How can we detect a black hole? [duplicate]

Question

If black holes are phenomena of very high density (gravitational singularities) which don't emit radiation how can we detect them so far away from us where so much other radiation can hide the black hole? Aren't there many objects and things in front of black holes that would obstruct our view?

Answer 1

There are so many ways we can either directly or indirectly detect black holes that this answer will be necessarily incomplete.

Although no light can escape a black hole, the effects of black holes on the space, matter, and activity around them is often very dramatic. One of the most common ways is via an accretion disk of matter spiraling around the black hole as it falls in. Here is an artist conception of that:

As the matter falls in, friction between the particles heat it up causing it to emit extreme quantities of x-rays and gamma-rays. There are also often jets on the poles of the rotating black hole and we can sometimes see those jets as they affect the surrounding matter:

Sometimes we can see a star get too close to a black hole and get torn apart due to tidal forces.

Sometimes there are stars orbiting a black hole very close which causes their orbit to be super-fast. This allows us to estimate the mass they are orbiting and determine that it must be a black hole.

Another trick for finding black holes involves gravitational lensing where the gravity of the black hole bends light from behind it as it passes by on its way to Earth:

Finally, soon we may be able to detect a black hole merge with another massive object via gravitational waves using LIGO.

This list certainly isn't complete!

Answer 2

This answer is kind of parallel to Brandon's, because I want to emphasise the point underlying these types of observations.

We will never be able to observe a black hole, because for external observers the formation of an event horizon takes an infinite time. This may seem a bit pedantic, but it's an important point because our aim is not to directly observe a black hole but rather to measure the properties of a system and from these infer that the system must form a black hole.

For example, take Sagittarius A$^*$, which is believed to be a supermassive black hole at the centre of our galaxy. We can observe stars orbiting it and from these observations calculate that it's mass is about 4.1 million Solar masses, and its size is less than about 6.25 light-hours. This does not prove that Sag A$^*$ is a black hole, because the event horizon radius for the mass of Sag A$^*$ is about 40 light-seconds. However we don't know of any way an agglomeration of mass with this density could stay stable for anything like the lifetime of the Milky Way so we infer it must have formed a black hole. We expect future radio telescope measurements to make the limits on the radius even tighter and increase our confident that Sag A$^*$ must be a black hole.

Alternatively take Cygnus X-1. We can estimate it's mass using various methods and get a mass in the range 10-20 Solar masses. We can place a limit on it's size by measuring the timescale of changes in it's X-Ray emission, and we get an upper limit of about 10$^5$km (a bit less than the Sun). This makes Cygnus X-1 at least a neutron star, but assuming our calculations of stellar equations of state are reliable no neutron star heavier than 3 Solar masses can resist collapsing into a black hole. So once again we can infer the existance of a black hole even though we can't directly observe it.

Brandon's answer gives lovely examples of the measurements we can make to get evidence that black holes exist. The key point to understand with all of these is that we are trying to place lower limits on the density of the observed object. If these limits are sufficiently high, and assuming our current understanding of the physics involved is correct, then we can infer that the object is in the process of forming a black hole.