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Have heard about BLACK HOLES?


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#1    zarvirus

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Posted 17 May 2006 - 03:26 AM

A supermassive black hole is a black hole with a mass in the range of a few times 105 to a few times 1010 (hundreds of thousands to tens of billions) of solar masses. It is currently thought that most if not all galaxies, including the Milky Way, contain a supermassive black hole at their galactic centers.

Supermassive black holes have some interesting properties which distinguish them from relatively low-mass cousins:

The average density of a supermassive black hole can be very low, and may actually be lower than the density of water. This is because the Schwarzschild radius is directly proportional to mass, such that density is inversely proportional to the square of the mass.
The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole.
Black holes of this size can form in several ways. The most obvious is by slow accretion of matter (starting from a black hole of stellar size). Another method of producing a supermassive black hole involves a large gas cloud collapsing into a relativistic star of perhaps a hundred thousand solar masses and up. The star then becomes unstable to radial perturbations due to electron-positron pair production in its core, and may collapse directly into a black hole with no supernova. Yet another method involves a dense stellar cluster which undergoes core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, it may be possible to construct primordial black holes directly from external pressure in the first instances of the Big Bang.

The problem in forming a supermassive black hole is getting enough matter in a small enough volume. This matter needs to have nearly all of its angular momentum removed in order for this to happen. The process of transporting this angular momentum outwards appears to be the constraining factor in black hole growth, and leads to the formation of accretion disks.

Observationally, there currently appears to be a gap in the population distribution of black holes. There are stellar mass black holes, generated from collapsing stars, which range up to perhaps 10 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these regimes appears to be a dearth of objects. However, some models suggest that ultraluminous X-ray sources (ULX's) may be black holes from this missing group.

Direct Doppler measures of water masers surrounding the nucleus of nearby galaxies have revealed a very fast keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these lines to measure the mass, and perhaps the spin of the black hole that powers the active galaxy's "engine".

Such supermassive black holes in the center of many galaxies are thought to be the "engine" of active objects such as Seyfert galaxies and quasars. The Max-Planck Institute for Extraterrestrial Physics and UCLA provided evidence that Sagittarius A* is the supermassive black hole residing at the center of the Milky Way based on data from the ESO[2] and the Keck telescopes. Our galactic central black hole is calculated to have a mass of 3.6 million solar masses.

In May 2004, Paolo Padovani and other leading astronomers announced their discovery of 30 previously-hidden supermassive black holes outside the Milky Way. Their discovery also suggests there are at least twice as many of these black holes as previously thought. It is currently believed that every galaxy contains a supermassive black hole at its center, with most of them being in an "inactive" state not accreting much matter. In contrast, there do not appear to be black holes in the center of globular clusters.

There appears to be a link between the mass of the supermassive black hole in the center of a galaxy and the morphology of the galaxy itself. This manifests as a correlation between the mass of the spheroid (the bulge of spiral galaxies, and the whole galaxy for ellipticals) and the mass of the supermassive black hole. There is an even tighter correlation between the black hole mass and the velocity dispersion of the spheroid. The explanation for this correlation remains an unsolved problem in astrophysics.

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#2    mandricius

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Posted 17 May 2006 - 03:32 AM

Uhmnnn interesting...can we time travel through them? blink.gif


#3    zarvirus

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Posted 17 May 2006 - 03:35 AM

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Uhmnnn interesting...can we time travel through them? blink.gif


How can you use black holes for time travel?
Black holes can be used to travel into the future only. So far as we know, our universe prohibits traveling into the past.

According to Einstein's theory of general relativity, and to experimental evidence here on earth assembled by Harvard physicists Pound and Rebka, in the presence of a gravitational field, an external observer would see a clock in a strong gravitational field tick more slowly. This is analogous to the famous time dilation effect in special relativity, except that in the 'gravitational redshift' effect no motion between the observer outside the gravitational field and the clock located within the field, is required.

What this means is that if you were traveling into a strong gravitational field and sending out pulses of light every second, an observer watching these signals from a great distance would see the interval between the pulses increase from seconds to minutes and then hours as the field got stronger and stronger.

Black holes are fantastic sources of very strong gravitational fields. What a distant observer would see as your clock got closer to the so-called Event Horizon of the black hole is that the pulse interval would increase without limit from one second to one month and longer. The frequency of the light pulses would also get longer as the light lost more and more energy struggling to get out from the vicinity of the black hole. As your friend finally entered the black hole by passing across its event horizon, the last photon capable of making it to infinity is emitted at almost infinite redshift, meaning that if you originally emitted a gamma ray with an energy of 1000 billion electron volts, buy the time your friend received it far away, it would have lost enough energy to become a radio photon with an energy of 0.00001 electron volts! So, if it took your friend 1000 hours to travel from where you are to the black hole, the last photon he sent you just before entering the black hole, would arrive at your location 1000 hours from now, but when you looked at the interval between the last two pulses he sent, you would see that they are not the one second interval you started out with, but say 1 or 2 minutes or more. But here's the rub. According to your infalling friend, he/she is still sending the pulses out once each second!

In other words, one second to your friend falling into a black hole is several minutes to you and, in essence, your friend is aging more slowly than you and is traveling into the future faster than you are. If he/she could manage to put on the breaks just before crossing over the Event Horizon and escape to rejoin you, you would note that his/her clock reads a much earlier time than your clock. To your friend, only 2000 hours may have elapsed, however, YOUR clock would read perhaps 10000 hours or several weeks have elapsed depending how close to the Event Horizon your friend could get before escaping. The tidal gravitational forces are enormous near small black holes the mass of the sun, so your friend would be shredded into spaghetti within a few hundred miles of the Horizon. For supermassive black holes of several billion solar masses, however, the tidal forces near the Horizon are very small and survivable. This means you could accidently find yourself passing across this one-way barrier, and only realize your mistake when you tried to escape and found it impossible.

In principle, if you could get within a few millimeters of an Event Horizon before escaping, you could essentially time travel years or millenia into the future as measured by outside clocks. According to your clock, however, perhaps only a few hour or days actually elapsed.

Note, all of the above numbers are pretty darn approximate and are given to qualitatively show the magnitudes of the effects.



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#4    Propellerhead

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Posted 17 May 2006 - 03:40 AM

hehe...do you get old by traveling to the future?  rolleyes.gif


#5    Waspie_Dwarf

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Posted 17 May 2006 - 03:50 AM

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hehe...do you get old by traveling to the future?  rolleyes.gif


No. From your point of view everyone else does.

"Space is big. Really big. You just won't believe how vastly, hugely, mind-boggingly big it is. I mean, you may think it's a long way down the street to the chemist, but that's just peanuts to space." - The Hitch-Hikers Guide to the Galaxy - Douglas Adams 1952 - 2001

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#6    Propellerhead

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Posted 17 May 2006 - 03:55 AM

If you wanna learn more about time travel and balck holes visit:
http://www.daviddarling.info/encyclopedia/B/blackhole.html thumbsup.gif



#7    Waspie_Dwarf

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Posted 17 May 2006 - 04:06 AM

The above link gives next to no information on black holes and time trave except this:

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A Kerr black hole is formed by rotating matter, possesses a ring singularity, and is of interest in connection with time travel since it permits closed time-like paths (through the ring). A Reissner-Nordstrom black hole is formed non-rotating but electrically-charged matter. When collapsing, such an object forms a Cauchy horizon but whether it also forms closed time-like paths is uncertain.


What it fails to say is that these are highly theoretical concepts (along with worm-holes which are mentioned in the next paragraph). Although mathematical possibilities there is no actual evidence that such things really exist.

"Space is big. Really big. You just won't believe how vastly, hugely, mind-boggingly big it is. I mean, you may think it's a long way down the street to the chemist, but that's just peanuts to space." - The Hitch-Hikers Guide to the Galaxy - Douglas Adams 1952 - 2001

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#8    zarvirus

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Posted 17 May 2006 - 04:14 AM

Quote


The above link gives next to no information on black holes and time trave except this:
What it fails to say is that these are highly theoretical concepts (along with worm-holes which are mentioned in the next paragraph). Although mathematical possibilities there is no actual evidence that such things really exist.


I agree...thats not a rich site for black holes and time travel disgust.gif

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