Exactly.PeterDonis said:More precisely, because of the "magnification" effect of the cavity, the contribution to the overall distance measurement of the distance from the ITM to the beam splitter is very small (a fraction of a percent). That is because the length of the cavity is magnified by a factor of 1000 or so while the ITM-beam splitter distance is not.
Thank you.It does for the LIGO team's analysis because their "fiducial" point is part of the overall construction of the coordinate chart they are using. It is true that, in principle, any choice of chart should give the same values for actual observables like the interference pattern at the photodetector, so in that sense the choice of fiducial point doesn't matter.
OK, I'll drop it at that then. However, I think the point that what is measured in practice is the phase delay resulting from the influence of the GW on the F-P cavity relates back to the original question. Thanks for your responses.Then that is off topic for this thread and you should start a new one.