Black (really black) holes

Recently the famous astrophysicist Stephen Hawkings hinted that his theory on black holes might have to be revised. For most of us though, the more obvious revisions are, ‘what exactly is a black hole?’ and ‘why is there so much interest in them?’

Abandon all hope ye who enter here

So what is a black hole? A black hole is an area of spacetime from which no matter can escape, not even light. Of course space & time in a black hole are so jumbled up that they do not exist in the sense that we know them. Nothing can escape a black hole because the gravitational force inside this region is so intensely strong, especially at the center or the singularity, that the density of any matter is almost infinite, which causes the velocity at which the matter can escape the gravitational force to be higher than even the speed of light.

Lets understand this further. Imagine that you are standing on the Earth’s surface and throwing an object in the air. The object will go up a certain distance and then turn back and fall to Earth. The harder you throw, the faster and the higher will it go, but will eventually come back to Earth. But if you some how throw it hard enough for it to escape Earth’s gravity, which by the way is about seven miles a second (so don’t even try to do so with your hands), it would never return. This velocity is the escape velocity.

Now if a body is compressed into smaller volumes, its density increases and thus its gravitational pull increases, due to which the escape velocity increases. So it now needs to be thrown at higher and higher speeds. The fastest that an object can move, theoretically speaking, is at the speed of light, which is 186 thousand miles a second, so if the escape velocity exceed this speed, nothing would be able to escape, not even light. This essentially is what happens in a black hole.

The beginning of the end

In order to understand how a black hole is formed, we need to understand the life cycle of a star. A star forms when a very large amount of gas, mainly hydrogen, collapses onto itself due to gravitational attraction. As the volume decreases, the gaseous atoms begin to collide with each other at increasing speeds and at increasing frequencies. This increases the temperature to a level where ultimately the hydrogen atoms do not collide but fuse to form helium. Heat and light are released during the reaction and a star is formed.

The heat generated increases the pressure of the gases and they expand, until the force of expansion equals the gravitational pull. The star then stabilises and can remain so for billions of years, but eventually it will run out of fuel and as it does so, the gravitational forces take over and it gets more and more difficult for the light & heat to escape. But once the star has reached a critical radius, the force become so strong that nothing can escape and a back hole is formed.

Of course not all stars become black holes. Whether it will form a black hole will depend on the star’s mass at its critical point as well as on the amount of outward pressure that it is able to exert to counter the gravitational pull. If the star is very massive, the immense gravitational pull will ensure a black hole, else it may turn into a white dwarf or a neutron star. Electrons in the case of a white dwarf and neutrons in the case of a neutron star resist the inward force of gravity. Thus although highly dense, they do not collapse into a singularity.

The point of no return

The surface of the black hole is known as the ‘event horizon’, beyond which the gravitational pull is so strong that no information can escape to the outside. It is thus like a one-way boundary, you can enter, but you cannot exit, ever. This boundary or the event horizon coincides with those rays of light that just fail to escape the black hole.

The German astrophysicist Kurt Schwarzschild had calculated the critical radius for a given mass at which not even light will escape; this radius is proportional to the mass of the black hole. Naturally, more the mass, more the surface area, for e.g. a black hole with a mass that is ten times that of our sun will have a radius of about 30 km while a black hole that is thousand times the solar-mass will have a radius of 3000 km. Thus if a black hole weighs 10 times another black hole, its radius will be 10 times more.
As to how big black holes can be, there are estimates that some black holes have masses 1,000,000 times that of the sun.

I want to see a black hole

It is not possible to literally see a black hole because no light can escape it, but we can see the effects of a black hole on nearby matter or objects. For e.g. if a black hole were surrounded by matter like gases or even stars, the quickening of their movements as they got closer to the black hole could be observed by the shifts in the energy of the x-rays that they emit.

However conclusive evidence can be obtained only once the gravitational waves that black holes produce, are studied. The problem is that we do not have sufficiently sensitive gravitational wave detectors yet. These wave detectors measure the vibrations in spacetime as black holes are formed or as they interact with other massive matters like a neutron star or even another black hole.

Don’t we have better thing to do?

Why do we study black holes?
__Curiosity – this is perhaps the strongest reason. If it exists, or even if it doesn’t, we need to know. Human evolution has been based upon this simple premise.
__ Since black holes have such massive gravitational forces, studying them will give us a better idea about how gravity works.
__If we are able to conclusively prove the existence of black holes, we would be more confident about theories, like those about the origin of the universe, which cannot be proved decisively.

And what exactly is this Singularity?

The singularity lies at the center of the black hole. Any matter that crosses the event horizon, is doomed to move towards the singularity, where it will be crushed to infinite density. In fact once the event horizon is crossed, any effort to escape will only accelerate the movement towards the singularity.

At this singularity, gravitational pull is so strong, that spacetime has no meaning in its normal sense. Even physical laws like General Relativity do not apply here, till date, no theory has been fully able to explain the functioning of the singularity.