The Britannica Guide to Particle Physics by Britannica Educational Publishing

Author:Britannica Educational Publishing
Language: eng
Format: epub
Publisher: Britannica Educational Publishing
Published: 2011-06-04T16:00:00+00:00

Much more than mere points, particles are more like the strings on a violin, with mass and charge determined by the string’s vibration. Don Emmert/AFP/Getty Images

During the decade from 1984 to 1994, many theoretical physicists strove to fulfill string theory’s promise by developing this abstract, wholly mathematical framework into a concrete, predictive theory of nature. Because the infinitesimal size of strings has precluded their direct detection, theorists have sought to extract indirect implications of the theory that might be testable. In this regard, the extra dimensions of string theory have proved a major hurdle. Imagining these extra dimensions as small and hidden is a reasonable explanation for their apparent absence. However, also because strings are so small, they would vibrate in every dimension, not just in the usual three dimensions. Studies showed that, much as the shape and size of a French horn affect the vibrational patterns of airstreams coursing through the instrument, the exact shape and size of the extra dimensions would affect how strings vibrate. And since the strings’ vibrations determine quantities such as particle masses and charges, predictivity requires knowledge of the geometrical form of the extra dimensions. Unfortunately, the equations of string theory allow the extra dimensions to take many different geometric forms, making it difficult to extract definitive testable predictions.

By the mid-1990s, these and other obstacles were again eroding the ranks of string theorists. But in 1995 another breakthrough reinvigorated the field. Edward Witten of the Institute for Advanced Study, building on contributions of many other physicists, proposed a new set of techniques that refined the approximate equations on which all work in string theory had so far been based. These techniques helped reveal a number of new features of string theory. Most dramatically, these more exact equations showed that string theory has not six but seven extra spatial dimensions. The more exact equations also revealed ingredients in string theory besides strings—membranelike objects of various dimensions, collectively called branes. Finally, the new techniques established that various versions of string theory developed over the preceding decades were essentially all the same. Theorists call this unification of formerly distinct string theories by a new name, M-theory, with the meaning of M being deferred until the theory is more fully understood.

Today, the understanding of many facets of string theory is still in its formative stage. Researchers recognize that, although remarkable progress has been made over the past three decades, collectively the work is burdened by its piecemeal development, with incremental discoveries having been joined like pieces of a jigsaw puzzle. That the pieces fit coherently is impressive, but the larger picture they are filling out—the fundamental principle underlying the theory—remains mysterious. Equally pressing, the theory has yet to be supported by observations and hence remains a totally theoretical construct.

In the next decade this could change. An intriguing outcome of theoretical developments since 1995 is the recognition that strings and the extra dimensions might be significantly larger than previously thought. Rather than being 10-33 cm, studies with the more

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.