https://arxiv.org/pdf/1701.04775.pdf DETECTION PRINCIPLE OF GRAVITATIONAL WAVE DETECTORS: 17 Jan 2017 GIUSEPPE CONGEDO With the first two detections in late 2015, astrophysics has officially entered into the new era of gravitational wave observations. Since then, much has been going on in the field with a lot of work focussing on the observations and implications for astrophysics and tests of general relativity in the strong regime. However much less is understood about how gravitational detectors really work at their fundamental level. For decades, the response to incoming signals has been customarily calculated using the very same physical principle, which has proved so successful in the first detections. In this paper we review the physical principle that is behind such a detection at the very fundamental level, and we try to highlight the peculiar subtleties that make it so hard in practice. We will then mention how detectors are built starting from this fundamental measurement elemen 6. Conclusions In this paper we have briefly reviewed the essence of GW detection at a very fundamental level, emphasising how the measurement element – comprising two test masses and a laser beam – is sensitive to GWs. This measurement element responds to fluctuations of the Riemann tensor integrated over the light path from emission to reception, which is a somewhat different approach compared to traditional calculations. The main advantage is the covariant gauge-free formulation, which eases the physical interpretation. The physical observable becomes the rate of change in the frequency shift, which is directly related to relative acceleration. However, this quantity also picks up other nuisance, e.g. non-gravitational forces that act on the test masses and disturb their free fall, and inertial forces that appear as observations are not made in an inertial reference frame. It is worth noting, though, that inertial forces do not play a significant role in ground-based detectors as the frequencies at which this affect becomes relevant are much lower than the measurement band. A different situation would be for space-based detectors as the effect falls in band – this should be taken into account and ultimately corrected for in the calculation of the detector’s response. We have reviewed, far from being exhaustive, how detectors are built as combinations of the fundamental measurement element. We have mentioned their main differences and peculiarities, how different combinations address different science questions, and yet the fundamental measurement principle is ultimately the same.
