Then There Were Five
It seems strings can be either open—with two end points like a line—or closed in a loop. When two open strings touch, they join to become one longer string. If the two end points touch each other, they join to form a closed string. So we have a theory of strings that include both open and closed strings.
However, if we consider only closed strings, we can never have open strings. If two closed strings contact, they form a single closed string.
The open string theory fits beautifully with quantum subatomic theory—at first glance. At low energies, there is the graviton, and exactly the proper gauge fields to form the photon, and all the proper bosons. Fermions are present as well and are of the proper ‘handedness.’
Ah, but all is not perfection with this so called Type I string theory. Besides requiring 10 dimensions, the symmetry group associated with the gauge particles is much larger than that found in the quantum world. And the particles are all massless, contrary to what we know of particles such as the electron and quarks.
In attempts to solve these problems, physicists worked to develop other string theories based on closed strings. Two came to the fore, called IIA and IIB. These had problems, however. They don’t have gauge particles and as a result fermions have no charge. Each had other problems as well, and they fell into disrepute until recently.
In their efforts to solve the problems with the Type II theories, physicists developed a hybrid “herterotic" theory. The excitation of closed strings appears like waves moving along the loop. Some move left and some move right, but they do not interfere with each other. The right moving waves are supersymmetric.
But the left moving waves have an exotic origin. It appears that these left moving waves are made up of tiny circles that are the missing 16 dimensions of the 26 dimension theory all curled up! This gives the left moving waves an additional 16 degrees of freedom in which to vibrate. And these extra degrees of freedom are manifested as gauge fields!
One question remained regarding the 16 dimensional circles. They created a torus as they moved around the string. But what size was the torus? It turned out there were two possible answers. One answer gives a gauge symmetry group similar to the Type I theory. The other gives a completely new fifth theory with a gauge symmetry group one-fourth the size of the other. This theory is called the E(8) x E(8) theory.
This theory became the toast of the string theory world for a time, because it was the one that seemed most prone to a process called compactification--that is compressing the sting’s 10 dimensions into the four we observe.
But further work developed new concepts such as membranes, manifolds…and an 11th dimension.
We talk about these in Part 2.