Gravitational Waves + impedance

Why isn't there an Impedance with gravitational waves?

http://www.scientificamerican.com/video/gravitational-waves-are-the-ringing-of-spacetime/

• Are you talking about acoustic impedance? – Danu Feb 22 '16 at 21:05
• What would impede spacetime? – Kyle Kanos Feb 22 '16 at 21:29
• Optical impedance is called the index of refraction; we also have acoustic impedance which is the density times the speed of sound in that material; and of course, electrical impedance. Now that we a method for finding gravity waves, gravitational surveys may eventually find gravitational impedance ... perhaps dark energy or dark matter. A research project for a cosmologist. – Peter Diehr Feb 22 '16 at 23:40
• Couldn't severely warped spacetime itself act as impedance? In other words in the vicinity of a black hole's event horizon? – docscience Feb 23 '16 at 0:04
• "Spacetime has a characteristic impedance ∼ $c^3/G$ (Blair, 1991)" - Advanced Gravitational Wave Detectors – Alfred Centauri Feb 23 '16 at 0:06

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 $10^{21}$.