Written by Jacob Henegan
Black Holes are some of the most perplexing things in the universe. A single point of such extreme mass that nothing, not even light, can escape. When they were first predicted by General Relativity, scientists assumed they were just a fluke in the maths, but now not only do we know that they exist, but they are so common that astrophysicists believe there to be over 100 million in our galaxy alone.
But since they give off no light, we can't actually see black holes, so how do astronomers find them?
Since black holes are 'black', and give out no light, astronomers can't observe them directly, they have to look not at them, but on their effects on the universe. There are 3 common ways to do this
Some black holes, such as Sagittarius alpha, the supermassive black hole in the centre of our galaxy, can be observed by the the stars orbiting them. Based on the speed and distance these stars orbit from, physicists can calculate its mass and location to a pretty high level of accuracy.
This technique is a really safe and accurate way of inferring black holes, but we need to have a lot of information about the surrounding region so it only works for nearby sources like those in our galaxy.
2. Gravitational Lensing
Einstein's theory of General Relativity tells us that objects with mass bend the fabric of spacetime itself, bending the path of light. Now, it takes a LOT of mass to make this a noticeable effect, but with supermassive black holes weighing in the order of 10^37kg, more than a million times the mass of our sun, the distortions are pretty big.
This causes an effect called gravitational lensing, where objects we would normally see as points, like stars, or distant galaxies, appear instead like streaks of light. The blackhole actually warps the ray of light as its moving towards us, and that's how we see it.
This technique, again, doesn't work for every black hole, it only works for Supermassive black holes, not the more common stellar mass black holes which are only a few to a few dozen times the mass of the sun, and we can only make good calculations from the effect if the black hole moves past the background, so we can compare them.
3. Accretion Disc radiation
The most common method of detecting blackholes, however, is by looking at the stuff falling into them. When stuff – whether its dust, asteroids, planets or stars, falls into a black hole, it usually doesnt fall straight in, but comes in at an angle. When this happens, the massive tidal forces from the black hole rip objects apart into dust particles, which then begin orbiting the black hole. The dust particles from everything falling into a black hole will form a disc, usually called an accretion disc. These discs are so dense and the particles are moving so fast that the friction between dust particles causes them to heat up to immense heats.
You know when metal gets really hot, it glows red or orange? And if you make something way, way hotter than that it can glow blue or white? Well these dust particles, just because of friction, are so hot, that they skip the visible spectrum altogether, and glow in X-rays! (good spot to throw in an EM spectrum animation)
By looking at the source of the light with an X-ray telescope, we can work out from the spectrum and brightness of the source whether it's a black hole and roughly how massive it is.
This technique works for black holes of all mass-ranges, but requires other matter to interact with the black hole and form a disc.
With these techniques, we can still only observe a tiny, tiny fraction of the billions of billions of black holes astrophysicists believe there to be in the observable universe, and there's still a whole lot we don't know about them, but as more techniques get developed and the technology improves, we're sure to learn a whole lot more about this big crazy universe we live in