1)
WHAT IS A BLACK HOLE:-
A black hole is a
region of space-time from which nothing, not even light, can escape. The
theory of general relativity predicts that a sufficiently compact mass will
deform space-time to form a black hole. Around a black hole there is a
mathematically defined surface called an event horizon that marks the point of
no return. It is called "black" because it absorbs all the light that
hits the horizon, reflecting nothing, just like a perfect black body in
thermodynamics. Quantum mechanics predicts that black holes emit radiation like
a black body with a finite temperature. This temperature is inversely
proportional to the mass of the black hole, making it difficult to observe this
radiation for black holes of stellar mass or greater.
Objects whose gravity
field is too strong for light to escape were first considered in the 18th
century by John Michell and Pierre-Simon Laplace. The first modern solution of
general relativity that would characterize a black hole was found by Karl
Schwarzschild in 1916, although its interpretation as a region of space from
which nothing can escape was not fully appreciated for another four decades.
Long considered a mathematical curiosity, it was during the 1960s that
theoretical work showed black holes were a generic prediction of general
relativity. The discovery of neutron stars sparked interest in gravitationally
collapsed compact objects as a possible astrophysical reality.
HOW ARE THEY FORMED:-
Black holes
of stellar mass are expected to form when very massive stars collapse at the
end of their life cycle. After a black hole has formed it can continue to grow
by absorbing mass from its surroundings. By absorbing other stars and merging
with other black holes, supermassive black holes of millions of solar masses
may form. There is general consensus that supermassive black holes exist in the
centers of most galaxies. In particular, there is strong evidence of a black
hole of more than 4 million solar masses at the center of our galaxy, the
Milky Way.
EVENT HORIZON:-
The defining
feature of a black hole is the appearance of an event horizon—a boundary in space-time
through which matter and light can only pass inward towards the mass of the
black hole. Nothing, not even light, can escape from inside the event horizon.
The event horizon is referred to as such because if an event occurs within the
boundary, information from that event cannot reach an outside observer, making
it impossible to determine if such an event occurred.
As predicted
by general relativity, the presence of a mass deforms space-time in such a way
that the paths taken by particles bend towards the mass. At the event horizon
of a black hole, this deformation becomes so strong that there are no paths
that lead away from the black hole.
To a distant
observer, clocks near a black hole appear to tick more slowly than those
further away from the black hole. Due to this effect, known as gravitational
time dilation, an object falling into a black hole appears to slow down as it
approaches the event horizon, taking an infinite time to reach it. At the same
time, all processes on this object slow down causing emitted light to appear
redder and dimmer, an effect known as gravitational redshift. Eventually,
at a point just before it reaches the event horizon, the falling object becomes
so dim that it can no longer be seen.
On the other
hand, an observer falling into a black hole does not notice any of these
effects as he crosses the event horizon. According to his own clock, he crosses
the event horizon after a finite time, although he is unable to determine
exactly when he crosses it, as it is impossible to determine the location of
the event horizon from local observations.
Daniel Strange, one of the PhD students working on the research, commented: “The great thing about the robots is once you tell them what to do they can do it very precisely over and over again — so a day later I can come back and see a fully made sample.”
