developments that particle physics has gone through in the last generation, the idea of a multiverse where every possibility is actualized at every moment is central to one of the leading theories. Certainly, when defining parallel universes, physicists prefer not to use the superficial approach I‘ve used to spice this article up. They strive to explain this extremely important issue through long, complicated formulas or experimental results that are beyond a layman’s understanding. Nevertheless, whether using the hardcore scientific approach or my choice of lighter examples, the consequence that matters most to us remains: Even if it‘s not yet completely proven, it’s likely we live in one component of the highly complicated system we call the multiverse.
From the discussion in the halls of theoretical physics to the daily conversations or the imaginative tracks of sci-fi, this concept can be called by other names: parallel universes, quantum universes, alternate universes, parallel worlds, or alternative realities. As you can imagine, the various theories in this area are numerous.
The Many Worlds Interpretation, also know by its popular acronym MWI, was developed by Hugh Everett of Princeton University in 1957. It was one of the breakthrough theories of the time. Starting in the 60s, it was developed through the contribution of physicists from all over the world. In a nutshell, the MWI talks about “a complicated structure where all possible pasts and futures are real and each represents a separate world or universe.” It’s similar to the concept of distinct universes, where each possible choice is realized. In other words, the previous understanding of reality as a single, unchanging linear history was radically changed by the MWI.
It’s useful to point out the names of two prominent experts in this field: Michio Kaku and Brian Greene. They stand out not only because of their studies into the structure of multiple universes but also because of their successful books, which translate these complex subjects into something everyone can understand. In particular, Greene’s book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos is one of the most impressive works of this decade in this field.
As you’ve no doubt noticed, this is only an introduction to this subject. We haven’t yet said anything about the flashy developments in the quantum world that support the existence of parallel universes. So, let’s just accept this as a beginning and resolve to continue our journey into parallel universes later.
After all, maybe we’ve already done this in a parallel reality. Who knows?