More facts about black holes

  

1. Introduction:

Black holes are one of the strangest, mysterious and the most dangerous things in space. These are also the heaviest and have largest mass than anything. We can imagine black holes as black discs or in our daily life black CDs or DVDs. We cannot see black holes usually but we can see them because of rotating disks around them known as accretion disks. We can also see them while exploding or sucking something.

2. What are black holes?

Black hole, cosmic body of extremely intense gravity from which nothing, not even light, can escape. A black hole can be formed by the death of a massive star. When such a star has exhausted the internal thermonuclear fuels in its core at the end of its life, the core becomes unstable and gravitationally collapses inward upon itself, and the star’s outer layers are blown away. The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity.

3. INSIDE A BLACK HOLE:

•       We basically understand what happens outside the black hole as you approach its event horizon, that infamous point of no return. The event horizon is where the escape speed exceeds the speed of light: you’d have to be going faster than light (which is impossible for any bit of matter) to escape the black hole’s gravity.

•       Inside the event horizon is where physics goes crazy. Calculations suggest that what the fabric of space time looks like inside a black hole depends on that particular black hole’s history. It might be turbulent, twisted, or any other number of things. One thing’s for sure, though: the tidal forces would kill you.

4. TIME INSIDE A BLACK HOLE:

Suppose an intrepid astronaut on the surface of the collapsing star sent a signal every second, according to his watch, to his spaceship orbiting about the star. At some time on his watch, say eleven o’clock, the star would shrink below the critical radius at which the gravitational field became so strong that the signals would no longer reach the spaceship. His companions watching from the spaceship would find the intervals between successive signals from the astronaut getting longer and longer as eleven o’clock approached. However, the effect would be very small before 10:59:59. They would have to wait only very slightly more than a second between the Astronaut’s 10:59:58 signal and the one that he sent when his watch read 10:59:59, but they would have to wait forever for the eleven o’clock signal. The light waves emitted from the surface of the star between 10:59:59 and eleven o’clock, by the astronaut’s watch, would be spread out over an infinite period of time, as seen from the spaceship.

5. OUR NEAREST BLACK HOLES:

This list contains all known black holes relatively near the Solar System (within our Milky Way galaxy). To make it easier to compare distances, our nearest star aside from the Sun – Proxima Centauri – is about 4.24 light years away and our Milky Way galaxy is 180,000 light years in diameter. According to the startling new results, black holes do not culminate in a singularity. Rather, they represent "portals to other universes," reports New Scientist. This new theory is based on a concept known as 'loop quantum gravity' (or LQG). ... With both the deep pit and the black hole, there is no "other side."

6. Falling into a black hole:

If you were to fall towards a relatively small black hole, then your gravitational forces before reaching the event horizon, and your body would be stretched out until you were ripped apart– a process sometimes called spaghettification. However, if the black hole were large, you would not notice when you had crossed the event horizon, except that distant objects would look distorted because light would be bent. It would be too late hurtle towards the singularity. An observer on the outside would not see this happening but would see your descent appear to slow down as you approached the event horizon and the light took longer to travel away. The light waves would also stretch out so that they reddened in colour. Eventually, all that would remain would be a dimming, reddened image of you frozen in time at the event horizon.

7. WHY BLACK HOLES ARE IMPORTANT:

A black hole at the centre of a galaxy is essential to the formation and stability of that galaxy. However, could it exist, a large rigid non-black-hole of the same mass might also be effective - albeit the reduction in gravitational wave emission from material that closely circled the black hole means that we would have ended up with very different galaxies from those we know today.

8. KERR SOLUTION:

The solution of Einstein’s equations describing the exterior of an isolated, spherically symmetric object (the Schwarzschild solution) is quite simple. Indeed, it has been found in 1916, immediately after the derivation of Einstein’s equation. In the case of a rotating body, instead, the problem (which is very relevant: astrophysical bodies do rotate) is much more difficult: we don’t know   any analytic, exact solution describing the exterior of a rotating star (even if we know approximate solutions).  But we know the exact solution describing a rotating, stationary, axially symmetric black hole. It is the Kerr solution, derived in 1963 by R. Kerr. We say that this metric describes a black hole, because it is a solution of Einstein equations in vacuum (Tµν = 0) and it has a curvature singularity covered by a horizon: like in the case of Schwarzschild space-time, everything falling inside the hole cannot escape.  We stress that while, thanks to Birkhoff theorem, the Schwarzschild metric for r > 2M describes the exterior of any spherically symmetric isolated object (a star, a planet, a stone, etc.), the Kerr metric outside the horizon can only describe the exterior of a black hole.

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