Editing Dirac string (section)

{{Short description|Unobservable spacetime curves needed to describe Dirac monopoles}}
In [[physics]], a '''Dirac string''' is a one-dimensional curve in space, conceived of by the physicist [[Paul Dirac]], stretching between two hypothetical [[Dirac monopole]]s with opposite magnetic charges, or from one magnetic monopole out to infinity.  The [[gauge potential]] cannot be defined on the Dirac string, but it is defined everywhere else. The Dirac string acts as the [[solenoid]] in the [[Aharonov–Bohm effect]], and the requirement that the position of the Dirac string should not be observable implies the [[Magnetic monopole|Dirac quantization rule]]: the product of a magnetic charge and an electric charge must always be an integer multiple of <math>2\pi\hbar</math>. Also, a change of position of a Dirac string corresponds to a gauge transformation. This shows that Dirac strings are not gauge invariant, which is consistent with the fact that they are not observable.

The Dirac string is the only way to incorporate magnetic monopoles into [[Maxwell's equations]], since the [[magnetic flux]] running along the interior of the string maintains their validity.  If Maxwell equations are modified to allow magnetic charges at the fundamental level then the magnetic monopoles are no longer Dirac monopoles, and do not require attached Dirac strings.