A black hole is a region of space from which nothing, not even light, can escape.A black hole is a super dense object that has an intense gravitational pull. There are two parts to a black hole, a singularity and a event horizon. If you were to take a slice of a black hole right through its center it would look like this:
The event horizon is where the force of gravity becomes so strong that even light is pulled into the black hole. Although the event horizon is part of a black hole, it is not a tangible object. If you were to fall into a black hole, it would be impossible for you to know when you hit the event horizon.
The singularity is not really a tangible object either. According to the General Theory of Relativity the Singularity is a point of infinite space time curvature. This means that the force of gravity has become infinitely strong at the center of a black hole. Everything that falls into a black hole by passing the event horizon, including light, will eventually reach the singularity of a black hole. Before something reaches the singularity it is torn apart by intense gravitational forces. Even the atoms themselves are torn apart by the gravitational forces.
It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.
How Are Black Holes Formed?
The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.
Imagine a star which is much more massive than our sun, and which has a mass, called the critical mass, which is large enough to cause a black hole to form. What keeps this star from collapsing onto itself and becoming a black hole? The answer is that there is an intense pressure caused by nuclear reactions within the sun. When the fuel that feeds the nuclear reactions gets used up the massive star cannot support itself anymore. It then collapses to form a black hole.
Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius.
This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects,
and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon.
There are actually many methods used to see if black holes really exist in our universe. The first method is to look for objects in our universe that have a lot of mass, but are very small. For example we can prove that there exists a black hole in an astronomical object called M87. This object weighs three billion times more than our sun, but takes up a volume no larger than our solar system.
Another method of finding black holes is to look for an acceleration of matter. Since black holes have such strong gravitational fields, they accelerate anything that gets near them to great speeds. Rapid acceleration of an object can be observed by looking for doppler shifts in the light given off by an accelerating object.
( Explanation: This artistic image is actually the signature of a supermassive black hole in the center of distant galaxy M84 - based on data recently recorded by Hubble's new Space Telescope Imaging Spectrograph (STIS). Very near black holes the force of gravity is so strong that even light can not escape ... but the presence of a black hole can also be revealed by watching matter fall into it. In fact, material spiraling into a black hole would find its speed increasing at a drastic rate. These extreme velocity increases provide a "signature" of the black hole's presence. STIS relies on the Doppler effect to measure gas velocity rapidly increasing to nearly 240 miles per second within 26 light years of the center of M84, a galaxy in the Virgo Cluster about 50 million light years away. The STIS data show that radiation from approaching gas, shifted to blue wavelengths left of the centerline, is suddenly redshifted to the right of center indicating a rapidly rotating disk of material near the galactic nucleus. The resulting sharp S-shape is effectively the signature of a black holes estimated to contain at least 300 million solar masses. Do all galaxies have central black holes? )
Cool Things About Black Hole
First of all, if you get close enough to a black hole you will see the back of your own head! This effect, called an Einstein ring, is caused by the intense gravity around a black hole. When you are near a black hole at certain distances the light that leaves from the back of your head will travel though space that is bent so much by gravity that it will enter your eyes.
Another cool thing about black holes is that they might be able to destroy information. The destruction of information is not allowed by quantum mechanics, so Hawking concludes that the usual rules of quantum mechanics cannot apply for black holes.
Black holes aren’t always dangerous.
Having said that, let me ask you a question: if I were to take the Sun and replace it with
a black hole of the exact same mass, what would happen? Would the Earth fall in, be flung away, or just orbit like it always does?
Most people think the Earth would fall in, sucked inexorably down by the black hole’s powerful gravity. But remember, the gravity you feel from an object depends on the mass of the object and your distance from it. I said the black hole has the same mass as the Sun, remember? And the Earth’s distance hasn’t changed. So the gravity we’d feel from here, 150 million kilometers away, would be exactly the same! So the Earth would orbit the solar black hole just as nicely as it orbits the Sun now.Of course, we’d freeze to death. You can’t have everything.
The nearest black hole is 1,600 light years away. That is about 16 quadrillion kilometers for Earth.
There is a super massive black hole at the center of the Milky Way galaxy. It weighs in at about 4 million solar masses. Luckily, there is no reason to worry. This giant sucker is over 30,000 light years away.
Time Travel through Blackholes??
The idea that spaceships might zip across the universe using black holes as a high-speed portal is a well-worn sci-fi cliché.
But the consensus among scientists of late is that black holes are so destructive, spaceships would be torn to subatomic bits if they tried such a thing.And moreover if time travel was possible why don't we have visitors from the future?.well there maybe,its just something that we dont know of.
Then again, maybe not. A new paper by University of Utah physicist Lior Burko, building on earlier work, raises the possibility that black holes may not annihilate everything, and that the potential for hyperspace travel is still open.
"One possibility is that black holes may allow us to travel to very remote places in the universe, or another universe entirely," said Burko in a telephone interview from his office in Salt Lake City. "It depends on the topology of the universe, which we do not know very well.... I'm not arguing it's a practical thing to do, but maybe in 1,000 years from now, maybe it would be simpler."
In Burko's scheme, black holes may be doorways to wormholes, theoretical constructs equivalent to tunnels, or shortcuts, between distant points of the universe, different points in time or even parallel universes.
Burko's ideas aren't new. Wormholes were popularized by Caltech physicist Kip Thorne in the 1980s, and were the interstellar vehicle of choice in Carl Sagan's influential novel
Contact.
But subsequent black hole studies have suggested it would be impossible to use them as wormhole portals. The interiors of black holes are so infinitely dense that they exert massively destructive, "tide-like" distortions on approaching objects, ripping them into their constituent subatomic particles.
In fact, this infinitely dense interior gives black holes their potential for space and time travel. Inside a black hole, the very fabric of the universe is collapsed into a point of infinite curvature -- known as a "space-time singularity," where the laws of physics no longer apply.
However, Burko, a 34-year-old physicist from Israel, has suggested that some black holes may not be as destructive as others. Under certain circumstances, black holes may have "Cauchy horizon singularities," which may not be destructive but still act as openings to a wormhole.
Earlier work by Burko and others suggested weaker singularities were present in rotating black holes.
In a paper published in the March 28 issue of
Physical Review Letters, Burko suggests that under certain conditions, hybrid singularities may exist. These hybrid singularities are composed of a strong sector, which is destructive, and a weak sector, which may not be. Any spacecraft entering the weak sector could possibly pass through without being damaged.
It's all theoretical, but the possibility of a weaker singularity doesn't rule out the potential of using black holes for interstellar travel.
"At the moment, we don’t have compelling evidence that this kind of hyperspace travel is disallowed," said Burko. "It doesn't mean, of course, it
is allowed, but we don’t have compelling evidence to the contrary."
Princeton physicist Richard Gott, author of
Time Travel in Einstein's Universe, said he hadn't read Burko's latest paper, but previous work on the subject reopened some interesting possibilities.
"It's certainly one of the possibilities that you can have a weak singularity," he said. "I characterize it like a speed bump. You hit it, and you come out in a new region. It could be a region of time travel, another universe or somewhere a great distance away. These are interesting possibilities and should be investigated further."
Burko noted that the theory rests on some unproven assumptions, and further work could debunk it. In addition, he said so little is known about quantum gravity -- the marriage of quantum physics and classic theories of gravity -- that as-yet-undiscovered laws may forbid hyperspace travel altogether.
Even if the idea of interstellar space travel through black holes is possible, traveling to a suitable black hole to try it out would be a problem.
Burko said the black hole at the center of the Milky Way may be a candidate, but it's 26,000-30,000 light years away. Traveling at near the speed of light -- the upper limit for an interstellar excursion -- the trip would take nearly 30,000 years.
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