Black Hole |
(Think about these carefully before you consult the answers below)
QUESTIONS
1) If black holes cannot be seen, how could there be any evidence of their existence? <answer>
2) Why won't the Sun become a black hole? <answer>
3) Are black holes forever? <answer>
4) Can anything live inside a black hole? <answer>
5) How do we know that we don't live inside a gigantic black hole?<answer>
ANSWERS
1) Although the black hole itself cannot be seen, things falling into it can, at least in theory. The appearance of something falling into a black hole would follow a specific sequence of events, that would be characteristic of the presence of a black hole. Unfortunately (or rather, fortunately!) there are no black holes close enough that we can actually see things falling into them. However, charged particles falling into black holes will emit characteristic types of radiation that will identify the presence of a black hole. We have observed this radiation from several sources in the sky, including an area known as Cygnus X-1 (the radiation is X-rays in this instance). So even though we can't see them, we can detect their presence. Like the old saying, "where there's smoke, there's fire."
2) Theoretically, there is no lower limit to the size of a black hole. Some scientists have hypothesized that "mini" black holes were created at the beginning of the Universe (the "Big Bang"). But if so, those were under very special circumstances. In today's reality, an object would have to have approximately three times the Sun's mass to have enough gravity to be crushed into a black hole. If some other mechanism were known to produce the unimaginable pressures involved, smaller objects could, in theory, become black holes. Fortunately, no such mechanisms are known and the general concensus is that none are likely to be found. There is no force in the universe that will turn our Sun into a black hole. If such a force were known, the Sun would become a black hole if it were crushed to about 4 miles (6.4 km) across, down from its present 865,000 miles (1, 390,000 km). Even the Earth could become a black hole if it could be crushed to just less than three-quarters of an inch across. Relax, it simply isn't going to happen!
3) Originally scientists thought that since nothing could escape a black hole, it would exist forever. However, some years ago the brilliant English physcist Stephen Hawking presented a strong argument that black holes slowly "evaporate" due to consequences of Quantum theory. By Hawking's reasoning, if a black hole is not constantly fed with new material, it will eventually leak out the material it ingested, and after billions or trillions of years (depending on its mass) it will cease to be. Any "mini" black holes that may have been created at the time of the Big Bang would have long since evaporated.
4) Strangely enough, the conditions for life may be possible inside a very large (massive) black hole. This is because the larger the black hole, the farther the Event Horizon will be from the center. This greater this distance, the less the gravity at the Event Horizon and the less pronouced the differential gravitational pull will be. Just inside the Event Horizon of a supermassive black hole, objects may not be immediately torn asunder, and spacecraft might remain intact. However, they will continue to fall toward the center and ultimate destruction unless they can somehow establish an orbit, within the black hole, around the center at a safe enough distance. This would likely still be only a temporary reprieve, however, as it still could not escape back into normal space.
5) If you define a black hole as an area of space, which under the influence of gravity, cannot be escaped, then we do live in a universal black hole. To some extent this can degenerate into a semantic argument, but if as Einstein figured, space is warped by gravity, then it is possible that the Universe has closed in on itself under gravity. In that case the Universe is a black hole. Even if the Univese is not closed, there still is no chance of "escape" and in that sense, it is a black hole.
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