What Is A Black Hole?

 Timeline of Stars.

Stars are massive fiery balls of burning gas but  this fierce display of burning fuels does not last forever. Eventually, a time comes when the star ends up all its fuel which powered the fusion. So the pressure due to burning of gasses also keeps decreasing as the time goes on. Then, the Gravity collapses the remaining matter together when the outward pressure of burning gasses is no more sufficient to counter balance the gravity of the star. For small stars (like our sun or the stars which don't have enough mass to undergo supernova) first bulge into a hot glowing red giant and then cools down as white dwarf where as for massive stars, what happens next is a display of extremes. First, the star explodes in a supernova, scattering most of its matter throughout the universe. This explosion is so bright that for a few days, the dying star outshines its entire galaxy. After the supernova explosion is over, the light fades and darkness returns and  the remaining matter forms an object so dense that anything that gets too close will completely disappear. The object formed is a black hole.

This is the first ever image of a black hole M87. 
Image source : jpl.nasa.gov

History of Black Holes.

 The notion of black hole dates back in 18th century.  In 1687, Isaac Newton published his historic paper "The Principia". In his book, he detailed the laws of motion and the universal law of gravitation. Newton derived the notion of escape velocity which is the launch speed required to break free from the pull of gravity. In 1783, the English clergyman John Michell found that a star which is 500 times larger than our sun would have an escape velocity greater than the speed of light. He called these giant objects “dark stars” because they could not emit starlight. Then this idea lay dormant for almost a century. After more than a century of silence over this topic in 20th century, Albert Einstein developed two theories of relativity that changed the way we see space and time: the special theory and the general theory. The special theory is famous for the equation E=mc^2. The general theory painted a new and different picture of gravity. According to the general theory of relativity, matter and energy bend space and time around it. Because of this, objects which travel near a large mass will appear to move along a curved path because of the bending of spacetime. We call this effect gravity. One of the consequences of this idea is that lightis also affected by gravity. After all, if spacetime is curved, then everything which either posses maas or energy must follow along a curved path, including light.

 Newton’s theory of gravity could be expressed using a simple formula where as Einstein’s theory required a set of complex equations known as the “field equations”. Just after few months of Einstein’s publication, the German scientist Karl Schwarzschild found a surprising solution. Solving  one the case of the field equations, an extremely dense ball of matter creates a spherical region in space where nothing can escape, not even light. This was a astonishing result, but a question raised that did such things actually exist? At first, the idea of a black sphere in space-time from which nothing could escape was considered purely a mathematical result, but one which would not really happen. However, as the years passed, our understanding of the life cycle of stars grew. It was observed that some of the dying stars became pulsars, another exotic object predicted by theory. These all pointed towards one thing - the existence of dark stars could actually be real. So, these alien spherical objects  were named “black holes,” and scientists began the quest for finding them, describing them and understanding how they are created. 

But there was an issue, how would you find an object in space which is completely black? Well, there was an elegant solution to it, as black holes have a large mass, they also have a large gravitational field. So, if we are unable to see a black hole, we can look for its gravity which it will be exerting on its neighbors. So, using this concept, astronomers searched for a place where a visible star and a black hole were in close proximity to one another. One such place is binary stars (it is a system of two stars orbiting one another). They can be spotted in many ways. We can search for stars that change position back and forth ever so slightly. Alternatively, if you observe a binary star from the side, the brightness will change when one star passes behind the other. So it’s possible that somewhere in space, there’s a binary star consisting of a black hole and a visible star. In fact, such binary systems have been observed! Astronomers have found stars orbiting an invisible companion. From the size of the visible star and its orbit, astronomers calculated the mass of its invisible neighbour. It fit the profile of a black hole. As we can’t see a black hole, is there a way to find its size? From Einstein’s field equations, we know that given the mass of a black hole, we can determine the size of the sphere that separates the region of no escape from the rest of space. The radius of this sphere is called the Schwarzschild radius (R_s)  in honor of Karl Schwarzschild. The surface of the sphere is called the event horizon. 

                     R_s = 2GM/c^2. 

If anything crosses the event horizon, it’s gone for ever hidden from the rest of the universe. This means, once you know the mass of a blackhole, you can compute its size using a simple formula. And it’s actually quite easy to measure the mass of a black hole. Just take a standard issue space probe and shoot it into orbit around the black hole. Just like any other system of orbiting bodies , like the Earth orbiting the Sun, or the Moon orbiting the Earth — the size and period of the orbit will tell you the mass of the black hole. If you don’t have a space probe handy, then compute the mass and orbit of a companion star and use that to find the Schwarzschild radius. Black holes come in many sizes. If it was made from a dying star, then we call it a “stellar mass” black hole, because its mass is in the same range as stars. But we can go bigger - much bigger.

 Properties Of Black Holes.

 Now, we are going to the center of a galaxy. Hundreds of billions of stars in a galaxy, all orbiting a central point. Scientists now assume that there exists a black hole in the center of most of which we call a “supermassive black hole,” because of its tremendous mass. The size may vary from hundreds of thousands to even billions of solar masses. For example, at the center of our own Milky Way galaxy is a supermassive black hole with a mass 4 million times that of our sun. Black holes have few properties that we can measure- like their spin, just like the planets, stars rotate. And different stars spin at different speeds. Imagine we can adjust the size of this star but keep the mass constant while if we increase the radius, the spinning slows down If we decrease the size, the spinning speeds up. But while the rotational speed can vary, the angular momentum never changes, it remains constant. Even if the star were to collapse into a black hole, it would still have angular momentum. We could measure this by firing two probes into opposite orbits close to the black hole. Because of their angular momentum, black holes create a spinning current in spacetime.The probe orbiting along with the current will travel faster than the one fighting it, and by measuring the difference in their orbital periods we can compute the black hole’s angular momentum. This spacetime current is so extreme it creates a region called the ergosphere where nothing, including light, can overcome it. Inside the ergosphere, nothing can stand still. Everything inside this region is dragged along by the spinning spacetime. The event horizon fits inside the ergosphere, and they touch at the poles. So in one sense, black holes are like whirlpools of spacetime.

The one last property of black holes we can measure is the electric charge. Mostly, we encounter with uncharged or neutral matter, a black hole may have a net positive or negative charge. We can easily measure this quantity by seeing how hard the black hole pulls on a magnet. But, in general charged black holes don't exist in nature because the universe is teeming with charged particles, so a charged black hole would simply attract oppositely charged particles until the overall chargeis neutralized.

 So, basically there are 3 fundamental properties of a black hole we can measure - mass, angular momentum, and electric charge. According to physicists, once we know these three values, we can completely describe the black hole. This result is humorously known as the “no hair theorem,” since other than these 3 properties, black holes have no distinguishing characteristics. 

How does it look inside? 

We haven't discovered the inside of the black hole yet because we can’t send a probe inside to take a look. Once any instrument crosses the event horizon, it’s gone. But! Don’t forget we have Einstein’s field equations. If these correctly describe space time outside the black hole, then we can use them to predict what’s going on inside as well. To solve the field equations, scientists considered two separate cases of rotating black holes, and non-rotating black holes. Non-rotating black holes are simpler and were the first to be understood. In this case, all the matter inside the black hole collapses to a single point in the center, called a singularity. At this point, spacetime is infinitely warped. Rotating black holes have a different interior. In this case, the mass inside a black hole will continue to collapse, but because of the rotation it will coalesce into a circle, not a point. This circle has no thickness and is called a ring singularity. 

Black hole research continues to this day. Scientists are actively investigating the possibility that black holes appeared right after the big bang, and the idea that black holes can create bridges called worm holes connecting distant points of our universe.