Last September researchers using the Chandra X-ray Observatory detected sound waves blaring from a distant black hole. That news prompted a lot of double takes. After all, astronomers always say sound can’t carry in a vacuum, and nothing, including sound waves, should be able to escape from a black hole. Even the makers of the movie Alien knew that “in space, no one can hear you scream.”
To understand how a black hole can hum, let’s review a little basic physics. Unlike light, sound has no independent existence. It is merely a disturbance in a medium. Jostle a bunch of atoms so that they in turn jostle others, and you have created an acoustic wave—just a fancy name for sound. The number of times per second that the atoms shift back and forth defines the wave’s frequency, experienced by the human eardrum and brain as pitch. The power of the ...
