Black Holes and Fundamental Laws of Physics

Black holes are structures in the fabric of space and time itself. And there are many such objects in the universe. Despite the mathematical complexity of describing black holes and various ways in which it forms, we also know that they possess a very striking simplicity.

In 2005, black holes were identified by the gravitational waves released from the collision of two massive black holes orbiting each other. In 1687, Sir Isaac Newton described the law of gravity, by observing an apple falling from the tree and then he observed that every falling object behaves in the same way.

Fig.n. Newton’s Diagram of a Planet with a mountain on it.

In the above figure drawn by Newton, there’s a planet with a tall mountain on it and imagine that you’re throwing a ball from point V with a lower velocity, it will fall on D having a lower distance from V. And as the velocity of throwing the ball is increased, it will fall farther from point D, say on E. And if you’re able to throw the ball with a velocity of 7.9km/sec, then the object will keep falling and returns to the initial point. He explained that the moon and other celestial bodies orbiting the earth are objects that are actually falling.

Fig.n. Explaining Gravity is not a force

The next chapter began in 1905, Einstein proposed the theory of relativity. The main aspect of this theory was that “The universe has a speed limit” which is about 3000km/sec which is the speed of light. In 1915, the proposed general relativity, in which he described that Gravity is NOT a Force. Gravity is the manifestation of curvature in space-time. It implied that matter curves space-time and space-time curved forms how matter moves. After the Einstein’s discovery first black hole was found. A black hole is a matter in space-time in which once you go inside the black hole, you can never escape. The boundary of a black hole is called Event Horizon. Now suppose you are driving a spaceship towards a black hole and after crossing the event horizon, no matter how much force you apply, you can’t escape. Similarly, if you light a torch from inside the black hole, the light will remain inside it as shown in the figure below.

Fig.n. Figure showing the event horizon of a black hole.

The concept of black holes started from the evolution of stars. Inside the star, there are two forces acting that is gravity which is pulling everything inside and the pressure which is pushing everything outside and both of these forces are in equilibrium. But as the fuel inside the star i.e. the nuclear fuel is exhausted and if the star is big enough, the star will compress and explode to form a supernova. And then there will be the core of the star that will keep collapsing in or out it and forms a black hole. Despite the complexity of formation of black holes, the final black hole that is created is specified by two numbers:

• ·         It’s Mass
• ·         It’s Spin.

Well precisely there are 3 numbers; the third one is the Electric charge. But it can be neglected as it is very small.

Fig.n. Black hole sucking matter from a star.

If black holes are black, how do we detect them? In the given figure, there’s a black hole and a star. And the black hole is spinning and you can see that it is continuing to strip off the matter from the star. And as it strips off the matter, and because the black hole is rotating, that matter gets swirl around the black hole and heats up and forms the plasma. These plasmas reach a very high temperature and start to emit X-rays. The first indirect attention of black holes came from measuring X-rays produced by black holes as in this picture. Through this approach, now we know that there are many black holes in the Universe and there’s also a massive black hole inside the Milky Way galaxy weighing 3 or 4 million times that of our Sun.

Now, let’s discuss the direct way. In 1916, general relativity predicts gravitational waves traveling at the spent of light. It says that when a matter moves through space-time, it produces gravitational waves. Similarly, the earth also releases gravitational waves, while orbiting the sun. But gravitational waves are small as gravitational forces are weak forces. Now, imagine two black holes are orbiting each other as shown in the figure below. 

Fig.n. Black holes orbiting each other.

And while orbiting they are emitting gravitational waves which are not very strong. But as they approach near to each other, the strain increases and then a stage comes when there is a merger and at that time the strain is the strongest. During the merger, the larger gravitational waves are generated. And after the merging of two black holes, a single one is formed where the strain becomes zero as shown in the figure below. The variation of strain in gravitational waves gives the idea about the presence of black holes in the Universe.

Fig.n. Graph showing variation in strain during merging of two black holes.

Until now we have found many black holes in the Universe but there are many more to be discovered. And still there’s one question which is unanswered and the question is that whether the black holes will bring nightmares for the Universe or they will lead us to some astonishing secrets of the Universe.