Today, astronomers can see out to a distance of approximately 42 billion light-years, our cosmic visual horizon, and we have no reason to believe that the universe ends there (Physics for Dummies, 2011). There exists the possibility that there are an infinite number of domains beyond our own. The majority of cosmologists today agree with the concept of a multiverse where the same laws of physics apply in all domains, but some dare to take the notion even further. Some physicians believe in the existence of infinite parallel universes with different laws of physics, alternate histories and other dimensions than ours (Science Universe, 2012). With this essay, I aim to pursue the parallel universe theory in order to determine its validity outside the realm of science-fiction. I will explore the justifications for it as well as the criticisms against it by consulting the theories of various cosmologists and quantum physicians. The ultimate reason for this pursuit is to determine whether or not the theory of parallel universes can be considered as an indisputable scientific fact or should rather remain in the world of fiction.
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Before proceeding, I need to state the limitations and scope of this paper and clearly disclose the structure in which the subject matter will be discussed. Firstly, I will define a parallel universe and provide some background information on the subject, as will be applicable to the essay’s content. Secondly, I will identify the theories that support the idea of parallel universes and organise them according to a cosmological or quantum physics perspective. Thirdly, I will identify the theories that criticise the idea of parallel universes and again organise them according to a cosmological or quantum physics perspective. Lastly, after analyzing both schools of thought, I will conclude my findings and determine if parallel universes are indeed fact or fiction.
Part 1: Parallel Universes: Definition and Background
To understand the concept of a parallel universe, one must first define the umbrella term it falls under, namely a multiverse. The multiverse theory claims that our universe is not the only one of its kind and that there exists many universes parallel to one another (The Theory of Parallel Universes, 2011). The universes that exist within this multiverse are called parallel universes. These multiple universes consist of everything and anything that can exist in matter, time, energy and space (The Theory of Parallel Universes, 2011). Other terms that are used to refer to parallel universes include quantum universes, alternative universes, alternative realities or parallel worlds. The following paragraphs will give an overview of some scientific theories on parallel universes in order to give an understanding of the argument and the logic that supports the notion. We will first look at the argument as approached from a cosmological perspective.
According to Ellis (2011), if the universe and the possible histories that take place within it are infinite and the number of types of DNA-based beings are finite, then this infinite universe will contain an infinite amount of copies of these finite DNA-based beings. He goes on to say that, given this argument, some of these living beings will inevitably follow very similar lines of history (Ellis, 2011). In lay terms, when given the prospect of an infinite amount of histories combined with finite types of living beings, these histories will repeat to infinitum. However, Soler Gil and Alfonseca (2013) do not agree that if there are an infinite amount of possible histories, that these histories will necessarily be repeated in parallel universes. They state that given this scenario, the amount of histories will always be greater than the number of living beings, therefore if these beings are infinitely repeated they will still experience different histories (Soler Gil and Alfonseca, 2013).
Given this brief background overview, it is evident that the scientific community is still heavily divided on the notion of the existence of parallel universes. We will now continue by evaluating more detailed descriptions of theories that promote and refute the existence of parallel universes. For the purpose of this paper, we will only draw from the fields of cosmology and quantum physics.
Part 2: Theories that promote the Existence of Parallel Universes
Quantum physics is the science that attempts to explain phenomena which cannot be explained by the regular laws of science and physics. The parallel universe theory was first formulated in 1956 by Hugh Everett. Everett formulated this theory in an attempt to substantiate his belief that every probable outcome of any decision we make, does actually happen. He claimed that even though we may choose option A in this universe, we will choose option B in a parallel universe (Everett, 1956). Everett’s theory is relatively new to the history of physics but it has already become a popular and controversial topic in the scientific community. This proposal was coined the Many-Worlds Interpretation. It essentially states that any object can be in any state at any time in a different parallel universe (Everett, 1956). This implies that the wavefunction of a state of being does not collapse at the moment of observation (like we observe in the Double-Slit Experiment later in the paper), but rather continues to evolve in a deterministic manner while simultaneously embracing all its possibilities (Everett, 1956). Everett (1956) notes that even though every possible outcome does exist simultaneously, they do not interfere with one another due to the fact that we are unable to observe these alternate realities.
The Cosmological Perspective
Max Tegmark, a well known modern cosmologist, states that if space is indeed infinite and inhabited by a finite number of living beings, then there is bound to be some identical occurrences that take place in different universes (Tegmark, 1997). Tegmark goes on to make use of a mathematical equation to calculate the probable distance of our nearest doppelgänger at any given time. His answer being approximately 1010115 meters. This theory coincides with a fundamental conjecture of cosmology, namely that the universe exists beyond the scope of our observation.
Tegmark formulated a categorisation that recognizes four different levels of the multiverse where each level builds on the previous one. Tegmark (1997) refers to the first level as the level “Beyond our Cosmological Horizon.” This refers to an infinite universe that contains Hubble volumes that concern themselves with realising all primary conditions. Tegmark (1997) states that an infinite universe will necessarily contain an infinite amount of Hubble volumes that are subjected to the same physical laws. Given this precedent, there are bound to be Hubble volumes that have identical configurations to ours in some universes. This statement is rooted in the cosmological principle that states that we do not possess a unique Hubble volume.
The second level concerns “Universes with Different Physical Constants” (Tegmark, 1997). He coins these universes as “bubble universes” and claims that our universe is just one of many bubble universes in existence. Tegmark (1997) then builds on the cosmic inflation theory in order to justify his argument that although the multiverse is infinitely stretching, there are some regions that cease to stretch. Once they become stationery, they form bubbles that may possess differing physical constants.
The third level is an expression of the “Many-Worlds Interpretation of Quantum Mechanics” (Tegmark, 1997). One feature of quantum mechanics is that there is no observation that can be predicted with absolute certainty, but there is rather a range of probable observations. By applying this feature to the many-worlds interpretation, one can deduce that each probable observation should be compatible with a different universe. Tegmark (1997) explains that the difference between level one and level three is that in level one our doppelgängers reside in a three-dimensional space whereas in level three they reside in an infinite-dimensional space.
The fourth level is referred to as the “Ultimate Ensemble” or the “Mathematical Universe Hypothesis” (Tegmark, 1997). He claims that due to mathematics’ abstract nature, one can use a mathematical structure to prove just about any “Theory of Everything” (Tegmark, 1997). Given this statement, one can describe any imaginable parallel universe at level four and include all other ensembles by simply using mathematics. By doing so, one will bring an end to the multiverse hierarchy and eliminate any probability of the existence of a fifth level.
Even though Tegmark does utilise some characteristics of quantum mechanics to promote the existence of parallel universes, he is fundamentally a cosmologist. There is, however, another physician that uses the field of quantum physics more extensively in this pursuit. In the next section we will evaluate Brian Greene’s argument for the existence of parallel universes from a quantum physics perspective.
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The Quantum Physics Perspective
Before discussing Greene’s application of quantum physics to promote the existence of parallel universes, we first need to recognize three observations about quantum physics. Firstly, energy has the ability to travel through space without having the need to cover the superseding distance (Physics for Dummies, 2011). This means that energy and quantum particles possess the ability to exist in more than one place at a time. This observation is clearly fundamental for the substantiation of the parallel universe theory from a quantum physics perspective. Secondly, all quantum particles exist in different places at any given time, even though we are unable to see them (Physics for Dummies, 2011). Given this observation, there is a possibility that doppelgängers may exist in different locations. Thirdly, quantum particles are influenced by the practice of observation when we observe them (Physics for Dummies, 2011), which plays an important role later in the paper when we discuss the Double-Slit Experiment.
Greene attempts to comprehend the origin of multiverses by utilising string theory. String theory speculates that the universe can be described in terms of small strings that vibrate in ten or eleven different dimensions that we are unable to see (Greene, 2011). Greene (2011) claims that string theory not only includes strings but also objects that resemble two-dimensional membranes. He goes on to suggest that we may be living on one of these two-dimensional surfaces and that other surfaces may be floating around us in space. This claim is currently without any emprirical evidence, but Green (2011) says that this can be tested in the Large Hadron Collider (LHC) at CERN. If Greene’s hypothesis is correct, then when particles are slammed together in the LHC, debris can be emitted off of our membrane surface and into the greater cosmos where our membrane is floating. This loss of debris will take away some amount of energy in our universe. We can then measure the amount of energy and compare it to the amount that was present before the collision. If there is found to be less energy afterwards, it would indicate that some energy had been launched into the greater cosmos (Greene, 2011). This particular outcome of the experiment will prove that Greene’s hypothesis is correct. However, until this experiment at CERN takes place and is found to be successful, Greene’s hypothesis remains pure speculation.
Part 3: Theories that refute the Existence of Parallel Universes
The Cosmological Perspective
Cosmologist, Paul Davies, is a firm believer that notions of parallel universes belong in the realm of scientific philosophy and not in physics. Even though all cosmologists accept the existence of other regions that we are not able to observe, most of them do not deduce the existence of infinite universes from this precedent. Davies addresses the topic in his New York Times opinion piece by saying that physicists should not believe a theory based on faith, but rather insist on hard scientific facts supported by empirical evidence: “The multiverse theory may be dressed up in scientific language, but in essence it requires a leap of faith” (Davies, 2007).
George Ellis is another cosmologist who shares the same sentiment as Davies towards the parallel universe theory. Ellis (2011) points out that the basic problem with the multiverse proposal is the existence of a cosmic visual horizon, as we will never be able to observe these universes even if they did exist. He also rejects the application of string theory, as attempted by Greene (2011), in order to substantiate the plausibility of the parallel universe theory. Ellis (2011) states that string theory is not yet a complete theory that has been tested or proven. He admits, however, that if string theory is ever proven correct then there will be a legitimate argument for the existence of a multiverse. Ellis (2011) concludes that multiverse proposals are merely scientifically based philosophical speculation and cannot even qualify as a well-defined scientific theory. He states that before the multiverse proposal can be considered to be a scientific theory, it needs to be coherent and not just a patchwork of different ideas (Ellis, 2011).
The Quantum Physics Perspective
The most common criticism of the parallel universe theory is the lack of scientific facts derived from empirical evidence collected by means of observation and experiments. It has proven difficult to acquire scientific proof for the existence of alternate realities for many reasons, one of these reasons being that by observing reality, we are also altering it. This fascinating notion is demonstrated by the famous Double-Slit Experiment (Live Science, 2012). Essentially, this experiment proves that energy and matter display characteristics of particles and waves, indicating the fundamental probabilistic nature of quantum mechanics. In a basic Double-Slit Experiment, we observe the light of a laser beam aimed towards a thin plate pierced with two parallel slits. The wave-like nature of the light results in the light waves interfering with each other while passing through the two slits. When observed from behind the plate, one sees bright and dark bands being created, which is unexpected if one were to assume that light only consisted of particles. Instead of acting like particles, the light becomes absorbed on the screen as if it consisted of photons or discrete particles (Live Science, 2012). The mysterious result of this experiment lead to the detection of the inherent probabilistic nature of individual photons.
When Deutsche (2001) conducted his version of the Double-Slit Experiment, he took the findings even further. He states that when we perceive a photon passing through one slit, a parallel universe might perceive a photon passing through another slit. According to Deutsche (2001), our universe is the reality of the tangible particles that we observe whereas the reality of the shadow particles is actually a parallel universe.
However, there are very strong arguments against Deutsch’s interpretation of the findings of the Double-Slit Experiment. It is impossible to confirm Deutsch’s claim that the photon goes through one of the slits and not both because an interference pattern emerges when we try to measure this (Sturman, 2011). Even though the standard way of approaching quantum mechanics does not provide a satisfactory explanation of the Double-Slit Experiment in terms of a single universe, it does not give precedent to an unverified theory of a multiverse that might explain it better. After observing that when a photon passes through the first slit, it interferes with a photon that might have passed through the second slit, Deutsch deduces that a photon must have passed through the second slit in a parallel universe (Sturman, 2011). This, however, is unsatisfactory due to the fact that Deutsch has not proven that the photon passes through one of the two slits even though it could have passed through the other slit. There is still a distinct possibility that the photon is passing through both slits at the same time in our universe (Sturman, 2011).
What I think Deutsche has forgotten in his pursuit to desperately prove the existence of parallel universes, is that the role of physics is to describe what we experience in our world. When we consider the notion of doppelgängers in different universes which we cannot observe, we are stretching the field of physics to be greater than our experiential reality. In my opinion, this cannot be considered science, but rather speculation.
Conclusion
After analyzing both schools of thought on the existence of parallel universes, I remain firmly unconvinced by this theory. The unverified cosmological arguments made by Tegmark (1997) and the futile attempts of Greene (2011) and Deutsch’s (2001) quantum physics approach did not succeed in convincing me that parallel universes exist. Even though I approached this concept with an open mind, one must still require empirical evidence before confirming any theory to be remotely factual. Given the evidence, I can confidently conclude that the case for the existence of parallel universes remains unproven for now. However, I have found the contemplation of a multiverse extremely enticing as a way to reflect on the nature of our existence. There is no doubt that the concept belongs in the realm of philosophy for now, but the lack of scientific proof does not leave it entirely worthless. We should rather embrace what multiverse proposals truly are – “scientifically based philosophical speculation” (Ellis, 2011). By doing this we can bring newfound legitimacy to the subject. Instead of trying to force it into the field of natural sciences, where it clearly does not belong, we should welcome it in the field of social science. I think that by doing this, we will enable the concept of parallel universes to be explored to its full potential without the boundaries set by the scientific method.
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