If you like series, comics or science fiction and fantasy books, you have almost certainly seen some plots involving parallel universes, other dimensions and multiverses. The concept is so widespread in popular culture that, perhaps, anyone without any contact with the classic media that approach the topic - such as comics and sci-fi books - must already have some notion on the subject, since there are even songs with fantasies about a world where our decisions differed from those we had here, in our reality.
The idea is really fascinating and tempting, so tempting that not even some of the most serious scientists can avoid enchantment: somewhere, in another universe similar to ours, a version of ourselves lives a life different from ours. That version got the dream job (which unfortunately we couldn't get here), has another family, adopted a nice cat or a well behaved dog, supports a team that wins more championships than the team we support here… and the possibilities are infinite!
This is precisely what part of the serious science behind the theories of parallel universes is about - the possibilities. It is that, according to some hypotheses about the mathematics of the universe, if there is a possibility that something will happen, such as, for example, a certain configuration of particles, it will happen. If we extrapolate this concept, it means that every possibility of decisions in our life must also become real. The problem is that, most of the time, we can only choose one of the options that life offers all the time.
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| In a multiverse, would the "Earths" be similar or completely different from each other? (Image: Playback / geralt / Pixabay) |
Well, for all infinite possibilities to happen, infinite universes must exist. And we have a lot of different hypotheses that deal with the subject, based on this or other premises. But, before dealing with this in more detail, we need to pay attention to some concepts. The universe we know contains everything we can observe - from the dust that is accumulating on your furniture to the stars, galaxies and black holes throughout space-time. So far so good, right?
This universe is also gigantic, about 93 billion light years in diameter, according to astronomers' estimates. It is impossible to explore all this immensity, because it continues to expand. But if it is this big today, does that mean it is finite? If so, is there nothing else? Does it repeat itself? Well, there may be something out there, like parallel universes, bubble universes, child universes, among other concepts. Cosmologists call this idea a multiverse - the sum of all universes that may exist.
But not all hypotheses are alike. Not all ideas from other universes suggest that there are other versions of ourselves. In addition, the proposals arise from different scientific currents, such as string theory and quantum mechanics, or the theory of cosmic inflation, which is currently widely accepted as one of the main hypotheses about the evolution of our universe. All of these theories predict the existence of a multiverse. Perhaps this is a strong indication that it does exist - but it is extremely difficult to take to an important stage in any science: observation.
What is parallel universe
In popular culture, we call “parallel universes” any worlds that exist beyond the conventional universe. But, for science, there are very specific terminologies for each explanation and concept of how and why such "universes" could be formed. Each of these hypotheses has its own explanations and arguments that almost convince us that, yes, there is more reason to believe that there is something beyond our cosmos than the other way around.
Specifically, the term “parallel universes” is often used by string theory researchers. According to the idea of strings, which aims to reconcile quantum physics with the Theory of General Relativity, we live in a reality where everything is formed by very small, imperceptible strings. Each string vibrates in a certain way, and together they form the particles. A variant of String Theory, called M theory, suggests that each particle is actually a tiny loop of string whose pattern of vibration determines what kind of particle it will be.
However, theory M requires that the universe has 11 dimensions, and so far, we can only detect four: three in space and referring to time. Proponents of M theory say that there may be other dimensions, but just as a being who perhaps lives only in two dimensions would be unable to see the third, we would also be unable to see the fifth, and so on. Well, in theory M, the four dimensions in which we live are called “brana”, and perhaps there are other branes, formed by certain dimensions, formed by the same strings.
Seems confused? Think of a stack of pancakes stuffed with honey. It is as if our universe exists in one of these pancakes, and the others are “on top” of ours. Honey prevents us from knowing about the other pancakes, but that doesn't mean they are not there. Scientists suggest that other four-dimensional branes would exist within an 11-dimensional space, and the best part is that, perhaps, a brane in higher dimensions will move and collide with our own brane. Proponents of the idea say it offers an interesting way to explain what motivated the Big Bang.
On the other hand, some ideas of parallel universes do not involve String Theory, and imply in some things, such as the impossibility of these realities meeting - even because they are parallel, as the name suggests. Two parallel lines, by definition, can never meet. Well, unless you twist the surface where they are, which would result in some collisions, even if the lines mathematically remain parallel. You can experiment with this by drawing parallel lines on a sheet and twisting them. This twist is something that scientists can look for to try to prove the existence of other universes, and it is definitely something that is proposed to argue about the existence of other dimensions.
But the task of looking at other universes is not an easy one, partly because the possibilities are many. To look for something, you need to know what it looks like, where it is, how it works, but there are many different hypotheses for parallel universes. Fortunately, there are also many scientists trying to find some evidence of some of them. MIT mathematician and cosmologist Max Tegmark, for example, categorized the possibilities into four:
- A parallel universe could not have anything qualitatively new and different than our own universe
- A parallel universe could have totally different fundamental laws of physics
- A parallel universe could have the same fundamental laws of physics, but it started with different initial conditions
- A parallel universe could have the same fundamental laws of physics, but different effective statutes
In addition to being a good tool for science fiction authors to create more innovative storylines, the four possibilities also give scientists some basis for working on their own research. And speaking of possibilities, let's understand how they work in favor of the idea of other realities.
Many worlds
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| Diagram of the Schrödinger box, all possibilities are superimposed until the observer checks the result |
Originating in the 1950s, the theory of many worlds postulates that parallel worlds constantly branch from one another, moment by moment. The correct name for this idea is the Interpretation of many worlds (or IMM), and it is an interpretation of quantum mechanics. It was initially formulated by Hugh Everett to explain some non-deterministic processes, and many different versions of this hypothesis came later, but with the same key concepts.
In his book “Something Deeply Hidden”, Sean Carroll uses the fable of the fox who tries to pick the grapes. Hungry, she tries to reach the bunch hanging on a vine, but without success. Refusing to admit failure, the fox turns around and says that the grapes didn't look so tasty and walks away. Carroll says this attitude sums up how physicists deal with the uncomfortable implications of quantum mechanics. And it goes further: the fox can pick grapes in other worlds. In fact, there are a number of possibilities for the end of the story, and each possibility generates a world totally separate from ours, hidden and inaccessible.
The idea fits like a glove in quantum mechanics, and it is tempting to explain some mysteries that, in fact, scientists observe and try to unravel - and the best, there is no need to resort to other eccentric ideas to explain these mysteries, because the hypothesis of many worlds it only uses the laws of physics that we already know. Quantum mechanics is the basic structure of modern subatomic physics and has already established itself firmly after almost a century of testing. In quantum mechanics, the world develops through a combination of two basic ingredients: the wave function and the realization of a possibility.
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| Hugh Everett (second from right) was the first to present the theory of many worlds |
The wave function, besides being totally deterministic, is one of the main mathematical formulas of quantum mechanics. It dictates that a mathematical expression that conveys information about a particle (which can also behave like a wave, and each type of particle behaves like a different wave) in the form of countless possibilities for its location and characteristics. The ingredient of the realization of possibilities, on the other hand, is simply when one of the countless possibilities of that wave comes true. At that time, all other possibilities are eliminated.
Perhaps you remembered Schrödinger's cat. The problem with the cat in the box is not just about the animal's chance of being dead or not - it's about the myriad possibilities and the fact that we can only infer which one has materialized through observation. When you open the box, we will certainly know if the cat survived, but until then, the countless possibilities are, shall we say, in suspension. We don't know, therefore, everything can be real at the same time. In quantum mechanics, this opens the door to several worlds, each with a cat inside its box. In each of the worlds, something different happened with the animal, and these worlds were generated until the possibilities ran out. This implies that in reality none of them have been eliminated, as quantum mechanics would originally say.
It is worth mentioning that there are different thoughts on the wave function. Albert Einstein, for example, argued that the wave function is only a temporary correction and that physicists will eventually replace it. Christopher Fuchs considers the wave function to be essentially subjective, so it is something that physicists should use only as a guide, rather than a name for a real feature of the subatomic world. Carroll says that the theory of many worlds is the most direct approach to understanding quantum mechanics. He says that there is a wave function, and only one, for the entire universe. When an event occurs in our world, the other possibilities contained in the wave function do not disappear, but new worlds are created, in which each possibility is a reality.
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| Diagram of Schrödinger's box |
Another way of looking at this approach is through the observation of a photon (particle of light). Theorists propose that if two people observe the same photon, they can come to different conclusions about its state, and yet both observations would be correct. This idea was presented by Eugene Wigner, in 1961. He says that a photon when observed in an isolated laboratory can be measured to find out if the polarization - the axis on which the particle rotates - is vertical or horizontal. It turns out that before the photon is measured, it exhibits both polarizations at the same time, that is, the photon exists in an "overlap" of two possible states.
After the person in the lab measures the photon, the particle takes on a fixed polarization, which is equivalent to opening the Schrödinger box. But for someone outside that closed laboratory who does not know the result of the measurements, the photon is still in a state of overlap. Weird, right? For this is the world of quantum mechanics, and perhaps this is one of the reasons why the explanation of the many many is so tempting, even for scientists: it offers countless possibilities to live with this craziness of the quantum world.
Of course, accepting these observations from quantum mechanics is very different from believing that there are other worlds, with other versions of ourselves, making different decisions. The idea of child worlds is a great extrapolation of the reality that scientists have realized. On the other hand, if we go back in time to the Big Bang, we will realize that everything is, in fact, viable. If in our universe the Big Bang happened from a singularity (an infinitesimal point where all the energy in the universe was contained) and the particles behaved in such a way that the cosmos was formed the way we see today, it may be that the Big Bang happened following other possibilities, generating other universes.
What is multiverse
With so many possibilities, scientists speculate that the infinite set of parallel universes or child worlds results in something called a "multiverse". Infinite universes are a consequence of the scientific theories reported above, so it may be that more than one proposal for these universes is correct. So, the set of these universes, whatever they may be, can be called multiverse. While some universes in the multiverse may be like ours, others may have totally different laws of physics, as we have seen before.
Other lesser-known proposals can be just as interesting as those described above. Perhaps the multiverse is a summation of all these bold suggestions.
Infinite universes
Scientists cannot say for sure what the shape of space-time is, the "mesh" that makes up our cosmos, but most likely it is flat. This does not mean that it is flat, but implies that there are no curves (besides the small curves imposed by the gravity of massive objects, such as black holes, but in general the universe is flat). Furthermore, it may be that the cosmos extends infinitely. But if it is infinite, it may begin to repeat itself at some point, because there are a finite number of ways in which particles can be organized in space and time.
So, if you look far away, you'll find another version of our galaxy, another version of the Earth, another version of you - or infinite ones of you. Some of these "clones" will do exactly what you are doing now, while others will be enjoying a vacation in Dubai. Are you jealous? Well, think that another version of you is in a much worse situation than yours, maybe it's already dead. If space-time is infinite, so are the possibilities (unlike quantum possibilities, which tend to be limited, just like the "many worlds" described above).
Since the observable universe extends only as far as light has had a chance to reach in the 13.7 billion years since the Big Bang (the same as 13.7 billion light years), we cannot see beyond that, and space - time beyond that distance can be considered a separate universe. In this way, an infinity of universes exists, side by side, like a giant patchwork of universes.
Counterpoints
Although some proven scientific theories may somehow lead us to the possibility of parallel and multiverse universes, scientists need a good deal of skepticism. After all, it is very tempting to use these hypotheses to get rid of troublesome problems such as the wave function of quantum mechanics, or even the incompatibility between quantum mechanics and general relativity. To validate the multiverse, it takes an absurdly ridiculous number of things to happen, and perhaps reality does not allow for something like this.
Consider the Big Bang and cosmic inflation, which is a well-accepted hypothesis by scientists that the universe has expanded exponentially, incredibly fast. This inflation, which occurred before the Big Bang itself, was a kind of configuration of the cosmos before the emergence of matter - when everything was just an energy inherent in space itself. It was an expansion at an extraordinary rate, without decreasing at all, in any corner of the space, and increasing as time passed - getting twice as big and farther, and then four times, eight, sixteen, thirty-two, etc. The number resulting from the expansion rate before the Big Bang would be the largest number you could ever imagine. This is probably the fastest speed you can get at the birth of any universe. It looks great, right?
Now, let's go back to one of the most popular hypotheses in other universes, that of the many worlds (that of quantum mechanics). The number of possible results for any interaction between particles anywhere in the universe tends to infinity. Not only that, but this infinity of possibilities appears instantly, as soon as the interaction takes place. This means that (if these possibilities, in fact, generate new universes, that is, new singularities, new Big Bangs, new cosmic inflations) this happens faster than the growth of that expansion rate of cosmic inflation.
In other words, the number of possible outcomes increases so quickly - much faster than an exponential increase, which is that of cosmic inflation - that, unless the inflation of the universe before the Big Bang occurred for an infinite period of time, there are no chances of parallel universes identical to ours. Well, this is just one of the possible counterpoints to consider. There are many others, just as there will be many other proposals to argue for the multiverse.
Only one cat matters
We may never know if there are other universes, or how many of them are possible. In fact, quantum mechanics itself, the basis for justifying the existence of many worlds, prevents us from knowing whether there are other versions of ourselves. Think again about Schrödinger's cat - when we open the box, all but one possibility falls apart for us. The cat will be alive or dead by the time we make the observation, just as the photon will be configured in an orientation as soon as we observe it.
So, for us, who are in this universe, it doesn't matter the other possibilities. They may have become real in other realities, but that does not affect us at all. The many other worlds may be parallel to ours, but they are as hidden as the results of the choices we have not made in our lives. So always choose the best options, be the best version of yourself, even though there may be others out there and, if possible, keep trying to pick the bunch of grapes on the vine.
Source: Astronomy, Nature, Space.com, Starts With a Bang
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