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)
“A writer is in danger of allowing his talent to dull who lets more than a year go past without finding himself in his rightful place of composition, the small single unluxurious ‘retreat’ of the twentieth century, the hotel bedroom.”
—Cyril Connolly (1903–1974)
“The nineteenth century planted the words which the twentieth ripened into the atrocities of Stalin and Hitler. There is hardly an atrocity committed in the twentieth century that was not foreshadowed or even advocated by some noble man of words in the nineteenth.”
—Eric Hoffer (1902–1983)
“The descendants of Holy Roman Empire monarchies became feeble-minded in the twentieth century, and after World War I had been done in by the democracies; some were kept on to entertain the tourists, like the one they have in England.”
—Ishmael Reed (b. 1938)
“Film is more than the twentieth-century art. It’s another part of the twentieth-century mind. It’s the world seen from inside. We’ve come to a certain point in the history of film. If a thing can be filmed, the film is implied in the thing itself. This is where we are. The twentieth century is on film.... You have to ask yourself if there’s anything about us more important than the fact that we’re constantly on film, constantly watching ourselves.”
—Don Delillo (b. 1926)