(ed.). Gravitational waves (IOP, 2001)(422s).

42 Gravitational-wave detectorsTable 3.1. (Continued)ElectromagnetismSimple detector: oscillating charge.Action is along a line, transverse to thedirections of propagation. Spin s = 1and two states of linear polarization thatare inclined to each other at an angle of90 ◦ .Strength of force ⇒ waves scatter andrefract easily.Local energy and flux well defined:Poynting vector etc.Multipole expansion in slow-motionlimit is straightforward, radiation reactionwell defined.Exact solutions, containing waves, areavailable and can guide the constructionof approximation methods for morecomplicated situations.General relativitySimple detector: distorted ring ofmasses. Action is elliptic in aplane transverse to the direction ofpropagation. Spin s = 2 and twostates of linear polarization that areinclined to each other at an angle of 45 ◦ .Equivalence principle ⇒ action dependsonly on h µν , which is dimensionless.Weakness of gravity ⇒ waves propagatealmost undisturbed and transfer energyvery weakly. Dimensionless amplitudeh is small.Equivalence principle ⇒ local energydensity cannot be defined exactly. Onlyglobal energy balance is exact.Multipole expansion different if fieldsare weak or strong. For quasi-Newtonian case fields are weak, and theresulting post-Newtonian expansion isdelicate. Radiation reaction is still notfully understood.Fully realistic exact solutions for dynamicalsituations of physical interestare not available. Extensive reliance onapproximation methods.