There is one thing that all of the many forms of electromagnetic radiation have in common.
It's their speed, the speed of light.
At the centre of black-holes are super-compact high mass objects called singularities.
These singularities have high gravities, so high that even light cannot escape from them.
This means that light, at all wavelengths, is captured within a black hole and we can't 'see'
a black-hole no matter how we look at one.
Just as moons, planets, and stars are gravity wells of varying strengths
(anything going too slow that comes too close will fall into them)
so too are black-holes gravity wells.
The area around a singularity, the part that is the black hole, has a type of boundary called an event horizon
which is calculated as the Schwartzchild Radius.
Within this radius the escape velocity is greater than the speed of light - hence the term black hole.
Outside of the event horizon the escape velocity becomes lower the further away you get.
This means that anything travelling slower than the speed of light may also be trapped in the gravity well and infall if it gets close enough.
There are three main ways of detecting black holes.|
These methods don't rely on observing the actual black hole, but the effect on surrounding matter by the singularity.
1. Infalling matter from an acretion disc emits radio and X-ray radiation which identifies possible black hole candidates.
Jets of radiation stream away from the black hole.
2. Gravitational lensing with no discernable galaxy as a lense may indicate the presence of a black hole.
In this image the gravitational lensing is due to the Galaxy Cluster Abel 1689,
black hole lensing will only occur when a black hole is directly between us and another object.
3. A high unseen mass as detected by the motion of a companion or nearby star may indicate the presence of a black hole.
This small movie file shows the motion of stars
around the black hole at the centre of the Milky Way
from 1992 to 2006.