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Black Hole at Center of Milky Way
kwertii writes: "The Washington Post reports new evidence that there is a black hole with the mass of 2.6 million suns at the center of our galaxy. The Chandra X-Ray Observatory happened to be looking at the presumed site of the hole at the moment it absorbed a comet, blasting x-rays off into space as a byproduct. The implication is that the Milky Way is slowly spiraling down into a giant galactic drain..."
Official website
Official press release
Story on CNN
actually the black hole will continue to grow as it swallows more matter. That is to say that the event horizon will continue to expand. This is the result of the increase in gravity as it becomes denser and denser. In effect it swallows some matter, expands, thus making it stronger and able to swallow more matter, and so on. In theory as long as there is matter flowing into the black hole then it could continue expanding to swallow the whole galaxy in time.
Steven Hawking explains this concept pretty well in his Brief History of Time
You are correct ... and you are also wrong.
Let me explain :-)
To an observer outside the event horizon of the black hole, the object never appears to actually cross the horizon, just to approach it more and more slowly as time goes on. In other words, the clock of an infalling observer will appear to run slower than the clock of an observer that does not approach the horizon. More generally, to a distant observer a clock in a strong gravity field will run slower than a clock he carries around with him.
Meanwhile, for the poor observer entering the black hole, as he approaches the horizon, the clock HE carries appears to continue ticking away at its usual rate, while his view of the universe slowly gets distorted, so that it looks like he is travelling down a tunnel towards the hole's surface. In a finite amount of time, he crosses the event horizon, and the "tunnel vision" he has of the rest of the universe shrinks to zero size. He doesn't notice his clock slowing down, and he eventually will hit the "bottom" of the hole.
Interesting fact: if he tries to fight the hole to prolong the time before he hits the bottom, he'll actually hit the bottom sooner than if he didn't fight.... of course, when you've already been ripped apart by the tidal forces, you wouldn't notice, but let's consider just and "ideal observer" :-)
This "strange" (some would incorrectly say "paradoxical") behavior of the same set of events appearing differently to two observers is one of the hallmarks of the "Theory of Relativity" ... but results like this where two people disagree qualitatively on the outcome can only occur when the two can never again communicate with each other. Otherwise, they will only disagree quantitatively on the outcome of an "experiment".
I've noticed that some people have a bit of confusion here about exactly what the effects of a black hole are. Here's are examples:
Q: What would happen to the orbit of the earth if all the matter in the sun were suddenly compacted into a black hole?
A: Absolutely nothing. A black hole which contains the mass of the sun would still also have the same gravity as the sun. The earth would continue to orbit as it always has.
Q: The galaxies stars orbit around the black hole.
A: This isn't proven. Some galaxies don't have any evidence of a black hole, yet theirorbit around a center of mass. In any case, the black hole at the center of our galaxy is 2.6million solar masses. This is NOTHING compared to the billions of stars in the galaxy, so the effect of the black hole of the actual shape and orbit of the stars is not significant.
Q: Doesn't it sound like someone has pulled the stats on this black hole out of their arse?
A: Not really, the size of this black hole has been measured in several ways, including observing very high velocity stars near the black hole. The motion of these stars betrays the existence and size of the massive object at the galaxy's center.
Q: Aren't black holes required for the formation of galaxies?
A: We don't know for sure yet. There are galaxies without black holes, so it might not be required. Of course, we might just not be detecting the black holes that are in those galaxies.
If tits were wings it'd be flying around.
Hawking didn't hypothesize the virtual particles, they are a neccesary feature in some aspects of quatum mechanics, especially QED (quantum electrodynamics), whose predictive validity has been established with incredible accuracy.
Hawking's contribution was thinking about how they might interact with black holes. Interestingly his theory was incomplete in that it rested on a major assumption that was not proved (it's math so "proved" is the right word) until quite recently. To be honest though we won't be certain about Hawking radiation till we have a good understanding of quantum gravity. Until then it's just a good hack trying to apply both quantum mechanics and general relativity to a problem, despite the fact that they are inherently incompatible theories.
As far as compressing things down, many physicists believe a black hole can't swallow anything whose de Broglie wavelength is greater than the diameter of the event horizion. De Broglie wavelength is a quantum mechanical property that in this context can roughly be thought of as a measure of something's intrinsic size. Once something gets pulled in, it would get compressed far smaller, but the black hole has to be able to catch it first. Electrons have a wavelength on the order of 10^-10 m, where as nuclear particles are about 10^-15 m. Schwarzschild radius is given by 2*G*M/c^2, which implies that a hole of 10^-15 m has about 6.7e11 kg of matter in it.
Thus you can't make a black hole out of a cat because a cat doesn't have enough mass to generate an event horizon that would encompass it's atoms. Besides we already wondering whether the cat is dead or alive, why subject him to anything else.
One final note, some of the plans for quantum gravity would replace the singularity with a highly compact structure of miniscule but non-zero volume. IIRC something with radius on the order of 10^-30 to 10^-34 m.