The notion of String Theory suggests that our Universe originated from a higher-dimensional and more symmetrical state with an overwhelming number of degrees of freedom. To resolve String Theory, we must eliminate its excess predictions, leaving only the Universe we can observe. However, the transition from that initial state to our current reality remains unresolved.

# The Elusive Quest for a Singular Universe

Many are struck by the elegance and power of String Theory upon first encountering it. The structure of our Universe reveals intricate patterns that adhere to distinct principles, particularly evident in the disparities between various theoretical components. For instance, we observe:

• A mismatch in the number of fermions compared to bosons.
• A predominance of matter over antimatter.
• A Universe filled with electric charges but devoid of magnetic charges.
• An abundance of left-handed neutrinos while right-handed antineutrinos are absent.

These discrepancies indicate numerous symmetries that one might expect to hold true, yet they do not. One could hypothesize that the three forces described by the Standard Model might unify under extreme energy conditions. Similarly, one might envision a corresponding boson for each fermion as proposed by supersymmetry, or even a "theory of everything" where gravity merges seamlessly with other forces at the highest energy levels.

At the heart of String Theory lies this compelling vision, yet it faces a significant challenge: there is no experimental or observational evidence to support it. The hope surrounding String Theory, upon close examination, can be likened to a shattered dream.

Theoretically, our Universe might possess more than three spatial dimensions, provided these "extra" dimensions remain below a certain critical size that has already been examined through experiments. Dimensions within the range of approximately 10^-19 to 10^-35 meters may still allow for additional spatial dimensions.

In theoretical physics, introducing a new element—be it a force, interaction, dimension, or coupling—requires two essential considerations. Firstly, it must align with existing theories and observations; introducing something previously ruled out is unacceptable in the field. Secondly, if the new component exists only at energy scales beyond our current capabilities, it must be eliminated before arriving at our current low-energy Universe. This presents a daunting challenge for String Theory, as the Universe we observe today is considerably less symmetric than its predictions.

Considering our Universe holistically reveals its complexity. It encompasses:

• Four fundamental forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
• The Standard Model particles, including quarks, leptons, gauge bosons, and the Higgs boson.
• Coupling constants that determine interaction strengths, which vary with energy.
• Four dimensions: three spatial and one temporal.
• The established laws of physics, notably General Relativity for gravity and Quantum Field Theories for the other three forces.

The unification of the weak nuclear force and electromagnetism into the electroweak force at high energies, achievable at certain particle colliders, is well-documented. Concepts like grand unification and supersymmetry propose the introduction of new particles and interactions, which could result in observable phenomena such as proton decay. The absence of such predictions in experimental outcomes imposes constraints on these theories.

String Theory extends beyond the realms of grand unification and supersymmetry. The goal of grand unification is to integrate the three forces of the Standard Model into a more extensive, symmetric framework. Rather than adhering to the known particles and interactions, grand unification seeks to nest the Standard Model within a broader structure.

While this concept may seem abstract, the group theory that represents the Standard Model is expressed as SU(3) × SU(2) × U(1). To unify these forces, one must utilize a larger group. Various approaches exist, such as Georgi-Glashow's SU(5) unification, which anticipates the existence of new super-heavy bosons that interact with both quarks and leptons. Alternatively, the Pati-Salam model incorporates right-handed particles, making the Universe symmetric regarding left and right.

However, the larger the framework, the more elements must be discarded, and the more intricate the explanations needed to clarify why these additional components do not manifest in our experiments and observations. The stability of the proton suggests that either the simplest grand unification model is flawed, or a more complex model must be adopted while evading constraints that dismiss simpler models.

When discussing unification and group theory within String Theory, the necessary group becomes vast. It may fit within an SO group but requires escalation to SO(32) or the exceptional groups E(8) × E(8), both of which are significantly larger. While this does not rule out the correctness of String Theory, it emphasizes the challenge of distilling a minimal Standard Model from such extensive groups.

The issue extends similarly to supersymmetry, which typically proposes superpartner particles corresponding to each Standard Model particle. The primary challenge lies in the expectation of additional particles appearing at energy levels that unveil the heaviest Standard Model particles. For instance, a second Higgs particle should exist below 1,000 GeV, along with a light, stable particle that has yet to be detected. Even without String Theory, N=1 supersymmetry faces substantial challenges.

In the absence of supersymmetry, the Standard Model equates to N=0. However, for String Theory to hold, nature must exhibit greater symmetry than that predicted by standard supersymmetry. String Theory encompasses a gauge theory known as N=4 supersymmetric Yang-Mills theory, necessitating the dismissal of further components to avoid contradiction with existing observations.

A significant hurdle for String Theory is its touted success: the integration of gravity. While String Theory seemingly allows for gravity to merge with the other three forces, it doesn't yield Einstein's four-dimensional tensor theory of gravity.

Einstein posited that gravity is determined solely by the presence of matter and energy. By incorporating all forms of matter and energy into General Relativity, the Universe's evolution—expansion, contraction, gravitation, and so forth—follows the stresses created by these entities. Gravity manifests in a four-dimensional tensor form, and while additional ingredients can be introduced, they cannot contradict existing observations.

What does String Theory provide instead? Unfortunately, it offers a 10-dimensional scalar-tensor theory of gravity rather than the required four-dimensional tensor theory. Consequently, the scalar aspect and six additional spatial dimensions must be eliminated.

Historically, alternatives to Einstein's General Relativity included Brans-Dicke gravity, which also incorporated a scalar component. Einstein's theory was pivotal in explaining the orbit of Mercury, particularly its perihelion precession of approximately 5600 arc-seconds per century. This precession was attributed to various factors, with Einstein's predictions accounting for a mere 43 arc-seconds.

By the late 1950s, some solar observations indicated that the Sun was not a perfect sphere but rather an oblate spheroid. Brans and Dicke argued that this deviation would generate an additional 5 arc-seconds of precession per century, differing from Einstein's predictions. Their solution was to incorporate a scalar component and a new parameter, the Brans-Dicke coupling constant. If this constant were about 5, the results would align correctly.

However, modern understanding confirms that the Sun is nearly spherical, meaning the previous observations were inaccurate. Current constraints dictate that this coupling constant must exceed approximately 1000, where the limit approaches standard General Relativity. For String Theory to be valid, this 10-dimensional Brans-Dicke theory must be reduced to Einstein's four-dimensional framework, necessitating the removal of six dimensions and the scalar term along with its coupling constant.

In summary, if String Theory holds true, it posits that our Universe began as a highly symmetric entity, markedly different from what we perceive today. At some early juncture of extreme energy, this Universe possessed 10 dimensions and a scalar component alongside the tensor aspect. It was unified within an expansive group, such as SO(32) or E(8) × E(8), and described by a maximally supersymmetric Yang-Mills theory.

If String Theory is indeed accurate, an unknown mechanism caused this ultra-symmetric state to break down dramatically, resulting in the disappearance of six dimensions and the scalar gravity component. The substantial unified group diminished significantly, leaving behind our relatively modest Standard Model, SU(3) × SU(2) × U(1). The breakdown of the supersymmetric Yang-Mills theory was so profound that no evidence for supersymmetric particles exists today—only the Standard Model remains.

This encapsulates the essence of String Theory: a grand vision that hopes to unlock the secrets of the Universe from an enormous, unbroken framework, akin to a box that, once correctly accessed, reveals a perfect model of our reality. In the absence of such a breakthrough, we must recognize String Theory for what it truly is—a speculative hypothesis.

While intriguing and promising, until we can derive our observed Universe from String Theory in a coherent manner, we must confront the reality that it remains an elaborate dream awaiting resolution.

Starts With A Bang is authored by Ethan Siegel, Ph.D., who is also the author of Beyond The Galaxy and Treknology: The Science of Star Trek from Tricorders to Warp Drive.