30 September 2011

What is a Black Hole?

What is a Black hole? Do they really exist? How big they are? These are the questions that I’ll try to answer in this article. A black hole is a region of space-time with very strong gravity that even light cannot escape. The gravity is so strong because the matter has been squeezed into a tiny space. Black hole created when a star is dying, most or all of its mass compressed into a small area of space causing infinite space-time curvature at that point. This point is called the Singularity. Nothing, not even light can escape from such massive space-time curvature.

Let me give you an example, if you take an object and squeeze it down in size, its gravity will increase and go up. If you keep compressing the object, you will have an object with so much gravity that the escape velocity is faster than light. To understand the concept more clearly you need to know what are the space-time and escape velocity?


In physics when we combine three dimensional space with time, then this coordinate system is called Space-Time. In this system time is playing the role for the fourth dimension. Using this approach and creating a single four dimensional coordinate, scientists are able to simplify various physics theories especially in the cosmology.

Escape velocity

The mass of the Earth creates gravity. When you throw a stone it will rise and travel a distance until the negative acceleration of gravity stop it and returns it to Earth. So if you throw the stone with enough initial upward velocity, it can overcome the gravity’s decreasing force and leave the earth behind. This initial velocity is called Escape Velocity. It is the minimum velocity an object must have in order to escape the gravitational field of the earth.

Escape velocity on the surface of the Earth is about 7 miles per second (11 kilometer per second), or 25,000 miles per hour.

1/2 mv2 = GMm/R

Where m is the mass of the object, M mass of the earth, G is the gravitational constant, R is the radius of the earth, and v is the escape velocity. It simplifies to:

v = sqrt(2GM/R)

A black hole is made of three main parts or layers which are:

  • Outer layer or Event Horizon
  • Middle layer or Inner Event Horizon
  • Center or Singularity

Outer layer or Event Horizon

The first and outer layer of a black hole is called Event Horizon. In this region the escape velocity is greater than the speed of light. Within the Event Horizon you would still be able to escape from the gravity.

Middle layer or Inner Event Horizon

The gravity in this layer is much stronger and does not allow any objects to escape from the gravity. At this point objects would begin to fall towards the center of the black hole.

Center or Singularity

The Singularity is where the black hole’s gravity is the strongest. In this region the space time curvature becomes infinite. The center of a black hole also has zero volume.

Black hole and General Relativity

General relativity or GR is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915. Einstein conducted that the observed gravitational attraction between masses results from their warping of space and time.

According to general relativity, a sufficient compact mass will deform space-time to form a black hole. After Einstein’s publication of general relativity in 1916, Karl Schwartzchild produced a solution to Einstein’s equation for a spherical mass.

rs = 2GM/c2

G is the gravitational constant, M is the mass, and c is the speed of light. From this expression, for a certain radius, the denominator of the term would become zero, which causes the term to blow up mathematically. With the help of Schwartzchild’s expression physicians were able to understand the black hole theory.

When a black hole form?

The most common way for a black hole to form is in a supernova, an exploding star. When a star with about 25 times the mass of the sun ends its life, it explodes. The outer part of the star screams outward at high speed, but the inner part of the star, its core, collapses down. If there is enough mass, the gravity of the collapsing core will compress it so much that it can become a black hole. 

According to the cosmologist, the smallest black holes formed when the universe began. Stellar black holes (black hole formed by gravitational collapse of a massive star) are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

Scientists also believe that supermassive black holes were made at the same time as the galaxy they are in. Astronomers think there is a supermassive black hole in the center of nearly every large galaxy, including our own Milky Way.

How light trapped by black hole’s gravity force when it has no mass?

Newton said that only objects with mass could produce a gravitational force on each other. Later Einstein discovered gravity is produced by a curved space-time. Then he found out that the mass and radius of an object actually curves space-time. We know the stronger the gravitational field of an object, the more the space around the object is curved. It means straight lines no longer straight if exposed to a strong gravitational field, instead, they are curved. Since light ordinarily travels on a straight-line path, light follows a curved path if it passes through a strong gravitational field. This is what is meant by curved space, and this is why light becomes trapped in a black hole.

What happens if we fall into a black hole?

Once you fall in a black hole you can never get back out and you probably would be dead before you get there. As you approach a stellar-mass black hole feet-first, the force of gravity on your feet can be thousands of times stronger than the force on your head and has the effect of stretching you and pulling you apart. This is what the scientists called the spaghettification. By the time you reach the black hole, you’ll be a thin stream of matter many miles long.

You may not even make it that far. Some black holes gobble down matter, stealing it from an orbiting nearby star or, in the case of supermassive black holes, from surrounding gas clouds. As the matter falls in, it loads up into a disk just outside the hole. Orbiting at enormous speeds, the matter in this accretion disk gets extremely hot (reaching millions of degrees). It will emit radiation, in particular high-energy X-rays. Long before the black hole could tear you apart you’d be burned by the light.

Let assume that you somehow survive this journey, then what will happen during your trip into the deep down of the black hole?

Once you pass the point where the escape velocity is faster than light, you can’t get out. This region is called the event horizon. That’s because no information from inside can escape, so any event inside is forever beyond our horizon and observable.

The nearest black hole is many lightyears away, so we don't have to worry about threats to the Earth. This is as close as you'll ever get to one.

How we can find black holes in the universe?

We know that no objects or matters even light can escape from a black hole’s gravitational force, so they are in absolute darkness. Then how we can find them in the universe?

Some stars form in pairs, called binary systems, where the stars orbit each other. Even if one of them becomes a black hole, they may remain in orbit around each other. By carefully observing such a system, astronomers can measure the orbit of the normal star and determine the mass of the black hole.

The other method for astronomers is trough the X-Ray. If a black hole is eating matter from a companion star, that matter gets very hot and emits X-rays. This is like a signature identifying the source as a black hole.

In fact, black holes are so good at emitting X-rays that many thousands can be spotted this way. EXIST is a spacecraft, designed by NASA to detect tens of thousands of black holes, some of which may be billions of light years away.

There is another question arises, are the black holes are really blacks? and here is the NASA’s answer to this question:

  • Surprisingly, black holes may not be totally black!
  • Infalling material can get hot enough to glow.
  • Sometimes black holes are so bright they can outshine an entire galaxy. Supermassive black holes can be so luminous we can see them from distances of billions of light years.
  • The birth of a stellar-mass black hole produces a flash of radiation so bright it can outshine entire galaxies, and be seen clear across the observable Universe!

Black holes and time travel

You probably have seen the Star Trek series and how the Enterprise uses the black holes to travel through the space-time. But in reality can we travel through the space-time?

It probably won’t work. There is a theory that a black hole can form a tunnel in space called a wormhole (because it’s like a tunnel formed by a worm as it eats its way through an apple). If you enter a wormhole, you’ll pop out someplace else far away. While wormholes appear to be possible mathematically, they would be violently unstable, or need to be made of theoretical forms of matter which may not occur in nature. The bottom line is that wormholes probably don’t exist.

White hole

From the general relativity, it is a hypothetical region of space-time which cannot be entered from the outside, but from which matter and light may escape and it is the reverse of a black hole.

Again from the point view of the general relativity, equations mathematically are symmetric in time. It means any solution of the certain equation and consider the time flows backwards rather than forwards. In this approach, you get another solution to the equation. Now if you apply this method for the solution of the black hole, you will get the White Hole

Since a black hole is a region of space from which nothing can escape, the time-reversed version of a black hole is a region of space into which nothing can fall. In fact, just as a black hole can only suck things in, a white hole can only spit things out. A conveniently-located wormhole would therefore provide a convenient and rapid way to travel very large distances, or even to travel to another Universe. - Ted Bundy, Associate Professor, physics department, Richmond University.

It is really amazing how the Universe works. In the end we can say black holes are the ultimate endpoints of matters. They twist space and time and along with our imagination. Amazing science facts!