The Great Debate: String Theory
String theory has seen better days. Once hailed as the most promising theory with perhaps a Theory of Everything just around the corner, it has recently been publicly criticized by several physicists as a potential dead end. Is string theory really hanging on by a thread? Or is it still tightly woven into mainstream physics?
The theory was first developed in the 1960s and 1970s to explain some behaviors observed in subatomic particles, the building blocks of matter. According to the theory, subatomic particles are made up of tiny strings that vibrate with specific resonant frequencies. Just like a violin string produces different notes based on its vibration, a string can vibrate in different modes that appear to the observer as different particles (such as a photon or an electron). Furthermore, these strings can split and combine amongst themselves, which would appear as the absorption and emission of particles observed in particle interactions.
These strings are appealing to many physicists because strings might explain the fundamentals of gravity. While gravity is the most prominent force we experience on a daily basis, to date it is perplexingly the one least understood by physicists. Many of the mechanisms behind it are still unknown, and to date it is the only one of the four fundamental forces (gravity, electromagnetism, the strong force, and the weak force) which has no theoretical connections to the others. With string theory, however, a particle known as a graviton would be created by its own specific vibrating string. The tiny graviton particle, theorized about in some branches of quantum physics, would produce the force of gravity.
For string theory to work, our universe needs to have many more dimensions than the three we know about: specific theories within string theory can call for 10, 11, or even 26 dimensions. A simplified analogy of these extra "hidden" dimensions goes like this: imagine our universe to be a hose, similar to one found in a garden or the back of a fire truck. When viewed from a great distance, the hose appears to be a one-dimensional line. But as we get closer, we notice that the hose has not one dimension but three: its length, width, and height. Similarly, extra dimensions in our universe would be visible only at extremely small distances to the strings or by watching particles interact at very short wavelengths.
The small scales illustrated by the analogy also illustrate a problem with string theory: the difficulty in finding experimental evidence. Science is an empirical subject, meaning all accepted theories are based on evidence to support them, and to date no version of string theory has ever made an experimentally verifiable prediction that could not be explained with another theory. Calculations with string theory also predict over 10500 different models of the universe, meaning to date there are over five times as many possible models as there are atoms in the universe, so it would be impossible to disprove the individual models within the theory.
Furthermore, string theory deals with very difficult mathematics- quite a statement in theoretical physics! Physicists are not yet at the stage where the theory can be accepted or discarded as a result. "Even the people who are the most expert in it do not have a good idea of the true shape of the theory," explains Clifford Johnson, a string theorist at the University of Southern California. "There is just so much more to do, so it can go either way."
Because of the rate of progress in string theory in recent years and decades, there has been increasing controversy in the physics community regarding how much an emphasis should be placed on the theory. "String theory is the least-successful idea in 20th century physics," says Lawrence Krauss, a physicist at Case Western Reserve University. Last year, Krauss authored a book titled Hiding in the Mirror: The Mysterious Allure of Extra Dimensions From Plato to String Theory and Beyond, where he emphasized his position as a skeptic when it comes to string theory. "It's a fascinating set of ideas but it hasn't really produced results. I have no problem with it being studied, but there's no evidence that it connects to the real world," he explains.
String theorists like Johnson, on the other hand, say it's too early to make a verdict. "Einstein worked for ten years on the theory of general relativity before the theory began to crystallize, which is an excellent example that these things can take a long time," he says. Johnson also cites the fact that string theory helps theorists understand several mathematical tools and techniques that are useful in other branches of physics. An example of one such tool is gauge theory, a class of physical theories that are useful in understanding the fundamental forces other than gravity.
So is there a verdict on string theory? Not yet: the theory still holds promise in the eyes of many, so it will be studied for some time to come. However, it is important for scientists to express their doubts or support for a particular theory, as this allows reflection and differing viewpoints. As a result, it is certain that the great string debate will continue for a long time into the future.
Greene, Brian. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. New York: W.W. Norton and Company, 2000.
Krauss, Lawrence. Hiding in the Mirror: The Mysterious Allure of Extra Dimensions From Plato to String Theory and Beyond. New York: Penguin Books, 2005.
Randall, Lisa. Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions. New York: Echo Press, 2005.
Smolin, Lee. The Trouble With Physics. New York: Houghton Mifflin, 2006.