String theory, sometimes called the Theory of Everything, is thought by some to be the unifying field theory Einstein sought before his death. It is the first mathematically sound theory that reconciles the world of the infinitesimally small, with the world known at large. It unites Einstein’s Theory of Relativity with quantum physics and offers a potential explanation for the Big Bang.
Prior to string theory, subatomic particles were envisioned as tiny balls or points of energy. This theory works on the premise that the tiniest subatomic bits that make up the elements of atoms actually behave like vibrating strings. The strings are so small that physicist Brian Greene has analogized that, if a single atom were enlarged to occupy the footprint of our solar system, a string would still be no larger than a tree.
Because these tiny vibrating strings are responsible for the properties of all matter, the cosmos has been likened to a cosmic symphony of superstrings. While poetically appealing, the strength of string theory is that it accounts for all four known forces in one elegant theory. These fundamental forces are gravity, the strong and weak nuclear forces; and electromagnetism.
One of the surprising elements of this theory is that it requires extra dimensions to be free of mathematical anomalies. Scientists added an extra six dimensions, initially, for a total of ten. The six dimensions were predicted to be contained in tiny curled up formations at every point within our three-dimensional space.
There was a problem, however: string theorists came up with several theories that all seemed to be correct. Ultimately, scientists found that adding an 11th dimension mathematically explained all of the seemingly different theories as different aspects of the same one. The one theory to rule them all is known as M-theory.
The 11th dimension of string theory predicts a new kind of string, stretched infinitely long to create what is termed a floating membrane, or brane. According to this theory, infinite branes exist that each support a separate but parallel universe. In this wildly exotic neighborhood, the “problematic” force of gravity was also explained.
While the Standard Model of physics had already united three of the known forces, gravity remained elusive. Part of the problem was that gravity was such a weak force relative to the others. String theory mathematically predicts that gravity is weak because it is only leaking here from a parallel universe.
This is possible, string theorists explain, because strings can be open or closed. Open-ended strings have one endpoint attached to the brane on which they reside, keeping matter contained within that brane. Human bodies are believed to be made from open-ended strings, which explains why people can’t reach into or interact with other dimensions. Close-ended strings, however, are like tiny rings, unattached to their brane, able to “leak” away from it.
Gravity is thought to be transferred via massless, hypothetical particles called gravitons. If gravitons were made from close-ended strings, scientists theorized, gravity might be leaking off our brane. It sounded good, but it didn’t work mathematically. The opposite hypothetical did work, however: gravity appears to be leaking to our brane from a parallel universe. Fantastically, this notion is mathematically sound.
String theory also offers a possible explanation for the Big Bang. It had long bothered scientists that although they could plot the stages of the Big Bang backward to the singularity, the initial cause for the event was without explanation. Now string theorists believe that two branes colliding could have caused the event.
The biggest challenge to the theory is that much of it is not provable. Scientists can’t test other dimensions, study migrating gravitons, or peek between the curtains of floating branes to witness a Big Bang event. For this reason, string theory has many detractors and critics. Some scientists believe that without the ability to prove the theory, it is not true science at all. Nevertheless, proponents seem confident that proof of various sorts will come with technological progress and time.