What is a Black Hole?

What is a black hole? Are they really exist?How big they are? These are the questions that I’ll try to answer in thisarticle. A black hole is a region in space-time with very strong gravity thateven light cannot escape. The gravity is so strong because matter has beensqueezed into tiny space. Black hole created when a star is dying, most or allof its mass compressed into a small area of space causing infinite space-timecurvature at that point. This point is called the ‘singularity’. Nothing, noteven light can escape from such massive space-time curvature.

Let me give you an example, if youtake an abject and squeeze it down in size, its gravity will increase and goup. If you keep compress the object, you will have an object with so much gravitythat the escape velocity is faster than light. You may ask yourself what arespace-time and escape velocity?

Space-time

In physics when we combine threedimensional space with time, then this coordinate system is called Space-Time.In this system time is playing the role for fourth dimension.

Escape velocity

The mass of the Earth creates gravity.When you throw a stone it will rise and travel a distance until the negativeacceleration of gravity stop it and returns it to Earth. So if you throw thestone with enough initial upward velocity so it can overcome the gravity’sdecreasing force and leave the earth behind. This initial velocity is calledescape velocity. It is the minimum velocity an object must have in order toescape the gravitational field of the earth.
Escape velocity on the surface of theEarth is about 7 miles per second (11 kilometer per second), or 25,000 milesper hour.

1/2 mv² = GMm/R
Where m is the mass of the object, M mass of the earth, G is the gravitationalconstant, R is the radius of the earth, and v is the escape velocity. Itsimplifies to:v = sqrt(2GM/R)

A black hole is made of three mainparts:

Outer layer or Event Horizon
Within the Event Horizon you wouldstill be able to escape from the gravity. Middle layer or Inner Event Horizon
The gravity in this layer is much strongerand does not allow any objects to escape from the gravity. At this pointobjects would begin to fall towards the center of the black hole. Center or Singularity

The Singularity is where the blackhole’s gravity is the strongest.

Black hole and General Relativity
General relativity or GR is a theoryof gravitation that was developed by Albert Einstein between 1907 and 1915. Einsteinconducted that the observed gravitational attraction between masses resultsfrom their warping of space and time. According to general relativity, asufficient compact mass will deform space-time to form a black hole. AfterEinstein’s publication of general relativity in 1916, Karl Schwartzchildproduced a solution to Einstein’s equation for a spherical mass.

r_s = 2GM/c²

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

When a black hole form?

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

Scientiststhink the smallest black holes formed when the universe began.
Stellarblack holes 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 anexploding star that blasts part of the star into space.
Scientists think supermassive black holes were made at the same time as thegalaxy they are in. Astronomersthink there is a supermassive black hole in the center of nearly every largegalaxy, including our own Milky Way.

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

Newton said that only objects withmass could produce a gravitational force on each other. Later Einsteindiscovered that gravity is produced by a curved space-time. Then he found outthat the mass and radius of an object actually curves space-time. We know thatthe stronger the gravitational field of an object, the more the space aroundthe object is curved. It means straight lines no longer straight if exposed toa strong gravitational field, instead, they are curved. Since light ordinarilytravels on a straight-line path, light follows a curved path if it passesthrough a strong gravitational field. This is what is meant by "curvedspace," and this is why light becomes trapped in a black hole.

What happens if we fall into a blackhole?

Once youfall in a black hole you can never get back out and you probably would be deadbefore you get there. As you approach a stellar-mass black hole feet-first, theforce of gravity on your feet can be thousands of times stronger than the forceon your head! This has the effect of stretching you, pulling you apart liketaffy. Tongue-in-cheek, scientists call this “spaghettification.” By the timeyou reach the black hole, you’ll be a thin stream of matter many miles long.

You maynot even make it that far. Some black holes greedily gobble down matter,stealing it from an orbiting companion star or, in the case of supermassiveblack holes, from surrounding gas clouds. As the matter falls in, it piles upinto a disk just outside the hole. Orbiting at huge speeds, the matter in thisaccretion disk gets extremely hot—even reaching millions of degrees. It willspew out radiation, in particular high-energy X-rays. Long before the blackhole could rip you apart you’d be fried by the light.
Butsuppose you somehow manage to survive the trip in. What strange things awaityou on your way down into forever? Once youpass the point where the escape velocity is faster than light, you can’t getout. This region is called the event horizon. That’s because no informationfrom inside can escape, so any event inside is forever beyond our horizon. Thenearest black hole is many lightyears away, so we don't have to worry aboutthreats to the Earth. This is as close as you'll ever get to one.

How we canfind black holes in universe?

We knowthat no objects or matters even light can escape from a black hole’sgravitational force, so they are in absolute darkness. Then how we can findthem in universe?
Some starsform in pairs, called binary systems, where the stars orbit each other. Even ifone 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 thenormal star and determine the mass of the black hole. The othermethod for astronomers is trough the X-Ray. If a blackhole is “eating” matter from a companion star, that matter gets very hot andemits 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 bespotted this way. EXIST is a spacecraft, designed by NASA to detect tens ofthousands of black holes, some of which may be billions of light years away.There isanother question arises, are the black holes are really blacks? and here is theNASA’s answer to this question:

Surprisingly,black holes may not be totally black!

· Infalling materialcan get hot enough to glow.

· Sometimes blackholes are so bright they can outshine an entire galaxy.

· Supermassiveblack holes can be so luminous we can see them from distances of billions oflight years.

· The birthof a stellar-mass black hole produces a flash of radiation so bright it canoutshine entire galaxies, and be seen clear across the observable Universe!

Black holesand time travel

You probablyhave seen the Star Trek series and how the Enterprise uses the black holes totravel through the space-time. But in reality can we travel through the space-time?
It probablywon’t work. There is a theory that a black hole can form a tunnel in spacecalled a wormhole (because it’s like a tunnel formed by a worm as it eats itsway through an apple). If you enter a wormhole, you’ll pop out someplace elsefar away. While wormholes appear to be possible mathematically, theywould be violently unstable, or need to be made of theoretical forms of matterwhich may not occur in nature. The bottom line is that wormholes probably don’texist.

White hole

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

“Theequations of general relativity have an interesting mathematical property: theyare symmetric in time. That means that you can take any solution to theequations and imagine that time flows backwards rather than forwards, andyou'll get another valid solution to the equations. If you apply this rule tothe solution that describes black holes, you get an object known as a whitehole. 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 whichnothing can fall. In fact, just as a black hole can only suck things in, awhite hole can only spit things out.” The white hole may be somewhere very faraway from the black hole; indeed, it may even be in a "differentUniverse" -- that is, a region of space-time that, aside from the wormholeitself, is completely disconnected from our own region. A conveniently-locatedwormhole would therefore provide a convenient and rapid way to travel verylarge distances, or even to travel to another Universe. Maybe the exit to thewormhole would lie in the past, so that you could travel back in time by goingthrough. All in all, they sound pretty cool.”- Ted Bundy, Associate Professor,physics department, Richmond University.

It is reallyamazing how the Universe works. In the end we can say black holes are theultimate endpoints of matters. They twist space and time and along with ourimagination.

Like I saidbefore our World is beautiful.