Black hole Edit
The supermassive black holes might create Quasars (see the article for a possible habitable zone).
In case of a conventional black hole, with mass of 15 times the mass of our sun, Sol, its radius should be around 10 km. Even if we talk about a radius, in fact this is the limit where light can still escape. Beyond that, escape velocity is beyond the speed of light. As so, black holes emit no light and no energy output (some scientists argue that they emit far infrared light, but still not much).
Around a black hole, matter is struggling to enter. As it gets closer, it is spinning faster and faster, creating an accretion disk and powerful magnetic fields. Some light can be generated from the accretion disk (as it gets compressed and heated), together with a lot of X - rays.
People usually think that a black hole is a very dangerous object. To see how dangerous they are, let's replace Sol with a black hole of similar mass. The diameter of such a black hole should be of 3 km. The main difference will be the lack of light and solar wind, but planets will keep on rotating like they do today. At enough distance, there is no difference between the gravity of a black hole and the gravity of a star. If you get closer (very close), you will experience the effect of its huge gravity. Even with current technology, a spacecraft can approach a 3 km-wide one and orbit at 100 000 km, without being destroyed.
Significance for progress of physicsEdit
Theory of relativity makes predictions about black holes incompatible with those of quantum mechanics, so studying black holes is the best way to discover quantum gravity/ a theory of everything. While large black holes (both stellar mass and supermassive) generate too little tidal force to produce measurable Hawking radiation, small black holes vaporize much faster. So it is important to find small black holes.
Small black holes with the mass of mountains to mountain ranges may have formed in the Big Bang, possibly en masse. One in or near our Solar System would be detectable by extremely sensitive telecsopes searching for gravity lensing recurring at certain points at Earth's orbit around Sol, and dispatch faster than chemical, possibly ion propelled probes to study Hawking radiation.
It is possible that even smaller black holes forms when solar wind hit Earth's ionosphere or be created in an accelerator. Lasers can be used for searching for their characteristic gravity lensing.
Very small black holes may also form from quantum foam. Searching for them would require very weak lasers (a few photons) to detect gravity lensing without disrupting the quantum foam, and the detectors for searching for Hawking radiation must be placed very close to detect the radiation before it disappears by the quantum laws that created the black hole. Also good vacuum and temperatures very near absolute zero is required.
Hosted planet Edit
Planets have never been detected around a black hole, but they might exist. Planets have been detected around Neutron stars, likely formed from debris left by their supernova. Since black holes are also created by a supernova, it is likely that planets exist there too. A planet orbiting a black hole would have no sun (except if the black hole has a stellar companion). Settlers on such a planet would see a circle in the sky without stars. The accretion disk, if it exists, may be visible. There is a very small chance that its light could sustain liquid water on a planet, because the amount of flowing matter is by far not a constant. X - rays and gamma rays can have a devastating effect on the planet. Another problem is that the planet can cross through the accretion disk and be assaulted by meteorites or even destroyed in the collision.
As in case of Neutron stars, planets around black holes are expected to lack hydrogen (and also water). Most likely, they are made of heavy elements.
Settlers around a black hole Edit
At a low level of technology, settlers would not find anything valuable around a black hole. The lack of light makes it unfriendly. It could be very hard even to locate a solid body around, for a base. Still, there is something that settlers might find useful. Black holes deform space and when they do this, they act like massive lens. With adequate correction, they can be used as the largest telescope. Since hydrogen is highly unlikely to exist there, only research facilities will be built. Scientists will have to import all they need.
A civilization with average technology will find a black hole more valuable for their economy. Planets that might orbit there are made of heavy elements. Unlike neutron stars, which are created by smaller supernovas, black holes are created by giant blasts. So, settlers might find in large amounts heavy elements like lanthanides or actinides and other rare metals. Without the thread of X - rays and gamma ray bursts, found around neutron stars, settlers will find far more easy mining around a black hole. A second advantage is that they will have no problems with garbage. Just dump anything into the black hole and the problem is solved.
The most advanced people will find a black hole as a main resource of energy. there are two ways to do this. First, by throwing something into the black hole, you can create an artificial accretion disk and a small artificial quasar. Then, you drain energy from it. The second way, more advanced, is to use powerful magnetic fields, to make a gravity pit through the black hole. This will allow some energy to exit, most probably under the shape of gamma rays. This energy can fuel-up a galactic civilization for thousands of years. Also, part of the diverted energy can be used for heating and illuminating a planet made for the workers at the power station.