Why is the universe the way it is? We have asked this question since ancient times and scientists are now optimistic that string theory can provide some answers. This modern idea describes the entire universe as an intricately woven collection of tiny vibrating filaments, or “strings” of energy.
If a string vibrates a certain way, it could be an electron, and if it vibrates another way, it could be a photon of light. These strings are so tiny that we currently have no way to observe them directly. If we imagined that an atom was the size of the entire solar system, a string would be no larger than a tree.
Physicists have typically thought of the constituents of the universe, such as electrons and quarks, as point-like particles. Even forces like gravity and electromagnetism were treated as particles by physicists. However, this led to strange anomalies when they tried to analyze extreme conditions, like during the Big Bang and inside of black holes. Beginning in the 1960s, physicists began to build theoretical models of the universe based not on particles, but instead on tiny strands or loops of energy.
String theory promises to resolve a dilemma that has plagued physics for the last century. Professor Brian Greene of Columbia University explained the problem to a packed Koffler Centre during his recent visit to U of T: “Quantum theory describes the small fantastically well. General relativity describes the big fantastically well. [But] these two theories conflict. Each says the other is wrong.”
Einstein proposed his general theory of relativity to remove the anomalies found in Newton’s laws of gravity. General relativity combines space and time into a four dimensional entity, called space-time, and treats gravity not as a force, but as an elegant warping of space-time. It describes an orderly, smooth-looking universe, and is very good at explaining the large scale, stars and galaxies. However, physicists found Einstein’s theory useless for explaining atomic structures and other small objects.
Around the same time, a group of physicists led by Niels Bohr founded quantum mechanics to explain the small stuff, atoms and subatomic particles. This theory does not account for gravity, but its predictions agree strongly with experimental results for the small-scale where gravity is weak and mostly unimportant. It envisages chaos in nature, and paints a rather turbulent universe in contrast to Einstein’s notions.
These two separate theories can explain most physical phenomena, but they still leave more work to be done in physics. Greene said, “If you don’t have a single theory that can embrace them both [General Relativity and Quantum Mechanics], that can allow them to operate in concert, then you’ll never be able to understand the realm of black holes, for example.”
The holy grail of modern physics is a unified theory, a theory that can explain both the large and small aspects of the universe. String theory could be this “theory of everything.” Greene asserts, “String theory shows us a way of putting them together, finally, in one cohesive package, one framework.”
The theory does have some strange aspects. Theorists believe that strings vibrate in dimensions beyond our familiar three dimensions of space and one dimension of time. String theory predicts a reality in which the universe exists in 11 dimensions, but we have evolved to naturally see only four of them. The notion of hyper-dimensions departs from classical thinking. Professor Amanda Peet of the department of physics at U of T notes, “People who’ve said there are extra dimensions of space have been labelled crackpots.”
Opponents of string theory argue that the idea is more philosophical than scientific because we currently have no experiments to test it. Greene defends the scientific validity of the theory, saying that just as we should not assess the acoustic quality of an unfinished Stradivarius, we should not dismiss string theory before it has a chance to properly mature.
Greene urges, “It’s a matter of looking at an extraordinarily promising theoretical construct, and knowing that it takes years of development before it can make contact with [experimental] physics. It’s up to us to work hard to try and develop the theory in order that it can do that.”
