How Things Work: String Theory
Atoms and molecules are what we commonly think of as the building blocks of our universe. Below that, most of us know that atoms are built of electrons, protons, and neutrons. Those of us who are more educated might even know that protons and neutrons are built of even smaller components called quarks. We think of these elements as infinitesimal points creating physical lines and borders known as the World Line, resulting in physical shape and appearance. But what about that which is below quarks? What defines the interactions between quarks?
In the past, most scientific circles agreed in a theory called the Quantum Field Theory (QFT), giving modern science the ability to build theories constant not only with quantum theory, the study of matter and radiation at the atomic level, but also with the special theory of relativity, Einstein?s description of the motion of particles near the speed of light. The problem is that although the gravitational force can be described by Einstein?s theory of General Relativity on a larger scale, QFT fails to explain gravity at the subatomic level and, instead, results in an answers on the range of infinity. In this application, there should be no infinites. Therefore, we need a new unifying theory to help explain this.
This is where string theory comes in. In string theory, the particles we usually associate with modern physics are replaced by strings. Strings come in two basic forms: open and closed. Open strings create the image of sheets or planes, while closed strings carve out tubes. Each of these strings freely vibrates, creating what we call its mode, or ?note.? Each note has a different mass, which we refer to as its ?spin.?
Spin is important because it allows us to classify string types. Strings that have an integer spin are known as bosons, while strings of half-integer spin are known as fermions. Bosons make up particles which carry forces, such as protons, gluons, and gravitons carrying the electromagnetic force, weak nuclear force, and gravitational force respectively. Fermions, on the other hand, make up the physical universe, comprising electrons and quarks. Although string theory gave us an extremely precise definition of gravity, getting rid of QFT?s problem of infinity, it only described bosons, leaving out a description of the physical universe.
String theory?s original hope was to be the unifying world theory that theoretical physicists have been searching for. This caused an extremely large problem. Besides causing many to burst into tears, it left much unanswered. Although gravity could finally be described by the interaction of two well-explained gravitons, it was difficult to apply it to the physical world. In order to solve this problem, a new theory was devised known as supersymmetry. When applied to the original string theory, we were able to attain an accurate description of fermions. From this, we derived what we now call superstring theory. This resulted in five new string theories, two involving open strings, one involving closed strings, and two involving a mix of superstring and string theories, known as heterotic string theory. This led to yet another problem: if we are trying to find one singular unifying theory, how can we have five distinct, accurately proved theories?
Before we continue, it may be beneficial to attempt to give a visual description of string theory. String theory predicts that there are up to 10 dimensions. We know from modern physics and the physical world that there are only three physical dimensions and one dimension of time. String theory, though, says that there are six other dimensions which are tightly packed together. Think about this as a simple geometric graph with x?, y?, and z-axes. For simplicity?s sake, let?s add a fourth axis for time. It is imperative to ensure that these four axes are evenly spaced from each other. Then let us just arbitrarily draw six more axes extremely tightly packed together somewhere on the graph. Imagine that we could make a 3D model of this graph. Take a piece of string and simply wrap it around the axis in which that string exists. For example, a string embodying a proton in a human might simply wrap around the first, second, third, and fourth dimensional axis, simply because that is where we exist. Although this is an extreme oversimplification, it is a good way to make a mental image. Now, we trudge forward.
Once again, scientists everywhere fell to their knees and cried. ?Why?!? was heard all around the world. Many then realized how little they knew about each of these string theories. Recently, though, new techniques have been devised allowing us to further explore these ideas. Through this research, it has been discovered that these five theories are actually extremely closely related. A new theory was devised in order to describe the relationship of the five original theories, called M-theory. Simply put, string theory has moved the boundaries of science forward in ways which no one had thought possible. From Newtonian to Einsteinian physics, we have made strides in theoretical physics which we never expected.