The Standard Model of particle physics has been widely accepted among theoreticians but the problem is that it doesn’t integrate all that well with Einstein’s Theory of Relativity. Gravity, more enigmatic than the other forces in nature, has remained a question mark for quantum mechanics. We seem to understand how gravity works on a large scale, but not in quantum-scale reality.
String Theory emerged in the late 1960’s. It replaces point-like elementary particles with shimmering one-dimensional strands. The bosonic string theory came first. It was limited to bosons and did not relate to fermions. (Fermions are half-integer spin particles that are constrained by the Pauli exclusion principle, meaning that two of them cannot occupy the same position in time and space. Integer-spin bosons do not have to conform to the Pauli exclusion principle.) Later, Superstring Theory posited a parallelism, supersymetry, between the two classes of particles.
String Theory seeks to harmonize quantum field theory and general relativity. Stephen Hawking among other contemporary theoreticians holds it to be a valid description of nature, and advanced mathematical and logical analysts has so far failed to find internal contradictions. Others, however, have criticized it on the ground that it does not provide a means for experimental validation. This may be subject to change, however, as high-energy particle colliders become capable or tracking larger-scale events. (In the strange world of quantum physics, larger particles such as the Higgs boson are more difficult to detect than smaller particles, because it takes more energy to make them trackable.)
String theory does not altogether do away with the idea of elementary particles. What it asserts is that the particles can additionally be conceived of as strings.
Strings can exist in open or closed loops, and they can bifurcate and recombine. These actions make for the emission and absorption of particles. Closed strings give rise to gravitons, while open strings are associated with the existence of photons.
Numerous versions of String Theory have been proposed including five distinct Superstring Theories. Each of the Superstring Theories appeared to be internally consistent, so it remained to discover which of them corresponded to the true situation. In the 1990’s, finally, it became apparent that the five theories were actually different limits to a single 11-dimensional construct, which is now known as M-Theory. This theoretical framework is what is now meant when we speak of String Theory.
Generically, String Theories replace point-like billiard ball particles with much smaller (in most versions) one-dimensional strings. Prominent among the several surmised qualities of these strings is that they vibrate. Each vibration initiates and sustains the existence of a particular particle. Strangely, the particle is not the string, but the vibration of the string. String parameters such as radius and length determine the rate and mode of vibration, which go on to underlie the existence of the variety of elementary particles. The string’s properties determine the particle’s properties, notably mass and charge.
Thus, from our point of view, what is real is not so much the string as the vibration of the string. The precise nature of the wave motion determines the particle’s mass, charge and spin, which is to say the type of particle that is created.
Strings may split and recombine, and these actions correspond to emission and absorption of the particle. One type of string vibration gives rise to a spin-2 particle that has zero mass. This particle, termed the graviton, comprises the force that appears to have all the properties of what we call gravity. (We have witnessed the discovery of the Higgs Boson. What we have to get used to is the fact that particles bring into being other particles, either radiation, bosons, or matter, fermions.)
The first version of String Theory was strictly bosonic. Theorists suggested the idea of supersymetry, which holds that there is a definite relationship between fermions, constrained by Pauli exclusion, and bosons, not so constrained. The assertion is that two or more fermions cannot occupy the same point in time and space.
M-Theory attempts, with great theoretical success, to unite the five versions of Superstring Theory into a single perspective. Experimental confirmation is lacking, but theoretically M-Theory is consistent. The problem is that strings are too small for us to detect using available particle-collider energies. This is rapidly changing and it is possible that within a generation remaining uncertainties will be resolved.
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