Observation of a Black Hole

As mentioned in the last post, black holes are black because light cannot escape them. In fact, black holes do not emit any radiation at all because of their extreme density. So exactly how can we observe a black hole?




There are a couple of ways. The first way is too look at the gas and dust that surround a black hole. This gas and dust are in what is called an accretion disk and as the accretion disk orbits the black hole (just outside the Schwarzschild radius), it does two things.

1. It falls into the black hole, in turn making the black hole slightly bigger. This is where the term accretion comes from as the material accretes onto the black hole.
2. The inner edge of the disk is very hot because it's moving very fast. The gas and dust gets so hot that the material begins to radiate energy and give off energy in the form of jets. We see these coming from the "poles" of the black hole and know that there is a black hole there.

Accretion Disk around a Black Hole with jets

Image Credit:

Christopher Wanjek/NASA

Also, if you observe the gas and dust, you can measure the orbital velocity of the gas. You can also determine the radial distance of the gas and dust from the central body. By using these two values, the mass of the central body can be measured. For any radius, a mass above a certain value for that radius will tell you that the body is a black hole using the Schwarzschild radius equation.




Another strange thing about black holes is that black holes are not permanent. Hawking radiation predicts that due to quantum effects, particles of the black hole near the Schwarzschild radius can escape. Over time, as more and more particles escape, the black hole can shrink in size. See more about Hawking radiation here.