Short explanation: A black hole is a region
in space where gravity is so immense that not even light can escape.
A black hole is the most fantastic object in the Universe. It is neither matter
(like stars, planets, and bicycles), nor radiation (like gamma rays, green
light, and radio waves). It is, in fact, a clot of gravity, residing in empty
space.
A black hole is formed when a star more massive than approximately 25 Solar
masses ends its life.
A star is the result of the equilibrium between two forces:
Gravity trying to pull everything together, and energy production trying to push
everything apart. When the star has burned all of its fuel, it will collapse.
For light stars, this collapse will stop at the state of a
white dwarf, while a heavier star explodes in a
supernova, leaving back a
neutron star.
However, if the neutron star is more massive than about 2 – 3 Solar masses,
nothing is able to prevent the
catastrophe: The collapse will continue until all that has not been blown away
by the supernova explosion has crumpled to a single point! The final
mass is
typically a few Solar masses, located not inside a very small volume, but in a
single point!
A s i n g l e p o i n t !
Near this point, Einstein's Theory of Relativity predicts that gravity is so
extreme that space itself gets severely curved,
attracting everything that gets too near. The closer one gets to the point, the
stronger the gravity, and thus the more speed is needed to escape. Inside the
so-called "event horizon", which for a one Solar mass black hole is 1.8 km from
the point, this necessary speed exeeds the speed of light, meaning that nothing,
not even light itself, can escape.
The definition of the black hole is the sphere "inside" the event horizon.
Not only space, but also time gets warped in the vicinity of a black hole, in the
sense that the closer one gets to the event horizon, the slower time wil run as
seen from far away. An astronaut falling towards the black hole will feel
nothing
special at the horizon itself, but will eventually be ripped apart by tidal
forces, ending in the point. However, from the point of view of an observer
located far away, the astronaut will fall slower and slower, never reaching the
horizon.
This fact implies several philosophically difficultly acceptable phenomena. What
takes, say, one minute for the falling astronaut, takes not only a very long
time, but forever (!) for the stationary observer. Moreover, if the falling
astronaut has a powerful rocket, so that he can go close to the horizon and then
return to his friends, he will find that while the journey took him, say, an
hour, a hundred years has passed in the "outside" world. In this way, a black
hole can be used as a time machine to travel forward in time (but not backward
— for this you must use other means...).
If you have further questions about this breathtaking scenario, or anything
else, please do not hesitate to write me at
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