- #1
jdougherty
- 25
- 0
Homework Statement
Show that a static, spherically symmetric Maxwell tensor has a vanishing magnetic field.
Homework Equations
Consider a static, spherically-symmetric metric [itex]g_{ab}[/itex]. There are four Killing vector fields: a timelike [itex]\xi^{a}[/itex] satisfying
[tex]
\xi_{[a}\nabla_{b}\xi_{c]} = 0
[/tex]
and three vectors [itex](\ell_{i})^{a}[/itex] orthogonal to [itex]\xi^{a}[/itex] that generate rotations and commute with [itex]\xi^{a}[/itex]:
[tex]
[\xi, \ell_{i}]^{a} = 0, \qquad [\ell_{i}, \ell_{j}]^{a} = \epsilon^{ijk}(\ell_{k})^{a}
[/tex]
Given a two-form [itex]F_{ab}[/itex] that satisfies the source-free Maxwell's equations
[tex]
\nabla_{[a}F_{bc]} = 0, \qquad \nabla^{a}F_{ab} = 0,
[/tex]
Let [itex]\eta^{a} = (-\xi^{b}\xi_{b})^{-1/2}\xi^{a}[/itex], and define the electric and magnetic fields as
[tex]
E^{a} = {F^a}_{b}\eta^{b}, \qquad
B^{a} = \tfrac{1}{2}\epsilon^{abcd}\eta_{b}F_{cd}
[/tex]
where [itex]\epsilon_{abcd}[/itex] is a volume form, so
[tex]
F_{ab} = E_{[a}\eta_{b]} + \epsilon_{abcd}\eta^{c}B^{d}.
[/tex]
In order for [itex]F_{ab}[/itex] to share the symmetries of the spacetime, its Lie derivative along any of the Killing fields must vanish. Since it's closed, that means
[tex]
0 = \nabla_{[a}(F_{b]c}\lambda^{c}).
[/tex]
for [itex]\lambda^{a}[/itex] any of the Killing fields.
This ought to be enough information to show that [itex]B^{a} = 0[/itex], but I can't quite get there.
The Attempt at a Solution
Let [itex]\kappa = \xi^{a}\xi_{a}[/itex]. Expanding the Lie derivatives a bit, and using the fact that [itex]\xi^{a}[/itex] is hypersurface-orthogonal, you can show
[tex]
\xi^{c}\nabla_{[a}F_{b]c} = -\kappa^{-1}\xi^{c}F_{c[a}\nabla_{b]}\kappa + \kappa^{-1}F_{c[a}\xi_{b]}\nabla^{c}\kappa \\
(\ell_{i})^{c}\nabla_{[a}F_{b]c} = \tfrac{1}{2}F_{cb}\nabla_{a}(\ell_{i})^{c} - \tfrac{1}{2}F_{ca}\nabla_{b}(\ell_{i})^{c}
[/tex]
The RHS of that second one is begging to be contracted against some [itex](\ell_{j})^{a}(\ell_{k})^{b}[/itex] to make use of the commutation relations, but I can't find an appropriate selection of indices. I also tried taking the Lie derivative of [itex]F_{ab}[/itex] expressed in terms of [itex]E^{a}[/itex] and [itex]B^{a}[/itex], and the problem then comes down to calculating the commutator of the Killing vector and each of [itex]E^{a}[/itex] and [itex]B^{a}[/itex]. However, it wasn't particularly illuminating.
Thinking about it loosely, the spherical symmetry should be what forces [itex]B^{a} = 0[/itex], so you'd think that throwing [itex]B^{a}[/itex] and the second Lie derivative expression together and shaking sufficiently vigorously would do it. The problem I have there is that there's no obvious (to me) way to mix those two elements together.