When massive objects move around (maybe two black holes orbiting eachother, for example), spacetime itself is changed. This "change" to the geometry of spacetime manifests as waveforms which propagate throughout the universe, and can be detected by measuring small changes in the distance of km scale laser beams, in the LIGO experiment (now advanced LIGO) . Hope this helps.
When massive objects move around (maybe two black holes orbiting eachother, for example), spacetime itself is changed. This "change" to the geometry of spacetime manifests as waveforms which propagate throughout the universe, and can be detected by measuring small changes in the distance of km scale laser beams, in the LIGO experiment (now advanced LIGO) . Hope this helps.