Our Universe could be a *Multi*verse?: Realistic Considerations

Some assumptions in science seem to integrate so harmoniously with our existence that we rarely question their validity. It is particularly challenging to think of truths, for example, that contradict the presence of three spatial dimensions, the unidirectional flow of time, or the law of conservation of matter. Daily experience underlies these conclusions while scientific experiments faithfully uphold their validity. Another seemingly infallible assumption is that our universe contains all that exists in space and time. Indeed, the term “universe,” derived from the Latin *universum *(“everything”), itself reflects the amount of faith we have in this belief. For most of human existence this confidence has gone unquestioned until the most recent decade of modern physics. Culminating in what is known as the *multiverse hypothesis*, some theoretical work in physics has accepted and even strongly advocated for the possibility that our “universe” is only one in a potentially infinite expanse of physically distinct universes. As outlandish as it may seem, the multiverse hypothesis is supported by a number of respected physicists, including Brian Greene, Stephen Hawking, and Neil deGrasse Tyson. Despite the support of these prominent experts, the idea of a multiverse requires a whimsical imagination which other physicists, including Roger Penrose and Nobel laureate Steven Weinberg, choose not to entertain, namely because of its ramifications on how we understand nature. The field of physics is thus divided when it comes to the possibility of multiple universes. Perhaps a coherent and thorough investigation of multiverse theory can clarify just how seriously we should consider this curiously unfamiliar idea of our “universe” being one in a series of many.

Before addressing the legitimacy of multiverse theory, we should understand the motivation for considering such a radical model in the first place. The origins come from pursuing the “theory of everything.” As the “Holy Grail” of physics, this theory is dreamt to resolve the elementary particles and four fundamental forces into one unified understanding of nature. Such an ambitious objective will, not surprisingly, sponsor arduous challenge. Since the mid-twentieth century, physicists have encountered persistent difficulty in integrating gravity with the three other forces; for some reason, the mathematics of unifying the forces yield tenaciously nonsensical solutions. As a result, physicists needed a theory that could tame the wild equations into a polished model of the universe. What they came up with was eccentric in imagination but suave in mathematics (a tradeoff physicists are often ready to make): string theory, which posits that the universe, at its most fundamental level, is composed of infinitesimally small objects, dubbed “strings.” Moreover, these strings will vibrate in specifically shaped dimensions to give us the elementary particles and forces of nature.

Undoubtedly, this idea is simply “weird” as physicist Brian Greene explained (2015). Not only is the fundamental component of physical existence now a puny string, but string theory even posits the existence of hidden spatial dimensions, a possibility which our daily experience with three spatial dimensions has never suggested.

Nevertheless, by claiming that all matter and energy is composed of one fundamental unit, string theory is remarkably able to explain everything from the subatomic, quantum scale all the way to the astronomical behavior of gravity. No other theory in the history of physics has been able to accomplish this feat. Moreover, string theory corroborates our current models of how the universe works, which strongly bolsters its credibility and candidacy as the theory of everything. The major impediment to officially awarding string theory the title of “theory of everything,” however, is that our experimental capabilities lag far behind the demands of the theoretical advancements. That is, strings and multiple dimensions are far too small to be directly detected by experiment. Fortunately, the theorists have delineated specific ways that particle accelerators, such as the Large Hadron Collider (LHC) in Geneva, may offer collision signals that lend support to string theory. As of yet, the theorists have not received conclusive evidence from the LHC. The hope is that this year’s LHC activity, augmented with two years of upgrades, will enable experimentalists to uncover some evidence of string theory’s foundations. Unless the LHC’s data suggest a blatant incompatibility with string theory, this concept of our universe being reduced to tiny strings vibrating in hidden dimensions will reign as the best “theory of everything” physics can offer.

But how is the multiverse related to string theory? It turns out that the concept of the multiverse emerges from the mathematics of string theory’s extra dimensions. The geometry is rigorous, but thankfully, the concept can be made accessible as follows: strings, the most basic component of nature, have specific vibrational energy patterns that give rise to the particles and forces of nature. By this logic, the properties of physics will depend on the strings’ vibrational patterns. But what then determines the specific vibrations of strings and thereby the properties of physics? As revealed by some esoteric yet illuminating mathematics, it is the *shape *of extra dimensions that control how the strings vibrate, and—by extension—how the particles and forces of nature behave. The simplicity of this conceptual model is what makes string theory so seductive, and why the last forty years of theoretical physics have been dominated by strings. The task at hand for string theorists even seems straightforward: find out the shape of the extra dimensions that gives rise to the physics we observe. Nature, however, does not let her secrets be so simply attained: string theorists have calculated that the set of possible shapes for the extra dimensions is so vast that we do not even have numbers to describe those magnitudes (Greene 2015). Too be more concrete, Bousso and Polchinski calculated that the number of possible shapes is on the order of 10500 (Wolchover 2013). Importantly, even a slight change in the shape of the extra dimensions would give rise to a physics that would not only make our universe drastically unfamiliar but even inhospitable for life. We have to ask the question then: why does our universe have extra dimensions that are shaped so improbably perfect that they sponsor the stability of our existence?

Well, there is a way to appease this question, but it tests the limits of imagination beyond comprehension. What if there isn’t one *single *right shape for the extra dimensions, as suggested by Stanford theoretical physicist Leonard Susskind? To put it another way, what if each possible shape of the extra dimensions gives rise to its *own *universe with its own idiosyncratic laws of physics? All the possible extra dimensional shapes would be then realized in a unique universe that collectively becomes a multiverse. The reason that we are in *this *particular universe is because the shape of its extra dimensions corresponds to a set of physical features that allow us to exist (Vilenkin 2011). Another universe, for example, with differently shaped extra dimensional might vibrate strings to make the electron slightly more massive or weaken the nuclear force; these changes would prevent star formation and therefore undermine the stability required to support life (Greene 2015). There is thus a delicate balance that our universe seems to demand when it comes to selecting the shape of the extra dimensions.

So with the multiverse theory, physicists can make sense of the statistically incredulous perfection of our universe. Of course, this tradeoff of appeasing a mathematical concern for a belief in infinite universes does not go without intense ramifications that must be resolved. From a theoretical standpoint, the existence of multiple universes is actually a mathematically sound consequence of string theory physics. In fact, mathematical simplicity is the greatest theoretical power of the multiverse hypothesis. If one element of physics has remained constant throughout the history of the field, it is the complete trust in mathematics, and therefore, it does not make sense to arbitrarily divorce physical theory from mathematics for the case of the multiverse, especially when this hypothesis complements current theoretical models in physics. Moreover, some critics hold that the multiverse dishonors Occam’s razor by introducing a whole set of parallel worlds just to solve a statistical problem. While true on a philosophical level, these claims fail to acknowledge that the mathematically simplest theories of the universe tend to advocate for the multiverse (Vilenkin and Tegmark 2011). Because physics is entirely founded in mathematics, Occam’s razor should be applied to the mathematical, not philosophical, discourse of the universe. Furthermore, MIT physicist Max Tegmark argues that the mathematical testability of theories is one method of scientific verification, and the most important one for physics (Vilenkin and Tegmark 2011). As an example, Einstein’s theory of general relativity itself gives rise to unfamiliar concepts that also are seemingly impossible to test, such as the existence of black holes. During the early twentieth century, general relativity critics even maintained that the theory was too abstrusely mathematical to be representative of natural truth. Today, however, general relativity and the existence of black holes have been corroborated with years of undeniable astronomical observations. Tegmark suggests that what the multiverse will be to string theory is akin to what black holes are to general relativity; soon we may accept the multiverse in a similarly well-established of domain of physics as black holes. How soon this endorsement can occur though will ultimately come down to the results of string theory experiments.

The most essential issue for integrating the multiverse into mainstream physics is its lack of experimental validation. While its theoretical insights are perhaps among the richest that physics has offered, the multiverse has been less exciting for the experimentalists. It should be clarified that this does not mean the multiverse cannot be experimentally proven. As one option, physicists could monitor the cosmic background radiation (the measurable thermal energy leftover from the big bang); any intense change would signify the collision of our universe with another one and directly support the multiverse hypothesis (Vilenkin and Tegmark 2011). The issue here is lack of guarantee that such an inter-universal collision would occur within the human lifetime. A more insistent method of verification relies on using the theoretical model of the multiverse to predict our observed constants in nature. In fact, this strategy has been used to statistically calculate the cosmological constant of our universe, which is essentially the energy density of empty space. Excitingly, astronomical observations in the 1990s have revealed our universe’s cosmological constant to actually be in tune with the value predicted by multiverse models (Vilenkin and Tegmark 2011). While not a direct confirmation of multiverse hypothesis, this finding could be the initial source of evidence for the multiverse. With more data garnered from the LHC’s particle collisions in the coming years, we may be able to validate more of the multiverse’s predictions of the properties of nature. Whatever the results may show us, the fact that we actually *can *test for the existence of parallel universes is in itself a remarkable feat of physics and human initiative.

As of this writing, in 2015, the multiverse hypothesis is still in a cautious infancy. While its supreme strength lies in its simple resolution of complex mathematics, the multiverse is conceptually hard to swallow. Whether or not we are ready to accept that our homely universe may actually have infinite neighbors, we should appreciate that the multiverse is a serious component of modern theoretical physics, especially in string theory. So far, no other theory can resolve the most vexing issue in physics—the unification of gravity with the other forces—and also still explain confirmed features of the universe. Ultimately however, experimental evidence is the authority on what we nourish in well-defined scientific knowledge and what we cast into

the playgrounds of science fiction. For now, the practical requirements to test string theory’s multiverse hypothesis push the frontiers of experimental design. Until we reach the point where we can confirm or dismiss the multiverse by experiment, the best attitude will be to keep an open and imaginative mind. For all we know, future generations may look back and label this time with the genesis of a paradigm shift: that we as humans began to transcend the provincial view that our single universe contains everything to realize that we inhabit a one island in a wide cosmic expanse of multiple universes.

Works Cited

Greene, Brian. “Why String Theory Still Offers Hope We Can Unify Physics.” *Smithsonian Magazine*. Smithsonian, Jan. 2015. Web. 16 Sept. 2015.

Vilenkin, Alexander, and Max Tegmark. “The Case for Parallel Universes.” *Scientific American*.

Scientific American, 19 July 2011. Web. 14 Sept. 2015.

Wolchover, Natalie. “New Physics Complications Lend Support to Multiverse Hypothesis.”

*Quanta Magazine*. Simons Foundation, 24 May 2013. Web. 17 Sept. 2015.

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