A black hole is an object in the Universe that contains so much matter, that its gravitational attraction force is so strong that nothing is able to escape from it, not even light. Einstein already predicted black holes based upon his General Theory of Relativity long before they were actually 'observed'.
Escape
velocity.
Newton has taught us
that all objects are attracted to each other by gravity, that is for instance
what causes objects to fall to Earth. If a rocket wishes to leave the Earth to space, than its
rising velocity must be greater than the speed with which the rocket would fall to Earth.
This specific velocity is called the escape
velocity (in this case from Earth).
The force of gravity is determined by the size of the mass and the
density of an object. The gravity of the Sun for instance is much larger
than that of the Earth. The larger and denser the matter, the larger
the escape velocity (each planet or star has its own escape velocity).
A super computer calculated the gravity field of a blackhole. The curves are the deformations of the spacetime. The colors indicate the speed of the local clocks. Clocks in the red areas tick much slower than those in the blue areas. |
Light is not
able to escape
At the end of their lifetime the heavier stars (with a density of about three times the Earth) collapse, the matter is continuously compressed. The star gets more and more compact, until the moment that it is so dense that the escape velocity is so great, that even the speed of light is too slow. Nothing is able to escape from the star anymore, not even light. The star has turned into a blackhole, with a singularity, a point of infinite density it the center. A blackhole is in a status of so called continuous gravitational collapse, were the present physical laws are not applicable. All matter that is attracted by the gravity forces of the blackhole will finally disappear in it. This is called the event horizon, everything that happens behind this horizon is shielded from the outside world. Also all the light that comes near it branches off so much, that it is taken into the blackhole. |
Spacetime stops.
Objects that have turned into a blackhole are super-massive, they have compressed so much matter that they have the mass of billions of stars, while their actual size is rather limited.
According to the General Theory of Relativity space is curved and time is delayed in the vicinity of matter, the greater the mass concentration, the greater the curvature. Near a blackhole the gravitational power of attraction is so strong that the space is curved into an infinite deep well. The outter limit of this curvature is called the Schwarzschildradius or event horizon.
Everything that crosses this event horizon, will finally run into the singularity and out of exhistance. Although for objects that have crossed the event horizon the clock will tick normally, someone looking from the outside will see everything near the blackhole in slow motion and eventually even frozen in time. Within the event horizon spacetime stops short.
| How can we observe
blackholes?
The blackhole itself is beyond our observations, because light nor other radiation is able to escape form it. The presence of the blackhole can only be demonstrated by its influence to its surroundings (the activities at the event horizon can be observed). A blackhole has such enormous gravity, that it can pull matter (gas) from a nearby star. This matter, that spirals towards the blackhole in an accretion disc (an area of accumulation of dust and gas) and finally disappears in it, is accelerated by the gravitational pull, it becomes more and more compressed and is heated up to extreme temperatures. During this process röntgen radiation is emitted by the matter in the accretion disc, which can be measured by satellites. At places of strong röntgen radiation, blackholes are assumed. |
One of the first pictures of the accretion disc of a blackhole made by the Hubble Telescope in 1995. The blackhole is located in the nucleus of galaxy NGC 4261, is at a distance of 100 million lightyears from Earth. |