The idea of string theory is that under sufficient magnification the Standard Model particles will be seen to be loops or segments of string. String Theory and Particle PhysicsĮxperimentally, the electron, quarks, photon, and other Standard Model particles are all points: no experiment has revealed any substructure, down to the distance scale of 10 -16 cm. What the "M" stands for is deliberately left unspecified, reflecting the unknown nature of the final theory "magic," "mother," "mystery," "membrane," and "matrix" have all been suggested. Since 1995 the term M theory has also been used, reflecting the fact that the string picture is believed to be just a stepping-stone to a final theory. String theory is equivalently referred to as superstring theory, reflecting the central role of supersymmetry. It is widely believed that the description of the theory in terms of one-dimensional building blocks is not ultimately the simplest or most complete but rather is a stepping-stone toward a more fundamental principle, which is being actively sought by string theorists today. String theory is still an incomplete theory. Another has been the understanding that string theory contains extended structures known as branes, which has led to new ideas for realistic models and for experimental and cosmological tests. One has been a new understanding of the quantum mechanics of black holes, resolving some long-standing puzzles. The ensuing period, known as the second superstring revolution, has produced many further discoveries. In 1995 Witten, extending results of Chris Hull, Paul Townsend, and others, identified the principle of string duality, which governs the behavior of strongly interacting strings. However, work at this time was limited to a certain approximation, known as perturbation theory, which applies only to small numbers of strings interacting weakly. The discovery of Calabi-Yau compactification by Philip Candelas, Gary Horowitz, Andrew Strominger, and Edward Witten and of heterotic string by David Gross, Jeff Harvey, Lance Dixon, and Ryan Rohm strengthened the evidence for string theory. This led to a tremendous wave of research activity, often called the first superstring revolution, as theorists who had been pursuing other approaches to unification began to develop string theory. In 1984 a discovery by Michael Green and John Schwarz, known as anomaly cancellation, showed that string theory could also describe quarks, leptons, and gauge interactions. In the following years it was discovered that string theory is actually a theory of gravity, that it implies a symmetry between bosons and fermions (which was named supersymmetry), and that it is free of the unphysical infinities that plagued all previous theories of quantum gravity. This idea was superceded by the 1973 discovery of the true theory of the strong interaction, quantum chromodynamics, but a small handful of theorists regarded string theory as a compelling idea and continued to develop it. It was first developed between 19 as a theory of the strong interaction: mesons such as the pion behave in some respects like open strings. The idea of building blocks that are one-dimensional, rather than zero-dimensional points, is rather novel, and it has had an odd history. String theory is believed to solve all of these problems. Further, ultimately matter and gravity should not be described by two unrelated theories but should be understood in a unified way. General relativity, when combined with quantum mechanics, suffers from several problems and paradoxes when applied to very short distances or to black holes. The Standard Model is based on a complicated pattern of particles and forces, similar to the Periodic Table of the elements, and this pattern must be explained. In the twentieth century, very successful theories of each were discovered: the Standard Model of matter and the general theory of relativity. Two of the central questions in physics are the nature of matter and the nature of gravity. This theory has not yet been experimentally tested, but it has attracted the attention of theoretical physicists from a wide range of fields because it unifies many of the central concepts of physics and resolves a number of longstanding theoretical problems. It is based on the idea that the basic building blocks of nature are strings, one-dimensional objects of zero thickness, which form either closed loops or open curves (Figure 1). String theory is a proposed unified theory of fundamental physics, incorporating both particle physics and gravity.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |