JHEP11(2016)037
The “electric” li m i t of these equations can be obtained as follows
(electric limit) A
t
= −ϕ , A
i
=
1
c
a
i
, Λ =
1
c
λ , c → ∞ with ϕ, a
i
, λ fixed. (2.2)
This results i n
∂
i
∂
i
ϕ = 0 , −∂
t
∂
i
ϕ + ∂
j
f
ji
= 0 , (2.3)
where f
ij
= ∂
i
a
j
− ∂
j
a
i
is the magnetic field. The contraction of the relativistic gauge
transformations leads to δ
λ
ϕ = 0 and δ
λ
a
i
= ∂
i
λ so that the scalar ϕ is invariant. The
equations (
2.3) respect a symmetry under Galilean boosts x
′i
= x
i
+ v
i
t, t
′
= t acting on
the the fields ϕ and a
i
as
ϕ
′
= ϕ a
′
i
= a
i
+ v
i
ϕ .
This follows from first performing a Lorentz boost on A
µ
and then taking the c → ∞ limit.
The “magnetic” limi t can be similarly defined by setting
(magnetic limit) A
t
= −˜ϕ , A
i
= ca
i
, Λ = cλ , c → ∞ with ˜ϕ, a
i
, λ fixed. (2. 4)
In this case the equations of motion (
2.3) reduce to
∂
i
˜
E
i
= 0 , ∂
j
f
ji
= 0 , (2.5)
where
˜
E
i
= −∂
i
˜ϕ − ∂
t
a
i
is the electric field. Gauge transformations act as δ
λ
˜ϕ = −∂
t
λ
and δ
λ
a
i
= ∂
i
λ so that the electric field is invariant. In this limit the potentials ˜ϕ and a
i
transform under G al i l e an boosts as
˜ϕ
′
= ˜ϕ + v
i
a
i
, a
′
i
= a
i
. (2.6)
In 3+1 dimensions the electric and magnetic limits are related by electric/magnetic dual-
ity [
12].
Finally we can define a third limit that has the advantage of allowing an off-shell
description. Consider the Maxwell action for A
µ
with an add i ti onal free real scalar field χ ,
L =
1
2c
2
∂
t
A
i
− c∂
i
A
t
(∂
t
A
i
− c∂
i
A
t
) −
1
4
F
ij
F
ij
+
1
2c
2
∂
t
χ∂
t
χ −
1
2
∂
i
χ∂
i
χ . (2.7)
The limit is given by
(GED limit) χ = cϕ , A
t
= −cϕ−
1
c
˜ϕ , A
i
= a
i
, c → ∞ with ϕ, ˜ϕ, a
i
fixed. (2.8)
By substitution in (
2.7) we obtain the action for Galilean electr ody n ami c s (GED)
S =
Z
d
d+1
x
−
1
4
f
ij
f
ij
−
˜
E
i
∂
i
ϕ +
1
2
(∂
t
ϕ)
2
. (2.9)
This action was first introduced in [
27] and the l i mit from which it arises is described
in [
13]. Under gauge transformations the fields trans for m as
δ
Λ
˜ϕ = −∂
t
Λ , δ
Λ
a
i
= ∂
i
Λ , δϕ = 0 .
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