WHAT IS A BLACK HOLE?

Kirti Jadhav

“None of the universal laws apply to them. They are the rebels who got away.”

 

Black holes are the weirdest objects you could ever stumble across in the universe. What makes them so weird is that we know very little about them because all the laws of physics break down near a gravitational singularity. In this article, we would like to provide an insight into what Black holes are. Let’s go back to the 18th century where Newton’s law of gravitation was popular and gravity was considered as a ‘force’ which caused matter in the universe to attract each other. Newton imagined that if you throw a cannonball hard enough, it will eventually escape the earth’s gravitational pull (the concept of escape velocity). In the year 1783, an English natural philosopher introduced the concept of dark stars. In a paper of Philosophical Transactions of the Royal Society of London, he wrote that there might exist some stars heavier than the sun, whose gravitational pull could be so strong that it’s escape velocity might be greater than the speed of light. Furthermore, Laplace referred to black holes as black stars, writing, “It is. . . possible that the greatest luminous [light-filled] bodies in the universe are invisible.” Unfortunately, dark star theory was based on the corpuscular theory of light and it was rejected.

 

 Years after that, Einstein proposed his general theory of relativity where he said that space-time is like a fabric. “Space-time tells matter how to move; matter tells space-time how to curve”. So, if you place an object having some mass, it will curve the space-time around it, and these curvatures describe the motions of the other celestial bodies revolving around the object. Inspired by Einstein’s general theory of relativity, a German physicist Karl Schwarzschild introduced the concept of something called ‘Schwarzschild Radius’.

It is the radius below which the gravitational attraction between the particles of a body must cause it to undergo irreversible gravitational collapse forming a black hole. This is the end of most of the massive stars in the universe. However, only the collapse of stars doesn’t need to form a black hole. If you crush the earth to the size of 2cm, it will form a black hole.

 

If you’re having a hard time visualizing, imagine that you are near the edge of the Niagara Falls. You will be safe if you row your boat fast enough and escape the water current. But, once you cross the edge you will go down the Niagara Falls. This is similar to a Black Hole. Now that you have a fair idea of what a black hole is, let’s take a look inside them. As you can see in the diagram above, the event horizon is what forms ‘the point of no return’. So, Black holes aren’t black, it’s said so because light cannot escape it. If you cross the event horizon, you will fall in the black hole. However, there are hypothetical particles called Tachyons, faster than light which could escape a Black Hole’s gravity. The singularity is the most mysterious part because space-time curves there infinitely. No one knows what happens inside a black hole. Many Sci-fi movies portray them as portals to another universe. They are an epitome of how amazing the universe is and how little we know. As Stephen Hawking once said, “Sometimes God doesn’t play dice, he sometimes confuses us by throwing them where they can’t be seen”. So remember to look up at the stars because somewhere something is waiting to be found!

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