String Theory

Tying Things Together with String Theory

The following is a research paper done for English. I could have done a lot better on it…

Point particles can only come so far in trying to connect all the different theories together into one, theory-of-everything. “…The universe is not a universe of points…” -Lillian Lieber (Lieber 58). By changing the point particles into strings, it will give them properties that are more flexible. String theory has come closer to that theory-of-everything in attempting to relate strong force, quantum field, gravitation, weak force, and the electromagnetic force theories (“String”). Even though string theory is hard to understand, it currently is the best theory explaining particle physics.

All matter is made up of atoms, and those atoms are made up of electrons, neutrons, and protons. Those are further divided into quarks and leptons. According to string theory, all units—quarks and leptons—can be broken down into smaller, one-dimensional loops, which vibrate at different frequencies to give the particle different properties (Green 14). These one-dimensional loops have no width—because they are one-dimensional—but have a length of 10^-33 cm (Pierre). This better explains the physics of the particles, because with strings, all matter is constant—they are all strings with different frequencies—instead of point particles just being classified “different” from each other.

Each string can be described as “open” or “closed,” depending on whether or not it is fully wrapped around a two-dimensional tube. This two-dimensional tube is called a worldsheet, and is an elapsed picture of the string as it travels through spacetime. There are five different sub-theories to string theory, and string dualities, which connect two or more of the sub-theories together. The five are IIA, IIB, I SO(32), SO(32) Heterotic, and E8 x E8 Heterotic (Pierre).

A D-brane is the end(s) on which the string is attached. Dp-brane stands for the D-brane in the number of dimensions—where p is that number—that the string exists. D-branes can exist from the -1^st dimension, to the number of dimensions that are spatial. A spatial dimension refers to the dimension through which one can move physically. There are ten total dimensions, one being time, and the other nine being spatial. D-1-brane is also called instation, and is where all space and time are fixed. D0-brane, or D-particle, is simply one point, with no dimension (height, width, radius), in space. D1-brane, or D-string, is where the strings exist, and all its points are on a one-dimensional line (Pierre).

With understanding the D-branes, the theories will make more sense. Type IIA contains D-branes 0, 2, 4, 6, and 8, and has closed strings, with a non-mirrored, N=2 supersymmetry. Type IIB contains just the opposite strings of IIA: -1, 1, 3, 5, 7. It still has closed strings, and has a mirrored, N=2 supersymmetry. Type I SO(32) contains D-branes 1, 5, and 9, with open and closed strings. It can be related to the 10-dimensional gauge groups with SO(32), and has N=1 supersymmetry. SO(32) Heterotic is similar, but does not have any D-branes attached to it, and has only closed strings. The E8 x E8 (E8²) Heterotic is similar to SO(32), but is related by the E8 x E8 10-dimensional gauge group. Type IIA, and IIB are related to each other by T-duality, which means that one has the radius R, as the other has its radius R^-1 or 1/R. S-duality relates the strong coupling limit with a weak coupling limit, such as in SO(32) Heterotic and Type I SO(32) (Pierre).

M-theory proposes even more promises than the theories previously mentioned. Edward Witten first proposed it in 1995 (“String”). It tries to connect each theory together, by taking a small portion of each; the Type IIA and E8 x E8 Heterotic theories are combined. M-theory tries to compact eleven dimensions onto a circle of ten dimensions. With each dimension comes a D-brane to match it. The D0-brane in M-theory would be the membrane alone, but shrunk down to a size of zero. The D2-brane would be an unwrapped membrane, and then the D4-brane would be a wrapped fivebrane on a circle. Last, the NS fivebrane would be an unwrapped fivebrane. The two excluded branes—D6-brane and D8-brane—are not yet defined, however the D6-brane could be described as the Kaluza-Klein Monopole, which is eleven dimensional supergravity on a circle. In a whole, the M-theory wraps up supersymmetry, eleven dimensional supergravity, and explaining quantum physics, while combining them with everything else (Pierre).

In the ten dimensional world, there are four defined dimensions—length, width, height, duration (time)—and six left for explaining. “How can the fourth dimension possibly be measured in the third dimension? … [By using] the true measuring devices, which are man’s mind…” (Norman 205). Within each four-dimensional point, there are six other dimensions, packed up in a Calabi-Yau space—a three-dimensional representation of six planes or dimensions. The Calabi-Yau space was named after Eugenio Calabi and Shing-Tung Yau (Green 207). “What’s pretty sure is that today we have to have six of the dimensions disappear. How things got to be this way is a little less clear.” –John Schwarz (Angier).

The only real problem with this theory is that it cannot be tested. All of it may seem logical, but science has proven that it is never consistent. As Peter Woit—a Columbia University mathematician—said, “It’s fine to propose speculative ideas, but if they can’t be tested, they’re not science” (Lemonick 2). Perhaps one day, scientists will be able to test and prove these theories, but as for now, they will hang on to string theory, which is the most logical.