Question

Mergers of two supermassive black holes will be major sources for observations of gravitational waves in the future. What do gravitational detectors aim to detect?

-Tiny shifts of fixed objects due to the rippling of space.
-Sound waves carried by cosmic ray particles.
-Tiny ripples in interstellar gas pressure.
-Modulated waves of electromagnetic radiation.

Answer 1

Gravitational waves are oscillations in the space-time geometry, they can be detected by measuring distances between freely floating objects. So, its's the first answer, but the objects are not really "fixed".