Realest black hole picture I could find on the internet
Since the Hubble Space Telescope was launched in 1990, there have been many observations of what are believed to be black holes, including the photograph below of a suspected black hole in the heart of the galaxy NGC 6251. But the subject of black holes began in theoretical physics, long before there were any observations by astronomers.
The advent of Einstein's General Theory of Relativity gave physicists a mathematical language for describing the gravitational force in a manner consistent with the constant speed of light. Most of what we believe we know about black holes has come from abstract theoretical models in general relativity.
But in order to observe black holes in Nature we need to know how those abstract theoretical models translate to a Universe filled with other stuff.
In the abstract theoretical model of black holes, a black hole is studied as if it were the only thing in the Universe. Using that approximation, the math of general relativity becomes doable, and we can make predictions about black hole behavior that are useful in understanding the black holes we see. In addition, we learn a lot of things about black holes mathematically that we may never get a chance to witness directly through observation.
In general relativity, the paths of light can be calculated for many different distributions of matter and energy using equations call the geodesic equations. The geodesic equations give us the paths that would be followed by freely-falling test particles. For example, a baseball after being hit by Sammy Sosa and before being caught by an eager fan would be a freely falling particle, travelling on a geodesic path through spacetime.
Light travels on geodesics paths through spacetime. When those geodesic paths cross the event horizon of a black hole, they never come back out. Interestingly, in a Universe where the energy density is never negative, this behavior of light leads mathematically to two very crucial properties of black holes:
- The surface area of the event horizon of a black hole can only increase, never decrease. This also means that although two black holes can join to make a bigger black hole, one black hole can never split in two.
- The pull of gravity at the event horizon is constant; it has the same value everywhere on the event horizon.