Twentieth Century
The quantum theory of magnetic charge started with a paper by the physicist Paul A.M. Dirac in 1931. In this paper, Dirac showed that if any magnetic monopoles exist in the universe, then all electric charge in the universe must be quantized. The electric charge is, in fact, quantized, which is consistent with (but does not prove) the existence of monopoles.
Since Dirac's paper, several systematic monopole searches have been performed. Experiments in 1975 and 1982 produced candidate events that were initially interpreted as monopoles, but are now regarded as inconclusive. Therefore, it remains an open question whether or not monopoles exist.
Further advances in theoretical particle physics, particularly developments in grand unified theories and quantum gravity, have led to more compelling arguments (detailed below) that monopoles do exist. Joseph Polchinski, a string-theorist, described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen". These theories are not necessarily inconsistent with the experimental evidence. In some theoretical models, magnetic monopoles are unlikely to be observed, because they are too massive to be created in particle accelerators (see below), and also too rare in the Universe to enter a particle detector with much probability. (According to these models, there may be as few as one monopole in the entire visible universe.)
Some condensed matter systems propose a structure superficially similar to a magnetic monopole, known as a flux tube. The ends of a flux tube form a magnetic dipole, but since they move independently, they can be treated for many purposes as independent magnetic monopole quasiparticles. Since 2009, numerous news reports from the popular media have incorrectly described these systems as the long-awaited discovery of the magnetic monopoles, but the two phenomena are only superficially related to one another. These condensed-matter systems continue to be an area of active research. (See "Monopoles" in condensed-matter systems below.)
Read more about this topic: Magnetic Monopole, Historical Background
Famous quotes related to twentieth century:
“Doubt, it seems to me, is the central condition of a human being in the twentieth century.”
—Salman Rushdie (b. 1947)
“The real passion of the twentieth century is servitude.”
—Albert Camus (1913–1960)
“In the twentieth century, death terrifies men less than the absence of real life. All these dead, mechanized, specialized actions, stealing a little bit of life a thousand times a day until the mind and body are exhausted, until that death which is not the end of life but the final saturation with absence.”
—Raoul Vaneigem (b. 1934)
“Advertising is the greatest art form of the twentieth century.”
—Marshall McLuhan (1911–1980)
“In the middle of the next century, when the literary establishment will reflect the multicultural makeup of this country and not be dominated by assimiliationists with similar tastes, from similar backgrounds, and of similar pretensions, Langston Hughes will be to the twentieth century what Walt Whitman was to the nineteenth.”
—Ishmael Reed (b. 1938)