Gravitational Waves + impedance

Question

Why isn't there an Impedance with gravitational waves?

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

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 00:04
"Spacetime has a characteristic impedance ∼ $c^3/G$ (Blair, 1991)" - Advanced Gravitational Wave Detectors — Alfred Centauri, Feb 23 '16 at 00:06
Pieter Diehr, I didn't know how to specify the type of impedance for gravitational waves that's where I just used impedance as greater term. But I have the 'feeling' that there always should be an impedance even with gravitational waves. Doscience: I was thinking exactly like that.
Alfred Centauri: I'll read up op on that, thanks! — Qinten, Feb 24 '16 at 10:07

score 6 · Answer 1 · edited Jan 08 '18 at 13:56

6

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 $10^{21}$.

edited Jan 08 '18 at 13:56

rob

89,569

answered Dec 21 '16 at 21:41

John Macken

61

1

Is there any proof that Space is a stiff elastic medium? – Bill Alsept Dec 21 '16 at 22:01
3

@BillAlsept Is the discovery of gravitational waves enough proof for you? – probably_someone Nov 25 '17 at 23:01
@probably_someone no, and light waves do not prove there is a medium for light either – Bill Alsept Nov 26 '17 at 00:29
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@BillAlsept What would it actually mean to prove that space "is" a stiff elastic medium? What kind of measurement would be necessary? – probably_someone Nov 26 '17 at 18:35
@BillAlsept but the electromagnetic impedance of space is well known and intimately tied to the EM wave equations as well. – geshel May 15 '21 at 18:05
@geshel I don’t agree that light needs a medium to travel on. Individual photons move through space. – Bill Alsept May 15 '21 at 18:22
I think you are conflating “medium” with some notion like an ether. Space is a “medium” and it has an impedance. If it had no impedance there would be no waves. – geshel May 15 '21 at 18:25

score 0 · Answer 2 · edited May 15 '21 at 18:11

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.

Gravitational Waves + impedance

2 Answers2