Monday, Sep 3 2001
Black Holes
- Divya ThakurDivya Thakur is a high school senior in Austin, TX who aspires to major in aeronautical engineering. Fierce with dreams and zeal, Divya is still looking for a way to combine her interests in science, writing and the White House (in that order), though still in vain. When asked for the likelihood of her finidng such a profession she quoted, "It is hard to find a cat in a dark room especially when there is no cat"-Confucious. Whatever her future might hold in store for her, she knows never to lose her main goal in life, that is to invent a new monetary value, a quadrillion, herself being the sole proprieter.
|
 |
Every rule has an exception, every grouping a dissident. Of all the stellar phenomena that plague our Universe, ad infinitum, Black Holes are one of the most perplexing. In their realm, Newton's Laws of Gravity do not apply, General Relativity has absolutely no relevance; the general laws of physics are significantly violated, and our usually harmonic colossal Universe is proved once again to be royally chaotic and unreliable. So what exactly are black holes? Why are they a cause of such a confoundation? Linger around further and acquaint yourself with these wonderful entities.
|
|
The Schwarzchild Black Hole
A black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull, not even light. To understand this concept we might have to look at gravity from a more simplistic perspective.
|
|
|
|
Ground Based Composite Visual/Radio View: The giant elliptical galaxy NGC 4261 is one of the twelve brightest galaxies in the Virgo cluster, located 45 million light-years away. Photographed in visible light (white) the galaxy appears as a fuzzy disk of hundreds of billions of stars. A radio image (orange) shows a pair of opposed jets emanating from the nucleus and spanning a distance of 88,000 light-years.
|
Imagine throwing a tennis ball into the air. As the ball rises higher, earth's gravitational pull will slower its speed until it starts to fall down again. Now if you threw the ball harder, it will go higher before it turns and falls toward the earth's surface. If you threw the ball hard enough you could make it escape the planet's gravity entirely. It could keep rising forever and the gravitational attraction would not be strong enough to pull it back down. The speed with which you need to throw the ball in order that it just barely escapes the planet's gravity is called the "escape velocity". As suspected, the escape velocity depends on the planet's mass: a more massive planet will have stronger gravity and thus a higher escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the escape velocity. The Earth's escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second (about 5300 m.p.h.).
As a body is compressed and its mass becomes increasingly concentrated into an ever smaller region of space, it's gravitational attraction increases, and consequently the escape velocity gets bigger. Things have to be thrown harder and harder to escape the planet's gravity field. The German physicist, Dr. Karl Schwartzchild, in 1916 found that when an enormous object goes through such a process, a point is reached eventually when its escape velocity becomes greater than the velocity of light. And according to Einstein's theory since nothing can travel faster than light, nothing can escape the object's gravitational field. Even a beam of light directed out into space will fall back to the ground. Such a phenomenon is called a black hole.
Black holes are conjectured to form once stars of two or three times the mass of the sun exhaust their supply of fuel. They begin to collapse inward with such tremendous force that even the powerful internuclear forces within the atoms of the star aren't sufficient to prevent it from continuing to fall in on itself until the entire mass of the star is concentrated at a point called a singularity. When the collapsing star passes its Schwartzschild radius(the point where its escape velocity equals the speed of light), it vanishes from view because its escape velocity has exceeded the speed of light.
|
|
|
HST Image of NGC 4261: A giant disk of cold gas and dust fuels a possible black hole at the core of this galaxy. Estimated to be 300 light-years across, the disk is tipped enough (about 60 degrees) to provide astronomers with a clear view of the bright hub, which presumably harbors the black hole. The dark, dusty disk represents a cold outer region which extends inwards to an ultra-hot accretion disk with a few hundred million miles from the suspected black hole. This disk feeds matter into the black hole, where gravity compresses and heats the material. Hot gas rushes from the vicinity of the black hole's creating the radio jets. The jets are aligned perpendicular to the disk, like an axel through a wheel. This provides strong circumstantial evidence for the existence of black hole "central engine" in NGC 4261.
|
The Strangeness that exists within
Immensely massive objects distort space and time, so that the usual rules of geometry cannot be applied. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very unusual properties. A black hole has something called an 'event horizon.' This is a spherical surface that marks the boundary of the black hole, where the escape velocity equals that of light. No knowledge of events beyond that boundary can ever be known by the outside world. You can pass in through the horizon, but you can't get back out. Once you've crossed the horizon, you're doomed to move inexorably closer and closer to the 'singularity' at the center of the black hole.
Within the singularity, matter is infinitely compressed into a region of infinite density. At the singularity, gravity is infinite. Space-time has become infinitely curved. At the present time, science has no tools to describe conditions within the singularity. All laws of physics lose meaning in such a region.
What would happen if one fell into a black hole?
Let's suppose you, the exceptionally daring commander of the starship Hapless get into your spaceship and journey into a Schwarzchild black hole. What would you experience?
At first, you don't feel any gravitational forces at all. Typical to any spaceship lodger, you will feel weightless because you are in constant free fall. Closing into the black hole's vicinity, you will pass the photon-sphere. This region usually situated at a distance of about 1.5 times the radius of the event horizon, is where the black hole's gravitational pull isn't strong enough to pull light into the event horizon, yet strong enough to prevent it from escaping. Photons of light are forever trapped inside this region, destined to orbit the black hole forever. Passing through the photon-sphere you would be flooded with intensely bright light. As you leave the brightness, you would find yourself in utter darkness. Your spaceship's velocity would increase to unimaginable levels. Approaching the event horizon, you would be subjected to "tidal" gravitational forces (the same that cause oceanic tides on earth) of astronomical proportions. Your feet would seem to weight uncounted trillions of tons more than your head. In a blinding instant, you would be disintegrated into atoms. You would then crash into the singularity, where your mortal remains would be summarily smashed out of existence.
What would you have seen as you were falling in? Images of faraway objects may be distorted in bizarre ways, since the black hole's gravity bends light. Even after you've crossed the horizon, you can still see things on the outside: after all, the light from the things on the outside can still reach you. No one on the outside can see you, of course, since the light from you can't escape past the horizon.
What will a far away observer see?
Let's say Dunlop, the captain of the starship Enterprise happens to be observing your hapless journey. The victim of an opitcal illusion, he will see things quite differently from you. Time, as you know it, will seem to pass at a horse and buggy pace for you. This is on account of the fact that the light you emit will be slowed by the formidable gravitational force of the black hole and will take a lot longer to climb back out. So as you get closer and closer to the horizon, Dunlop will see you move slower and slower. In fact, the radiation you emit right as you cross the horizon will hover right there at the horizon forever and never reach him. So in Donlap's eyes you will never quite reach the horizon of the black hole. Of course, you've long since passed through the horizon, but the light signal telling her that won't reach her for an infinitely long time.
Now in practice, you will actually become invisible to Dunlop after some time has elapsed. Light is "redshifted" to longer wavelengths as it rises away from the black hole. So if you are radiating visible light at some particular wavelength, Dunlop will see light at a wavelength longer than what you emit. The wavelengths get invariably longer and longer as you close in with the horizon. Eventually, it won't be visible light at all: it will be infrared radiation, then radio waves. Inevitably, the wavelengths will be so long that he'll be unable to observe them.
So what is the significance of black holes in cosmology? It is simply yet another reminder of the inexplicabilities that lie in abundance in our interminable universe. To date, most of our knowledge of these mysterious objects remains theoretical. The distortions in the structure of space-time caused by these objects give rise to many strange and almost inconceivable phenomena, white holes, worm holes, time travel and all the goodies our small minds can delve.
Works Cited
Graphics credit
 View
and Post comment on this article
The contents of the article are Copyright © of
the author and may not be reproduced in any form without prior
written permission from the author.
|
