Nib asked for
Seeing what cannot be seen
A black hole gives off no light of its own, but it betrays itself through what it does to everything around it: glowing infalling gas, orbiting stars, bent light, and rippling spacetime.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the dark hole
Pages 2 to 4 left us with a puzzle: if light cannot escape from inside the event horizon, a black hole is dark, and there is nothing to photograph. The way around it is the same way you notice wind without seeing air: you watch what it does to other things. Everything we know about black holes comes from effects on their surroundings, not from light that leaves them.
We never see the hole itself; we see what it does to its surroundings.
a common mix-upIf a black hole gives off no light, it must be impossible to detect.
2glowing gas
The first clue is glowing gas. Gas and dust falling toward a black hole do not drop straight in; like water circling a drain, they spiral and pile into a flattened disk. Friction and compression in that crowded disk heat the material to millions of degrees, so it shines brightly, often in X-rays, before it crosses the horizon. The glow comes from just outside the point of no return, which is why the light can reach us.
Infalling gas heats up and glows just before it crosses the horizon.
The light we see from this disk comes from inside or outside the event horizon, and why?
Show the answer
Outside. Light made inside the horizon cannot escape, so the glow must come from gas still outside it.
3fast orbits
The second clue is motion. Near the center of our own Milky Way, astronomers have tracked individual stars for decades and watched them whip around an invisible point at enormous speeds, completing orbits in just years. Orbit speed depends on how much mass is pulling, so those speeds reveal a mass of about four million Suns packed into a region far too small and dark to be anything but a black hole. Here gravity simply holds the stars in tight, fast orbits around something unseen.
Stars racing around empty-looking space reveal a huge, compact, dark mass.
4bent light
The third clue is bent light. Gravity bends the path of light passing near any mass, and a black hole bends it strongly. When a black hole drifts in front of a more distant star, the star's light curves around it and the star can briefly appear brighter or distorted. This is gravitational lensing, and it lets us weigh a dark object by how much it warps the light behind it.
A black hole's gravity bends light from stars behind it, brightening or distorting them.
a common mix-upBlack holes only matter to light that falls into them; light passing nearby is unaffected.
5ripples
The fourth clue is a different kind of signal altogether. When two black holes orbit each other and merge, they shake spacetime itself, sending out gravitational waves: ripples that travel at light speed. In 2015 detectors measured such a ripple for the first time, a stretching of space far smaller than an atom's width. Each detected pattern tells us the masses of the colliding black holes, so we can detect them without any light at all.
Merging black holes ripple spacetime, and detectors feel those gravitational waves.
Why can gravitational waves reveal black holes that give off no light?
Show the answer
They are ripples in spacetime caused by the masses moving, not light, so they do not depend on the holes shining.
6four clues
So here is the whole picture: one dark center, and four ways it gives itself away, glowing gas, fast orbits, bent light, and ripples.
So a black hole is invisible only to the eye. Its gas glows, its gravity steers stars and bends light, and its collisions shake space, so it can be found by what it does.