  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 element.
