Scientists say they have finally detected gravitational waves, proving Einstein right again. Here's what we know:

— Gravitational waves are extraordinarily faint ripples in the fabric of space and time, caused by cataclysmic events such as star collisions. Albert Einstein predicted them in 1916 as part of his general theory of relativity.

— A $1.1 billion set of instruments detected such a wave last September from a distant collision of two black holes, according to an international group of scientists.

— It's being hailed as a historic achievement because it means astronomers will now be able to "hear" the universe in action. The evidence of gravitational waves is captured in audio form. The discovery announced Thursday consisted of a single, faint "chirp."