If one has ever played at a slot machine in a casino, one knows that the money put inside usually disappears once that lever is pulled. There are no returns and you are left looking foolishly at the machine and inevitably trot off to get some more spare change in the hopes of recovering your coins. The perfect term to describe a slot machine in a casino is a black hole.

What then is a black hole? We have all heard about them, some of us have a vague notion about a black hole but what does it imply? To put it in a single sentence, a black hole is a region in space-time which has a gravitational pull so strong, that not even light can escape it. That sentence probably raises more questions. And there are answers. And, as always, the profound genius of Albert Einstein is deeply involved.

In 1915, Einstein developed his theory of general relativity which incorporate 10 equations that describe how gravity is the result of space and time being curved by matter and energy. The simplest way to perceive gravity as a result of an interaction between matter and space-time is to imagine a sheet of rubber held taut at all ends. If a ball is placed in the center, there will be a bulge in the sheet. If a smaller marble was placed on the sheet, it would inevitably fall towards the large ball. This, according to Einstein, is how gravity works. If you imagine the Sun to be the ball in the center and the marble to be Earth, you can understand how gravity works and is actually a result of this humongous mass of the sun literally bending space and time.

Now coming back to Enistein’s field equations, Karl Schwarzschild found a solution to these equations that described the gravitational field of a point mass. This, indeed, is what a black hole essentially is. What the solutions said were that there could theoretically be some regions in space where a point could have infinite density and zero volume and this point is called a singularity.

The existence of black holes was predicted in 1916 but at the time scientists were not even certain of the existence of other galaxies. It took Edwin Hubble to prove conclusively that other galaxies existed and that the universe is far larger than we even imagined. And that wasn’t all; his work also led to the theory of metric expansion of space. What that means is that space itself is expanding (incidentally the 2011 Nobel prize in physics was awarded to scientists who proved that this expansion is occurring at an accelerating rate). This has profound implications. If the universe is expanding, that means that there must have been a time when it was infinitesimally small. And this conclusion is what makes black holes even more interesting.

How does a black hole come to be? Let’s suppose you have a table and you start to load it with heavy books. The internal force of the table will keep it propped up to a point when the books are too heavy and the table collapses to the ground under the force of gravity, which is pulling down the books towards the center of the earth. Now in space there is no “ground”. If a spherical body is so overloaded, there will be a point when it’s internal force is utterly powerful against the forces of gravity and it collapses – into itself.

A black hole is formed when certain types of stars die. A star is nothing but a giant nuclear reactor in space. The fusion process in the core of a star converts matter into energy and also resists the force of gravity – which tends to make the star collapse in on itself. But something’s got to give and when the star eventually runs out of fuel, gravity wins. The core collapses in on itself so that a mass of several times the mass of the sun is squeezed into a point of infinite density – a singularity. The outer layers of the star are shed in a spectacular supernova explosion, which sometimes outshine the entire parent galaxy. What remains is the black hole.

N0w the black hole is not just the singularity itself. There is another important term that concerns black holes and that is the event horizon. The escape velocity of earth is 11 km/sec. That means that a spaceship has to exceed this speed to break out of earth’s gravitational field. The escape velocity of the sun is around 600 km/sec. Generally, the heavier the object, the higher the escape velocity. What if you had an object that was so heavy and dense that it’s escape velocity exceeded the speed of light? Since nothing can go faster than the speed of light, nothing can escape the gravitational pull of this dense object and this is another way to understand the black hole. The event horizon of a black hole is a region around the black hole beyond which light cannot escape. Nothing inside the event horizon can be perceived by an outside observer. If we send a probe to a black hole, once it has passed the event horizon, it is lost to us forever. And strangely, inside the event horizon, space and time do not make sense. Time literally stops.

When we see an object, why we see it is because light rays are reflected from that object. But in a black hole, any light that falls onto it, can never escape it. Hence a black hole can never be seen. How then can a black hole be detected? For one, if there is a black hole between the observer and a region of space, the light from the space beyond the black hole is bent by the gravity of the black hole. This gravitational “lensing” points to the existence of a black hole.

So there’s the lowdown. Black holes are quirky objects in space which mess up space-time. But what makes them really interesting and ties them so deeply to our own questions about the origin of the universe? Theories now suggest the existence not of just our own universe, but many universes – a multiverse in which our universe is just one.

What happens inside the singularity? Physics as we know it loses it’s meaning. Could it be that a black hole contains a wormhole to another point in our universe or another universe altogether? What if, at the end of a black hole is a white hole? A singularity into which nothing can go in. Now the true implications of the theory of metric expansion of space come to the fore. If space is currently expanding, at some point it must have been a singularity. Everything was expelled outwards from this singularity. Could the creation of our universe merely be the outcome of the formation of a black hole in another? Could the millions of black holes that exist in space point to the creation of millions of different universes? One can only dream about finding out.

Till then, I shall go get my quarters for the slot machines.