|MadSci Network: Physics|
Dear Spencer, Great question! I could write hundreds of pages in response, but I will not. Instead, because others have already written hundreds of pages on this topic, I will give you an extremely condensed answer and point you to further elaborations and background material. When we try to describe the matter and forces of Nature in terms of zero-dimensional point particles, the calculations give nonsensical results. One way around this problem is to describe the fundamental objects of Nature as extremely tiny one-dimensional strings. The physical properties of the object (mass, spin, etc.) are associated with the way that the string vibrates. There are five (and only five) types of superstring theory which are free of mathematical inconsistencies. The adjective "super" means that the string theory entails both fermions (matter, like quarks and electrons) and bosons (force carriers, such as the photon and gluon), and is thus capable of describing everything in the Universe. Type I - the first to be discovered, this is the only superstring theory which contains both open and closed strings (think of broken and unbroken rubber bands) which can link up with one another or break apart. All the other types deal exclusively with closed strings. The open strings can carry charges, labels which identify the particle, on their free ends. Type IIA and Type IIB - the first is "non-chiral" and the second is "chiral". Non-chiral means that a certain particle in the theory can spin along its direction of motion or opposite to its direction of motion. In the chiral theory, this particle can only spin one way. Another word for chirality is "handedness". For example, most screws are right-handed because they advance when rotated clockwise. If all the screws in your theory were right-handed, then your theory would be chiral. If your theory contains both right- and left-handed screws, then your theory is non-chiral. Of the five superstring theories, only Type IIA is non-chiral. Type II theories have two "supersymmetries", while the other three theories have only one supersymmetry. Briefly, a supersymmetry is a relation between fermions and bosons. SO(32) Heterotic and E8 x E8 Heterotic - "heterotic" means that vibrations propagating around the closed string in one direction are distinct from vibrations propagating around in the opposite direction; that is, they correspond to completely different particles. The first is based on a symmetry called SO(32), which is the 32-dimensional analog of rotations. For example, a two-dimensional circular disk can be rotated in the plane and still appears the same -- it has symmetry SO(2), for rotations in two dimensions. A sphere in three dimensions has rotational symmetry SO(3). The second is based on two copies of the E8 symmetry. This theory describes two parallel universes which can only interact with each other through the gravitational force. We inhabit one copy; the other copy is referred to as the "shadow" universe. A shadow planet could in theory exist very close to Earth, but we would not be able to see it because its shadow photons would not interact with our eyes. In fact, shadow matter could pass right through ordinary matter because the shadow quarks and shadow electrons do not "feel" ordinary quarks and electrons. We could, however, feel the shadow planet's gravity. These five superstring theories and eleven dimensional supergravity (which is not a string theory) are related by duality transformations. That is, one theory in a certain limit looks like another theory in a different limit. Because it is possible to transform any of these six theories into any other, they all appear to be limiting cases of one all-encompassing theory dubbed "M-theory". References: The Search for Superstrings, Symmetry, and the Theory of Everything by John Gribbin, ISBN 0-316-32614-3. http://www.superstringtheory.com http://www.physics.ucsb.edu/~jpierre/strings/ Try your own web search for "string theory" and/or "M-theory". --Randall J. Scalise http://www.phys.psu.edu/~scalise/
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