A simulaton of gravitational lensing caused by a black hole moving past a background
galaxy.
Hawking Radiation
In 1975 Steven Hawking published what was at the time a shocking result. When quantum
theory is taken into account black holes emit "Hawking radiation", consisting of
photons, neutrinos, and to a lesser extent all sorts of massive particles.
Virtual particle pairs are constantly being created near the horizon of the black
hole, as they are they are formed continuously throughout space. Normally, they are
created as a particle-antiparticle pair and so they quickly annihilate each other.
But if they form near the horizon of a black hole, it is possible for one of the
pair to fall in before the annihilation can happen, in which case the other one escapes
as Hawking radiation.
Therefore a black hole will eventually radiate away all of its mass unless there
is replacement matter falling into the black hole. The mass loss is gradual at first
but then becomes faster and faster as the black hole shrinks. Eventually the black
hole explodes. However, only black holes with masses less than about 1011 kg (the
mass of a small mountain) can evaporate in less than the age of the Universe.
The Kerr Metric
A rotating black hole is described by the Kerr metric.
The Big Bang Singularity
There is a more more fundamental relationship between black holes and the Universe
as a whole. The whole universe can be thought of as a singularity.
Supermassive Black Holes
The centres of galaxies often host super-massive black holes.
The central black hole is always one thousandth the mass of the host galaxy.
Artists impression of a black hole issuing a jet - NASA
Black holes are fascinating objects
Black holes dramatically warp and curve space and time in their vicinity. Near the
edge of a black hole time passes ever more slowly compared to how it elapses far
from the black hole. Indeed it seems that time comes to an end at the central point
of a black hole. This central point is called a singularity. According to general
relativity, a singularity is a point of infinite density. The idea of any object
possessing an infinite density is, of course, impossible and has led to the breakdown
of the theory. At present there is no clear solution to this problem, because as
yet there is no clear theory of quantum gravity.
How do we know they exist?
As far as we know, Black holes are the weirdest objects in the universe. But we have
every reason to believe that these objects really exist. Because no light can escape
from them, you can't see black holes directly but their existence can be deduced
based on how they affect their surroundings.
For example, the supermassive black hole at the centre of our galaxy has a observable
effect on the nearby, surrounding stars, known as “s-stars”. We are able to deduce
the mass of this black hole from the orbits of the s-stars, some of which are orbiting
at 11,000,000 miles per hour!
Gas from companion stars in a black hole binary may form an accretion disc around
the black hole. This gas gets very hot and generates x-rays that can be observed
here on earth.
Black Holes
What is a Black Hole?
Black holes are one of nature’s most extreme and amazing creations. A black hole
is a star that has collapsed under its own gravity. They become a spherical region
of space which has such a powerful gravitational pull, that anything which ventures
too close, including light itself, is unable to escape.
The term “hole” implies a flat object, but black holes are actually spherical.
The possibility of such objects was first contemplated as far back as the 18th-century.
But the modern study of black holes was initiated by Einstein's insights into the
gravitational force, embodied in his general theory of relativity.
How is a black hole created?
When stars have used up all of their nuclear fuel they can collapse under the pressure
of their own weight. If the star has sufficient mass, its collapse will create a
black hole. To create one out of the Sun, you would need to squeeze the Sun to less
than 2 miles across. For the Earth you would need to squeeze it to half an inch across!
Journey Into a Black Hole
Let’s take an imaginary trip into a black hole. We will visit Cygnus X-1 in our starship
the TUR 1.
As we approach the black hole, our instruments can detect a massive object just like
any other star. We also notice some Hawking radiation being emitted from the object.
Of course, we can’t actually see the black hole itself but it is surrounded by an
accretion disc of material from its binary partner star. This material is so hot
it shines in the x-ray region of the electromagnetic spectrum.
Binary Black Holes
Emit gravitational waves.
The Event Horizon Telescope
A proposed telescope to see the shadow of black hole at the centre of the galaxy.
The Event Horizon telescope
(2004) by B.J.Carr