# Term Paper: String Theory the Fundamental Forces of Nature

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String Theory

The fundamental forces of nature include the electromagnetic force, the strong nuclear force, the weak nuclear force, and gravity. One or some combination of these forces, applied to matter, is responsible for everything we can observe in the physical world. The story of strings as a plausible theory within the realm of physics stems from attempts to unify these forces under specific circumstances. The concept of unification is implied by the theory of the Big Bang. Essentially, if we are to accept that the entire universe -- all matter, space, and time -- originated from a single point, then we are forced to also conclude that major patters we observe today were also singular. In other words, the separate branches of physics must, at some point immediately following the Big Bang, have been indistinguishable from one another. It is a logical assessment of the situation, and has set physicists on the path to identifying circumstances under which the four forces behave identically.

Generally speaking, considering the enormous amount of matter that was concentrated in a tiny space after the Big Bang, the setting for unification is contained by almost unbelievable energy levels. Efforts to achieve this -- mathematically -- have been reasonably successful and are termed Grand Unified Theories, or GUT. So far, physicists have managed to unify the electromagnetic, weak, and strong interactions into one formalism.

These theories, of which there are several versions, have not yet passed major experimental tests. However, most physicists agree that the general ideas and techniques common to all these theories are probably correct and that only the implementation of the symmetries to achieve unification is still subject to debate." (Calle 585).

So, branches of physics that were once considered irreconcilable are now regarded as different aspects of a single all-encompassing rule.

Nevertheless, the fourth force -- gravity -- continues to frustrate physicists' labors to generate a Theory of Everything. The problem originates from the fact that gravity, as treated by Einstein, is a gauge theory, while the other three forces have been accurately described by quantum field theory. The task, clearly, is to develop a theory of gravity that can be incorporated into the quantum understanding of nature.

Additionally, quantum theories have been produced to explain observations made by physicists concerning the properties of particles. Essentially, mathematical models and formulas have been constructed that could never have been generated through pure theory alone. The mathematics of quantum mechanics is not by any means "traditional," and can be more accurately thought of as a set of rules or standards that best describe the causes of what can be observed. There is no intuitive aspect of these rules. Out of these observations have come a complex assortment of entities making-up particles like protons and neutrons; these entities are called quarks. Manipulation of quarks helps to explain the links between the three forces within GUT, but application of these particles introduces additional difficulties.

Basically, the most glaring shortcoming of GUT involves the messenger particles of the prevailing forces. If the particles are to be treated as points, with no value in any dimension, infinities arise. These infinities are introduced,

From the fact that messenger particles with ever-higher energy cluster in the regions closer and closer to particles of matter. Infinite quantities occur because there is no limit to how close the messenger particles can get to the particle of matter that is their source; because the source particles are, in standard theory, mathematical points of zero size, this means there is no limit to the energy of the closest messenger particles." (Davies 253-254).

Stated differently, the particles that act in favor of the four major forces approach unbounded energies as they get ever-closer to particles of matter. This, obviously, cannot be so, and must be a failing in our understanding of particles. One approach to rectify this problem is to trash the notion that particles are point objects.

It should not be surprising that, historically, physicists have made numerous attempts to describe particles as occupying some volume in space.

Attempts to treat particles such as electrons as little spheres, instead of mathematical points, go back almost a hundred years. These early ideas were not successful because they were inconsistent with the theory of relativity. The novelty of the more recent suggestion is that particles are extended in space in only one dimension. They are not point particles, nor blobs of matter, but infinitely thin strings." (Davies 254).

This is the fundamental notion of string theory: classical point particles are actually infinitesimally small lines, or perhaps, loops. The lines have a finite length -- thus, eliminating the problems associated with infinities -- and these "bigger particles, which are made up of quarks, [are] a bit like little pieces of string, with a quark at each end of the string." (Filkin 257). By contrast to the early attempts to describe particles apart from mathematical points, string theory only extends particles in one dimension, rather than three.

Furthermore, this approach possesses the added bonus of uniting quarks and leptons with messenger particles. "This supersymmetry brings together all quantum particles, including the messenger particle of gravity or graviton, as components of a single master superfield. The gauge particle responsible for supersymmetry is called the gravitino, with spin 3/2." (Calle 586). Accordingly, Einstein's theory of gravity is not abandoned by string theory, but merely extended by what has come to be called supergravity. This overall unification of particles under string theory has caused it to be labeled superstring theory -- thus, making the supersymmetry it implies explicit.

Superstring theory is most attractive from the mathematical consequences it brings about. To begin with, "At low energies the strings move about as if they were particles, and so mimic all the qualities that have been so successfully by standard theories for decades." (Davies 255). Logically, this is essential because the unification of the fundamental forces only takes place under extremely high energies; the flexible nature of strings allows for this merger. Another consequence is that at these high energies the strings begin to "wiggle, and thus drastically modify the high energy behavior in such a way that the infinities are quenched." (Davies 255).

The prevalence of superstring theory has, in fact, been gradually conquering physicists' search for the Theory of Everything since the mid-1980's. Its appeal is not difficult to grasp: it eliminates the nonsensical conclusions reached when high energies are applied to point particles. Yet, the theory's gaining popularity is impeded by implications that bring into question the fundamental ways physicists have come to view the universe. Namely, "Superstring theories may have these much sought after properties only if they live in universes that have many more dimensions of space than the three we are familiar with." (Barrow 130).

Expanding the generally accepted number of physical dimensions may seem counter-intuitive to most people, but the idea's roots go back much farther than the origins of string theory. "In 1919, the German mathematician Theodore Kaluza generalized Einstein's gravitational field equations to a five dimensional space-time. In this representation, the extra fifth dimension produced a set of equations that turned out to be Maxwell's equations for the electromagnetic field." (Calle 586). So, by playing around with the number of dimensions, Kaluza found electromagnetism to be nothing more than a consequence of gravity exhibited in a dimension so small that it is unobservable. This theory was added to a few years later, "In 1926, the Swedish physicist Oscar Klein extended and cleaned-up Kaluza's theory and calculated the radius of the extra fifth dimension to be about 10^-30 cm." (Calle 586). The resulting "Kaluza-Klein" theory attracted a number of followers, but proved to be ultimately too restrictive and inapplicable to any of the other forces.

Superstring theory, however, not only suggests the existence of…
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