"Feynman’s gravitational wave detector: It is simply two beads sliding freely (but with a small amount of friction) on a rigid rod. As the wave passes over the rod, atomic forces hold the length of the rod fixed, but the proper distance between the two beads oscillates. Thus, the beads rub against the rod, dissipating heat."
Why the argument of atomic forces does not hold in the case of interferometers? Am I wrong on thinking of them as chemically bound structures with fixed mirrors at the end? In this case I do not see what is playing the role of the bead, here. I treat the laser beam as merely the meaning to measure change in a arm via interference. In simple words, why a passing GW should oscillate the arm(s) of the interferometer but does not if a rod (that of the sticky bead argument) is considered? Please try to answer at the same level at which my question is posed, if possible.
EDIT: perhaps I should take the quote from Wikipedia less literally. I would say that both the rod and the beads of the sticky bead machinery oscillate. Just the beads do so more freely as there are almost free masses (ideal little friction). But the rod does oscillate to, just in less extent... Is this the key to dissolve my doubt?