Show Maxwell's Eqns. on a Cauchy Surface (Wald Ch. 10 Pr.2)

In summary, the problem at hand asks to show that ##D_a E^a = 4\pi \rho## and ##D_a B^a = 0## on a spacelike Cauchy surface ##\Sigma## of a globally hyperbolic spacetime ##(M, g_{ab})##. By using the expression ##E_a = F_{ab} n^b## for the electric field, we can simplify the first equation to ##D_a E^a = {h^a}_b {h_a}^c \nabla_c E^b##. After some manipulations and using Maxwell's equation, we get the final expression of ##D_a E^a = 4
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ergospherical
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This problem is Wald Ch. 10 Pr. 2.; it asks us to show that ##D_a E^a = 4\pi \rho## and ##D_a B^a = 0## on a spacelike Cauchy surface ##\Sigma## (with normal vector ##n^a##) of a globally hyperbolic spacetime ##(M, g_{ab})##. Using the expression ##E_a = F_{ab} n^b## for the electric field gives ##D_a E^a = {h^a}_b {h_a}^c \nabla_c E^b##. I replace ##{h^a}_b {h_a}^c = (\delta^a_b + n^a n_b)(\delta_a^c + n_a n^c) = \delta_b^c+ n_b n^c = {h_b}^c##, thereby obtaining\begin{align*}
D_a E^a &= {h_b}^c \nabla_c (F^{bd} n_d) = \nabla_b (F^{bd} n_d) + n_b n^c \nabla_c (F^{bd} n_d) \\
&= n_d \nabla_b F^{bd}+ F^{bd} \nabla_b n_d + n^c n_b n_d \nabla_c F^{bd} + n_b n^c F^{bd} \nabla_c n_d
\end{align*}The third term vanishes because ##n_b n_d \nabla_c F^{bd} = n_{(b} n_{d)} \nabla_c F^{[bd]} = 0##. Also, since ##n^a## is orthogonal to ##\Sigma##, the condition ##n_{[b} \nabla_c n_{d]}## holds i.e. ##n_b \nabla_c n_d## is totally antisymmetric, and the sum of the second and fourth terms is ##F^{bd} \nabla_b n_d + n_b n^c F^{bd} \nabla_c n_d = F^{bd} \nabla_b n_d - n_c n^c F^{bd} \nabla_b n_d = 2F^{bd} \nabla_b n_d##. Using Maxwell's equation ##\nabla^a F_{ab} = -4\pi j_b## on the first term gives\begin{align*}
D_a E^a &= -4\pi n_d j^d + 2F^{bd} \nabla_b n_d \\
&= 4\pi \rho + 2F^{bd} \nabla_b n_d
\end{align*}Does ##2F^{bd} \nabla_b n_d = 0##, i.e. is ##\nabla_b n_d## symmetric? I can't see why this should be so. (I thought about ##0 = \nabla_b(-1) = \nabla_b(n_a n^a) = n^a \nabla_b n_a##, but this doesn't seem to help).
 
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ergospherical said:
Does ##2F^{bd} \nabla_b n_d = 0##, i.e. is ##\nabla_b n_d## symmetric? I can't see why this should be so. (I thought about ##0 = \nabla_b(-1) = \nabla_b(n_a n^a) = n^a \nabla_b n_a##, but this doesn't seem to help).
This is up to a sign the second fundamental form, which is symmetric. With mathematical notations you can do the following. Let ##n## be the normal, and ##X,Y## tangent to the surface. Then differentiating ##g(n,Y)=0## you get ##g(\nabla _X n,Y)+g(n,\nabla _XY)=0##. So your expression is the negative of ##g(n,\nabla _XY)##, which is symmetric because ##g(n,\nabla _XY)=g(n,\nabla _YX)+g(n,[X,Y])##. The last term is zero because the comutator is also tangent to the surface.
 
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FAQ: Show Maxwell's Eqns. on a Cauchy Surface (Wald Ch. 10 Pr.2)

What are Maxwell's equations?

Maxwell's equations are a set of four fundamental equations that describe the behavior of electric and magnetic fields. They were developed by James Clerk Maxwell in the 19th century and are a cornerstone of classical electromagnetism.

What is a Cauchy surface?

A Cauchy surface is a three-dimensional hypersurface that intersects a given spacetime at exactly one point. It is used in the mathematical formulation of general relativity to divide the spacetime into a "before" and "after" region.

How are Maxwell's equations related to Cauchy surfaces?

Maxwell's equations can be written in a covariant form that is valid on any Cauchy surface. This means that they can be used to describe the behavior of electromagnetic fields in the context of general relativity.

What is the significance of Maxwell's equations on a Cauchy surface?

Maxwell's equations on a Cauchy surface allow us to study the behavior of electromagnetic fields in the context of general relativity. This is important for understanding the role of electromagnetism in the structure and evolution of the universe.

How can I visualize Maxwell's equations on a Cauchy surface?

Maxwell's equations on a Cauchy surface can be visualized using mathematical tools such as tensor calculus and differential geometry. These can be used to create diagrams and graphs that illustrate the behavior of electromagnetic fields in different spacetimes.

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