I'm currently reading the chapter in Schutz about gravitational waves (though knowledge of/access to Schutz shouldn't be necessary to answer my question). After demonstrating how a passing gravitational wave will cause the proper distance between two points to change temporarily, he says: "A frequent question is, if space is stretched, why is a ruler (which consists, after all, mostly of empty space with a few electrons and nuclei scattered through it) not also stretched, so that the stretching is not measurable by the ruler?" He explains this by saying that the changing metric due to the wave manifests as a tidal force acting on particles at the two points in space. As in classical mechanics, this force will pull the particles apart or push them together. However, rulers (and other physical objects) are made of matter which is held together by electromagnetic, etc., forces. These forces are much stronger than the tidal forces induced by the wave. Thus, it's only the distances between things that gets stretched, rather than the things themselves. This makes good sense to me. But then he goes on to discuss various ways of trying to measure gravitational waves. In particular, he goes on at length about interferometers. But the end mirrors of interferometers aren't just floating in space (at least, not yet). Apparati like LIGO are made of large tunnels made of matter, to which the mirrors are fixed. So, by the earlier arguments, shouldn't the distance between the mirrors remain unchanged since they're attached to matter that's held together by other forces? How are these detectors able to (in theory) detect an incoming wave?