Bohm-Aharonov effect
A phenomenon in which the topological non-triviality of a gauge field is measurable physically [a1]. Moreover, this topological non-triviality, which can be expressed as a number , say, is a global topological invariant and so is not expressible by a local formula; this latter point being in contrast to a simpler topological invariant such as the dimension of the underlying space, which is deducible locally.
To demonstrate this effect physically, one arranges that a non-simply-connected region of space has zero electromagnetic field
. This electromagnetic field
is related to the gauge field
by the usual relation
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where the usual differential form language has been used for and
so that, if
are local coordinates on
, one has
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Given this and
, one can devise an experiment in which one measures a diffraction pattern associated with the parallel transport, or holonomy, of the gauge field
round a non-contractible loop
in
(cf. also Holonomy group). The action of parallel transport on multi-linear objects, viewed either as vectors, spinors, tensors, etc., or as sections of the appropriate bundles, is via the operator
, where
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Differential topology provides immediately the means to see that is non-trivial. The argument goes as follows:
so that the vanishing of
gives
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Hence determines a de Rham cohomology class
and one has
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It is clear from Stokes' theorem (cf. Stokes theorem) that the integral only depends on the homotopy class of the loop
. In addition, the loop
determines a homology class
, where
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This means that the integral (which can be taken equal to the number
) is just the dual pairing
between cohomology and homology, i.e.
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An experiment to test this for the electromagnetic field was done in [a2]. The experimental setup is of the Young's slits type, where electrons replace photons and with the addition of a very thin solenoid. The electrons pass through the slits and on either side of the solenoid an interference pattern is then detected. The interference pattern is first measured with the solenoid off. This pattern is then found to change when the solenoid is switched on, even though the electrons always pass through a region where the field is zero.
Mathematically speaking one realizes the solenoid by a cylinder so that
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The loop is the union of the electron paths
and
.
Figure: b110690a
A schematic Bohm–Aharonov experiment
The Bohm–Aharonov effect shows that the gauge potential is a more basic object than the electromagnetic field
.
Geometrically speaking, is a connection and
is a curvature. Hence, in purely geometrical language one can describe the situation by saying that a flat connection (i.e. one for which
, cf. Parallel displacement) need not be trivial if
is not simply connected, i.e. if
. This non-triviality of flat connections when
extends to the non-Abelian or Yang–Mills case.
For a Yang–Mills -connection
(with
non-Abelian), the curvature
is given by
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Further, if is defined on a bundle
over a manifold
and if it is assumed that, as in the Abelian case,
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then the integral only depends on the homotopy class of
, which is also denoted by
(the context should prevent any confusion with homology classes). Adding to this the fact that the parallel transport operator
has the property that
, one can use
to define the following linear mapping:
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One can check that so that, as
varies, the flat
-connection
on
gives a representation of the fundamental group
in
; it is easy to verify that if
under a gauge transformation
, then this
acts on
via the adjoint action
. In other words, it acts by conjugation, i.e.,
. But this means that the representations defined by
and
are equivalent, thus the gauge equivalence class of flat connections
is characterized by an element of the quotient
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Hence the moduli space of gauge-inequivalent flat connections is the space
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The holonomy group element is also of central interest elsewhere. It occurs in the study of the adiabatic periodic change of parameters of a quantum system described in quantum field theory, [a3], [a4], where it is called a Wilson line or Wilson loop, and in mathematics. One of the most important of these concerns the quantum field theory formulation of knot invariants [a5], such as the Jones polynomial [a6]. Flat connections also play a distinguished part in this theory.
See also Quantum field theory; Knot theory.
References
[a1] | Y. Aharonov, D. Bohm, "Significance of electromagnetic potentials in quantum theory" Phys. Rev. , 115 (1959) pp. 485–491 |
[a2] | D.R. Brill, F.G. Werner, "Significance of electromagnetic potentials in the quantum theory in the interpretation of electron fringe interferometer observations" Phys. Rev. Lett. , 4 (1960) pp. 344–347 |
[a3] | M.V. Berry, "Quantal phase factors accompanying adiabatic changes" Proc. Roy. Soc. London A , 392 (1984) pp. 45–57 |
[a4] | B. Simon, "Holonomy, the quantum adiabatic theorem and Berry's phase" Phys. Rev. Lett. , 51 (1983) pp. 2167–2170 |
[a5] | E. Witten, "Topological quantum field theory" Comm. Math. Phys. , 117 (1988) pp. 353–386 |
[a6] | V.F.R. Jones, "A polynomial invariant for knots via von Neumann algebras" Bull. Amer. Math. Soc. , 12 (1985) pp. 103–111 |
Bohm-Aharonov effect. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Bohm-Aharonov_effect&oldid=22149