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Gravity wave interferometers are built to detect tiny distance changes between mirrors. Is this because it would take light more time to travel between them if the distance increases, or because the total number of wavelengths of light between them would change, with a passing gravity wave? I ask because I don't know if a gravity wave would also affect (eg. stretch) the light between the mirrors as it passes or not. TNX.

Mike Herman wrote:Gravity wave interferometers are built to detect tiny distance changes between mirrors. Is this because it would take light more time to travel between them if the distance increases, or because the total number of wavelengths of light between them would change, with a passing gravity wave? I ask because I don't know if a gravity wave would also affect (eg. stretch) the light between the mirrors as it passes or not. TNX.

Gravity waves are different from gravitational waves; you mean gravitational waves.

Your question is difficult to answer. My guess is that (to lowest order) the light wave acts as a test particle, and is unaffected by the gravitational wave. A gravitational waves does change the proper distance of the two interferometer arms differently. Hence, light travels different distances in the two arms before recombined at the detector (this is how an interferometer works).

Markus Schwarz wrote:Gravity waves are different from gravitational waves; you mean gravitational waves.

It is ridiculously easy to confuse these two. Pretty bad nomenclature, really. Right up there with planetary nebula.

Markus Schwarz wrote:

Mike Herman wrote:
Gravity wave interferometers are built to detect tiny distance changes between mirrors. Is this because it would take light more time to travel between them if the distance increases, or because the total number of wavelengths of light between them would change, with a passing gravity wave? I ask because I don't know if a gravity wave would also affect (eg. stretch) the light between the mirrors as it passes or not. TNX.

Gravity waves are different from gravitational waves; you mean gravitational waves.

Your question is difficult to answer. My guess is that (to lowest order) the light wave acts as a test particle, and is unaffected by the gravitational wave. A gravitational waves does change the proper distance of the two interferometer arms differently. Hence, light travels different distances in the two arms before recombined at the detector (this is how an interferometer works).

• The light wave acts as 'constant speed of light' test particles in which
  1) time, 2) the frequency of oscillation, & 3) the wavelength (= c/f) are all unaffected.
  
  Special relativity motion in the z direction distorts t & z ... but x & y are unaffected.
  A weak gravitational wave in the z direction distorts x & y ... but t & z are unaffected.

The proper relative distances (a.k.a., the simple relative spatial distances) of the two (free falling) interferometer arms do oscillation out of sync and (provided that the gravitational wave oscillation time is much slower than the back & forth travel time of the photons) it simply takes longer (i.e., more constant wavelengths) for the photons to traverse in the longer direction than in the shorter one.

Resonant-mass gravitational wave detectors work somewhat differently.

Thanks. My confusion is based on the interpretation of expanding space as causing the "stretching" of the CBR wavelength over time. If expanding space can stretch wavelength, I assumed a gravity (gravitational?) wave might do the same thing. If so, that could "hide" the varying distance between interferometry mirrors. But if it's not the wavelength but the travel time that signals a difference, I can see where a passing gravity wave might still be detected.