You can tell a lot about a human being's ancestry from their general characteristics. A child can have their father's eyes, their mother's smile, or maybe even their grandfather's male pattern baldness (thanks, grandpa).
However, black holes have few defining characteristics — as theoretical physicist John Wheeler put it, "black holes have no hair" (much like your humble author). Of course, though, testing a child's parentage based on physical features is far too subjective — that's typically where DNA tests come in. Such tests can offer a far more scientific way of checking a person's lineage, and new research suggests an analogous ancestry test for black holes.
Rather than relying on a cheek swab or a little blood, however, these cosmic DNA tests utilize tiny ripples in the fabric of spacetime called gravitational waves, first proposed by Albert Einstein 110 years ago.
