“Gravitational-wave astronomy is fundamentally different than ordinary astronomy. Most astronomy is based on light, which is an electromagnetic wave: a vibration in electric and magnetic fields. Gravitational waves are vibrations in the structure of space-time, which travel at the speed of light. Like sound, the frequency—the “tone”—of a gravitational wave often depends on the size of the system producing it. LIGO is particularly attuned to “high-pitched” waves made by pairs of black holes or pulsars right before they collide.”