While you are reading this article, what might another 'you' be doing right now? Would it be possible to talk to them? What if they are wealthier than you or have fulfilled all your dreams? These are just a few of the thousands of questions one could ask. Is the universe we live in the only one, or are there other universes out there? Could there be other seas, that is, other universes? Of course, this question is one of the most popular topics in science fiction. Today, we will talk about the fascinating, exciting, mysterious, and thought-provoking theories of parallel universes, multiverses, and bubble universes. These theories are not merely fictional subjects confined to books, series, or movies but are seriously researched and studied in the realm of real science. Since scientists began to better define the universe we live in, philosophers, dreamers, and science fiction writers have been contemplating the idea of parallel universes. The "universe" as we know it encompasses everything — planets, stars, galaxies, space, and even time itself — excluding nothing. According to astronomical measurements, our universe spans approximately 93 billion light-years. That's why we can't help but wonder. Speaking of movies, I suppose there’s hardly anyone who hasn't watched "Doctor Strange in the Multiverse of Madness". If you haven't seen it yet, I recommend watching it as it relates to the topics we will discuss here. Now, let's first delve into the theory of parallel universes.
Parallel Universe Theory
First, it is important to know this: Scientists report that the world we live in was formed through the sudden explosion of a particle smaller than an atom in a time frame shorter than a second. The theory of parallel universes is also based on this sudden explosion. Scientists think that as a result of the Big Bang, which occurred about 13.8 billion years ago and is considered the beginning of the universe, another universe was created — a reflection of our image where time flows backward. In other words, they suggest that not only our universe exists, but multiple universes do. Yet, the main problem lies in assuming these universes exist but not being able to communicate with them or their versions of us. So, what is the term "parallel universe," and how did it emerge? What is the origin of parallel universes? Let’s explore this together.
The Term "Parallel Universe"
The term "parallel universe" refers to the hypothetical collection of different, observable universes defined as a multiverse. This concept constitutes a part of what we call the "multiverse theory." The known universe, which we have been able to observe with telescopes for years, spans about 93 billion light-years. However, this universe corresponds to only a tiny fraction of the hypothesized multiverse. The multiverse may be finite or infinite and includes all possible universes, encompassing everything — all space, time, matter, energy, physical laws, and changes. These universes are also referred to as "parallel universes" or "alternative universes."
According to one classification, the universes forming the multiverse are interconnected and part of a system defined by specific physical and mathematical laws. In this model, universes are assumed to share a common origin and be related to each other. On the other extreme of the spectrum, a completely separate model suggests that all possible worlds exist simultaneously and equally, known as "modal realism." This philosophical view first emerged in the 1970s and was later proposed in the 1990s by physicists and mathematicians, suggesting that the known universe corresponds to a mathematical system and that all classes of mathematical systems are equally real. These distinct universes began to be called "parallel universes" or "spiritual universes." As mentioned earlier, some believe these universes are connected to our known universe, but how such connections occur remains unexplained.
The Birth of the Parallel Universe Theory
The concept of parallel universes was first introduced by American physicist Hugh Everett. Over time, it became one of the most intriguing and widely discussed theories in quantum mechanics, both in popular culture and scientific platforms. It was proposed that numerous universes could exist independently, never interacting with each other. However, scientists like Dr. Robin Hanson from George Mason University have argued that, contrary to popular belief, parallel universes are not independent but interact with one another.
When universes interact, smaller universes may split apart or be swallowed by larger ones. In quantum mechanics, the famous double-slit experiment shows that whether a photon behaves like a wave or particle depends on the observer's consciousness. This phenomenon's realization may depend on the observer's presence, even though there is no physical change in the system itself — the change only occurs in the potential and actuality of these properties.
The Origin of Parallel Universes
We can almost hear you asking, "Where do parallel universes come from?" Here are five physical explanations that attempt to answer this question:
1. Many-Worlds Interpretation
The interpretation supported by theoretical physicist Sean Carroll is this: You may have heard of Schrödinger's cat experiment in quantum physics. In this experiment, a cat is placed in a completely sealed box along with a poison bottle. The hammer that could break the bottle and kill the cat is connected to a radioactive atom.
According to Heisenberg's uncertainty principle, it is impossible to predict when a radioactive atom will decay into a less radioactive atom — it happens randomly. If the atom decays, the hammer breaks the bottle and kills the cat. If not, the cat lives.
Furthermore, if the atom is entirely isolated from the outside world, it enters a superposition state — both decayed and undecayed at the same time. This has been observed in microscopic atoms in real-world experiments.
However, in the macroscopic world, like a cat and a glass bottle, this state does not apply: If you open the box, you see either a dead cat or a living one. But when the box is closed and isolated, the cat must be both alive and dead simultaneously.
The many-worlds interpretation arises from this. For example, an electron in quantum physics may go right with a 30% probability and left with a 70% probability. As physicist Richard Feynman demonstrated, we observe the electron going either right or left. Yet mathematically, to calculate its trajectory, we must account for both possibilities. According to Carroll and others, this suggests many worlds exist.
In short, if the electron went left in our universe, there must be a parallel universe where it went right. Thus, alongside "the universe where you married that person," there's another where you married someone else. If you're a physicist here, there might be an alternate universe where you're a garbage collector.
2. String Theory
You can learn more about this topic in our previous articles, where we have already covered it in detail. String theory, which aims to unify quantum physics with relativity to create a theory of everything, is the most promising framework. According to string theory, the fundamental particles making up the universe are actually tiny, one-dimensional strings.
Thus, all physical forces — electromagnetism, gravity, weak and strong nuclear forces — can be explained through strings. However, there's one problem: For string theory to work, the universe must have ten spatial dimensions and one time dimension.
But in our universe, we observe only three spatial dimensions and one time dimension — in other words, a four-dimensional universe. So, where are the other seven dimensions?
According to M-theory, a modern version of string theory, the extra seven dimensions are curled up tightly, like a pill bug, making them invisible to us. How is this related to the multiverse? Let’s explain:
In an 11-dimensional cosmos, there are 10500 ways to arrange the strings that form the universe! In short, according to string theory, there are at least 10 to the 500th power universes. One of them is the one we live in. While this is an overarching concept, when scientists refer to the multiverse, they usually mean string theory.
3. Brane Cosmology
Brane cosmology is a multiverse theory derived from physics. According to this theory, there are no alternative copies of us doing other things in other worlds, but there are physically parallel universes that truly exist.
According to brane cosmology, our four-dimensional universe (including time) is one of an infinite number of universes aligned like slices of bread, stacked side by side or vertically layered, within at least a five-dimensional cosmos.
Our universe is, of course, four-dimensional, but just as a lamp post appears one-dimensional when viewed from afar, universes aligned like toast slices in the cosmos appear as thin, flat, two-dimensional sheets when seen from a distance. (If you look closely at a traffic light post, you can see an ant walking around it in circles, realizing that it is indeed three-dimensional.)
The Big Bang
According to brane cosmology, two parallel universes occasionally collide, causing numerous big bangs. These collisions create new observable universes, just like the one we live in, within these endless paper-strip-like universes.
In fact, these two infinitely long universes floating parallel in 11-dimensional space might have collided repeatedly at various points. Each collision could have created a new big bang and given birth to a separate universe.
Since these universes are so far apart that light can never reach between them, they can be considered distinct observable universes, aligned side by side on top of the main universe (the mega-universe), yet disconnected from one another.
4. Inflation Model
There is another way to create an infinite number of observable universes within the void that makes up the cosmos: the inflation model. Let’s briefly explain this.
The universe was formed by a big bang. We understood this through cosmic microwave background radiation and relativity theory, effectively proving the big bang's existence. However, matter and energy are spread evenly throughout the universe. Yet, in a universe formed by an explosion, we would expect matter to clump in specific areas, not to be evenly distributed.
We Inflated Like a Balloon
According to the inflation model developed by Alan Guth in 1979, due to quantum uncertainties, our universe rapidly expanded like a balloon for a very brief period and expanded faster than light. (Let's remember that right after the Big Bang, the universe was microscopic in size.)
Therefore, microscopic lumps of matter and energy, like tiny scattered bubbles in a small universe, were almost evenly distributed across today's universe. Thus, the inflation model explains why matter and energy are spread almost uniformly across the cosmos.
5. How Can We Test This?
You might say, "You mentioned five points, but only explained four." You're right, but there is a reason: The different universe models are essentially answers to four fundamental questions we ask about the universe. The fifth point is not another model, but the most difficult question of all: How can we prove the existence of parallel universes?
If parallel universes exist, which model is the correct one? What separates scientific theories from fairy tales is falsifiability — in other words, for these theories to have scientific merit, they must be testable. Just as we have repeatedly tested Einstein’s theory of relativity.
Multiverse Theory
The universe we live in may not be the only one. "In fact, our universe might be just one of the infinite universes that make up the multiverse." Modern physics proposes several theories on how the universe came into being, but one of the leading theories suggests that after the Big Bang, during a rapid expansion phase called "cosmic inflation," new universes formed and spread out into space.
How Did This Term and Theory Originate?
The term "multiverse" was first used by William James in 1895. The multiverse is not a theory itself but a model that encompasses parallel universe theories. It represents the hypothetical totality of different possible universes, whether finite or infinite, including their nature and their potential relationships. Our universe is only a small part of this multiverse. Emerging from quantum physics, the multiverse concept points to parallel universes where alternative versions of us might exist. However, not all multiverse models require copies of ourselves. Some models propose an infinite variety of universes, none of which are copies of ours. Alongside prominent supporters like Stephen Hawking, Steven Weinberg, Brian Greene, Max Tegmark, Andrei Linde, Michio Kaku, David Deutsch, Leonard Susskind, Raj Pathria, Sean Carroll, and Neil deGrasse Tyson, there are also scientists like Jim Baggott, David Gross, Paul Steinhardt, George Ellis, and Paul Davies who argue that the multiverse model is philosophical rather than scientific and critique its scientific validity. Physicists often refer to string theory when discussing multiverse models. Another supporting concept is brane cosmology, which proposes parallel universes without copies of ourselves.
Stephen Hawking, who passed away on March 14, shared his views on space based on his studies and articles on multiverses in an interview:
"It is unlikely that there is a single, unique universe. All findings, while not infinite, show us that other universes are possible." He added, "I believe life on Earth faces the threat of extinction from disasters such as nuclear war, genetically engineered viruses, and other dangers. I don't think humanity has a future unless it goes to space. That's why I want to encourage people to look to the stars."
Bubble Universe Theory
Another theory for the multiverse is "eternal inflation." Based on research by cosmologist Alexander Vilenkin from Tufts University, we must consider space-time as a whole. While the Big Bang stopped expanding certain regions of space (including our universe), other regions continue to expand. Therefore, we can imagine our universe as a bubble floating in space.
What makes this theory interesting is that other universes could have very different physical laws from ours. This is because they are not connected to each other.
In essence, this idea proposes countless universes inflating like bubbles within space-time. Some regions, like our universe, have stopped inflating, while others continue. Thus, our universe can be seen as just one of many bubbles in space-time, each potentially governed by its own unique set of rules. Imagining and contemplating this scenario makes it all the more fascinating and mysterious. Ultimately, these studies on multiverse theories continue to spark excitement both in the scientific community and in popular culture.
Orhan Açıkgöz
Yücel Cultural Foundation
Voluntary Author
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