What are Superstrings?

Superstrings, or superstring theory, is an exciting field of physics sometimes called The Theory of Everything. It is thought by many to be the elusive unifying explanation Einstein sought that could account for all known forces in the universe.

Until superstrings came along, scientists had two opposing theories for how the laws of nature behaved: Einstein's General Theory of Relativity and Quantum Mechanics.

General Relativity explains the world as we know it on a rather massive scale. It describes spacetime as a fabric warped by mass accounting for orbital systems, galaxies and the force of gravity. But these laws break down at the quantum level where a subatomic particle cannot be measured in terms of its exact position in space at a given time. It is also as likely to move backward in time as it is to move forward, and can even appear to be in two places simultaneously. The world of the infinitesimally small is so bizarre, scientists coined the term "quantum weirdness" to describe it.

The problem for physicists was to come up with a theory that would unite the world that we know with the quantum world. One explanation to account for all four known forces: gravity, the strong and weak nuclear forces, and electromagnetism. Superstrings might be that answer.

Through mathematical equations it became obvious that the way we had previously thought of particles as "points" or "little balls" of energy was inaccurate. These tiny bits of matter actually behaved more like wiggling, vibrating strings. Strings are so small that Brian Greene, a physicist and proponent, explains that if a single atom were the size of our solar system, a string would only be the size of a tree. Yet strings make up all matter from the quantum level up.

The way strings vibrate determine the specific properties of each and every particle, likening the universe to a cosmic symphony of superstrings. But to rid the theory of mathematical anomalies, six extra dimensions were necessary. The six extra dimensions form tiny, curled up 6-D shapes at every point within our space. Inside these 6-D shapes are the strings of superstring theory. The six extra dimensions, plus our three, meant there were really 9 dimensions. Add one more for time, and the total was 10 dimensions. As surprising as this was, it wasn't the end.

In 1995 differing theories of superstrings presented a conundrum until M-theory united them. The only catch? M-theory mathematically required an 11th dimension. This presented a new picture of strings whereby, given enough energy, a string could stretch to become an extremely large floating membrane, called a brane for short. Branes can have different dimensional properties and grow as large as a universe. In fact, according to the theory, our entire universe exists on a floating brane -- just one of several floating branes that each support their own parallel universe. Each brane represents one slice of a higher dimensional space or bulk.

Though the Standard Model of the 1970's already united three of the four forces in a unified theory, gravity could not be reconciled with the three quantum forces. But a breakthrough in superstrings encompassed the elusive force of gravity, whispering of the Holy Grail of physics. If a massless hypothetical particle responsible for transmitting gravity -- the graviton -- exists at the quantum level as a closed string, this would present a direct gravitational link to the theory of superstrings.

The theory predicts strings can be open or closed. Open strings, or strings that resemble little wiggling hairs have at least one endpoint "attached" to the membrane like a trolley car is attached by a top cable to an electric line. Strings can move through the brane but cannot leave it, explaining why we can't physically see out of, or reach out of our dimension. The atoms that make up our bodies are composed of open strings that have attached endpoints to our 3-D membrane. Another way to look at it is to consider a movie screen. People on a screen appear to be three-dimensional, but they cannot actually reach off the screen into our 3-D world. They are stuck in their 2-D world, just as we are stuck in our 3-D world and cannot reach into neighboring dimensions. Scientists refer to this as degrees of freedom.

But the graviton is different. As a closed string or loop without attached endpoints it was theorized that it might be able to escape our 3-D brane and seep into other dimensions. This would explain why gravity is many times weaker than the other forces.

However, what if the inverse were true? What if gravity on a parallel brane is as strong as the other forces, but is weaker here because it is only leaking into our dimension? Mathematically, the theory of superstrings again worked beautifully and finally put forth a plausible explanation for the weakness of gravity while uniting it with the other three forces.

There was just one hurdle left: The unifying theory should also be able to explain the Big Bang. Four physicists traveling together on a train casually tackled this subject. One of them put forth the question, What would happen if two branes collided? The plausible mathematical answer turned out to be the Big Bang.

Detractors of the theory of superstrings point to lack of proof and the difficulty in providing it. Is it just a beautiful mathematical construct? A philosophy? Or a true explanation of our world? No other theory has come close to mathematically unifying all four forces, much less additionally providing an explanation for the Big Bang. But proving that other dimensions exist -- floating branes and parallel universes -- has been a major sticking point.

Nevertheless, believers of the elegant theory are eager to see it proven, and scientists have since found that there may be observable proof of astronomically large strings. Thus, the theory of superstrings continues to gain ground. In the end, if successful, from 11 dimensions to parallel universes, from the swirling galaxies to quantum soup, superstrings might just truly be The Theory of Everything.