A black hole is like a library. Each bit of information that falls into it is a book. The more books, the heavier the whole building and the more it trembles when shaken. The accumulated information (the “memory burden”) changes the hole’s vibrations when it is disturbed — say, by a merger with another black hole.

A new parameter — the fraction of filled “storage”—becomes as fundamental as mass or spin. The idea dates back to Stephen Hawking and Jacob Bekenstein, who linked black holes to entropy (a measure of information capacity).

This can be tested by analyzing gravitational waves from mergers using spectroscopy — decomposing the signal into frequencies. A similar effect is reproduced in the lab with clouds of cold atoms. The most surprising part: a black hole’s trembling can reveal what it “fed on” billions of years ago.