In the dark outskirts of our Galaxy, far from bright star clusters, millions of black holes lurk. They are invisible: they don't shine and don't noticeably accrete matter. Conventional search methods, designed for dense stellar fields, are useless here. A different approach is needed.
Astronomers have proposed using the very fabric of space as a detector. When a black hole passes in front of a distant galaxy, its gravity acts as a lens: light is focused, and the source's brightness briefly increases. But to notice this effect, the source must be very compact — otherwise the amplification would be too weak. Tiny regions of ionized hydrogen (H II regions) in other galaxies turn out to be ideal targets. Their size, about ten light-years, matches the size of the gravitational lens of a stellar-mass black hole.
The key clue is hidden in the spectrum. When the lens acts, narrow peaks appear at specific wavelengths in the galaxy's overall light — emission lines of hydrogen and other elements. Importantly, gravity doesn't distinguish colors. All lines are amplified equally, preserving their usual ratios. It's as if someone turned up the volume without touching the equalizer. Real flares, such as those from supernovae, change the color ratios. So the spectrum immediately gives the trick away.
The method has a weakness: the most compact H II regions, which give the strongest amplification, are heavily dust-obscured. Dust absorbs visible light, so the signal must be caught in the infrared and radio bands. But even with perfect instruments, the chance of catching such an event is minuscule — roughly one per million years of observing a single galaxy. Yet the very existence of such a method gives hope for mapping the invisible population of black holes in the most desolate corners of the Milky Way.
The size of a typical H II region that astronomers are looking for is about 10 light-years. That's roughly the distance from the Sun to the nearest star, but the entire region glows like a giant neon ball.
In Michael Crichton's novel "Sphere," an alien signal is encoded in the spectrum of a distant star — here, the black hole's gravity itself momentarily "decodes" hidden details of distant galaxies.