String theory is the umbrella name for a set of several theories now being studied by physicists as candidates for the long-sought "theory of everything" (TOE). To find out more about string theory's history and development, see The History of String Theory. This page and those that follow will describe the various aspects of string theory, from its fundamental qualities to implications just now beginning to be understood.
Within the framework of string theory, the familiar fundamental particles such as electrons and quarks are not really particles at all - they are actually tiny vibrating strings of about the Planck length (about 10-33 cm). As such, they are undetectable to our particle accelerators, which would have to utilize energies about a million billion times greater than those accessible at present in order to determine that what seems to be a point particle is in reality a string.
Strings may represent the "bottom layer" of the universe's fundamental constituents. That is to say, although strings have extent in space, they are not made up of anything else - they are the last level of the sub-structure of the universe. However, there are subtle theoretical suggestions that strings may in fact have a substructure in their own right, thereby simultaneously destroying their chance at being the "fundamental" components of the universe and opening fascinating new doors of intricate theoretical physics. Either way, string theory seems capable of either providing itself as a final theory or leading us to a more complete theory that is a TOE.
It should be noted here that string theory provides for many different types of strings. There are both open and closed strings, with closed strings forming a loop and open strings, logically, having both ends free - a cut loop. For reasons that will later become apparent, the universe seems to incorporate closed strings, although open strings may also be present. Recent research has also revealed that "strings" may actually have many different dimensions, from the one-dimensional strings originally postulated to a two-dimensional membrane to many analogous structures in higher dimensions. Physicists have taken to calling strings "branes," and defining each's dimensional extent with a number, i.e., a one-brane is a one-dimensional string. These will also be explored later in the text.
String theory, as stated above, postulates the existence of tiny vibrating strings that correspond to the observed elementary particles. Strings can undergo an infinite number of different vibrational patterns, called resonances, whose evenly-spaced peaks and troughs fit exactly along its spatial extent. By analogy, the strings of a guitar can similarly undergo an infinite number of vibrational patterns that meet the same requirement, though we only come in contact with a few of them. These recognizable vibrations are perceived by human ears as different musical notes. Similarly, the vibrations which strings undergo not only correspond to, but actually create, the different masses and charges observed in the various elementary particles. In other words, an elementary particle's precise properties are caused by the vibrations of its string.
This connection is best illustrated for the mass of a particle. A vibrational pattern's energy is related to its amplitude, or the maximum height of a wave peak (or depth of a trough) and the wavelength, or the distance between one peak and the next. Greater amplitude and greater wavelength correlate with greater energy - that is, the more frenetic the vibrations of the string, the greater energy it has. Since energy is related to mass by Einstein's famous equation E=mc2, high vibrational energies correspond to high-mass particles.
|These vibrations are extremely low in energy and would therefore correspond to low mass.||These vibrations are moderate in energy and correspond to medium mass.||These vibrations are very high in energy and correspond to very large mass.|
All the known fundamental particles - including the messenger particles - have properties resulting from these types of vibrational patterns. This fact is one of the most attractive and unifying aspects of string theory - it postulates that all particles are made of the same "fabric," as opposed to the particle-physics view that each elementary particle is in effect "cut from a different fabric" (Brian Greene, The Elegant Universe).
The tension of ordinary strings - like guitar strings - is determined by plucking them. If we could pluck a superstring, we could determine the tension in the string - the one value string theory requires as "input." However, since strings are so tiny, we cannot pluck them experimentally. Using an indirect approach involving the postulated properties of the graviton, John Schwarz and Joël Scherk calculated that the strength of the force transmitted by a messenger particle is inversely proportional to its string tension. Since the graviton transmits the extremely weak gravitational force, its tension is enormous - about 1039 tons, or the Planck tension. This intrinsic "stiffness" of strings has several significant consequences.
First, it is the huge tension of a string that causes its contraction to an infinitesimal size - the Planck length of 10-33 cm.
Second, the high tension results in the typical energy of a string being extremely high. (Strings, even familiar macroscopic ones, are harder to set in motion when they have high tensions - try plucking a guitar and a piano string. Since higher-tension strings are harder to set in motion, they have correspondingly higher energy. If you took physics, recall inertia.) This tells us that a string's energy is determined by its vibration (more frenetic = higher energy) and its tension. Additionally, like all vibrations, strings' vibrations exist only in distinct units (recall the quanta of light), so that each string has a minimum energy denomination and an actual energy that is a whole-number multiple of this quantity. Actually, the minimum energy is proportional to its tension and its whole-number multiple is proportional to its amplitude. Therefore, since a string's tension is huge and its minimum energy is proportional to that tension, the minimum energy is also enormous. Strings' minimum energies are actually whole-number multiples of the Planck energy (roughly 1000 kilowatt-hours), which, translated into mass, yields the Planck mass (ten billion billion times that of a proton; roughly 1/100 of 1/000 of a gram; about the mass of a grain of sand).
Of course, the Planck mass is enormous by elementary-particle standards - as mentioned above, it is ten billion billion times the size of a proton. How can strings, with average energies corresponding to the Planck mass, create the observed elementary particles with much smaller masses and lower energies? "Quantum jitters," natural consequences of the uncertainty principle (recall quantum foam) can actually have negative energy associated with them, allowing their energy to cancel that created by string tensions, thus giving rise to the smaller, more mundane energies associated with the familiar elementary particles. In fact, some vibrational patterns have quantum jitters that exactly cancel the energy derived from their tension, yielding a massless particle - such as the graviton.
These facts lead to an obvious question, actually the third consequence of string tension: since strings can vibrate in an infinite number of ways, is the number of corresponding "elementary" particles therefore infinite? Each of the infinitely many string vibrations does in fact correspond to an elementary particle. However, all but a few (the currently observed particles) will be extremely heavy - many times heavier than the Planck mass. Our particle accelerators cannot possibly create this much energy; the most powerful creates less than a millionth of a billionth of the Planck energy. Nevertheless, we may be able to verify them experimentally. The early universe had enough energy to produce vast numbers of these particles. Although most would by now have decayed into successions of ever-lighter particles, there is a chance that some have survived to the present day and could be detected by our instruments.