Gravitational Waves + impedance Why isn't there an Impedance with gravitational waves? 
http://www.scientificamerican.com/video/gravitational-waves-are-the-ringing-of-spacetime/
 A: Spacetime is a very stiff elastic medium which is capable of propagating gravitational waves. The impedance of spacetime is
$$
Z_s = \frac{c^3}G = 4 \times 10^{35} \rm\,kg/s.
$$This impedance appears in two books on gravitational waves. The most recent is titled "Advanced Gravitational Wave Detectors" edited by Blair, Howell, et al (page 52). The gravitational wave designated GW150914 had measured intensity of about 20 mw/m$^2$ at 200 Hz.  If this was a 200 Hz sound wave, it would be very loud. However, the large impedance of spacetime meant that the displacment of spacetime ($\Delta L/L$) was only about 1 part in $10^{21}$. 
A: The $10^{35}$ kg/s value is valid at the Planck length. Wave impedance is scale dependent, has to be to do work. Invariant impedances (quantum Hall, centrifugal,...) are topological, communicate only phase. Gwave impedances at scales accessible to us differ from those at the Planck length. This is discussed in a recent presentation to a workshop on storage rings and gravitational wave detectors/sources, organized by LHC folks looking at what to do a decade or two from now.
