6 Fill in all the algebra implicit in the paragraph leading to Eq. (9.21)
Let us go to a Lorentz frame for the background Minkowski spacetime (i.e. make a background Lorentz transformation), in which the vector U upon which we have based the TT gauge is the time basis vector U = o. Then Eq. (9.19) implies Ao = 0 for all . In this frame, let us orient our spatial coordinate axes so that the wave is traveling in the z direction, k > (,0,0,). Then, with Eq. (9.19), Eq. (9.12) implies Az = 0 for all . (This is the origin of the adjective 'transverse' for the gauge: Av is 'across' the direction of propagation ez.) These two restrictions mean that only Axr, Ayy, and Axy = Ayx are nonzero. Moreover, the trace condition, Eq. (9.18), implies Axx = --Ayy. In matrix form, we therefore have in this specially chosen frame
Gravitational radiation
0 0 0 0 0 Axx Axy 0 0 Axy -Axx 0 0 0 0 0
(9.21)
There are only two independent constants, ATT and ATT. What is their physical significance?