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HIM
Thursday, November 24th, 2005, 10:48 PM
Black holes have always fascinated me. This is probably due mostly in part to their mysterious nature. We really know very little about them and much of what we do know has come from Einstein and his General Theory of Relativity and from Dr. Steven Hawking and his extensive research on black holes. It's hard to find a lot of information on them, but I found this site to be quite interesting and informative. The author does an excellent job of explaining hard to understand concepts.

http://cosmology.berkeley.edu/Education/BHfaq.html

Some additional information on General Relativity.

http://cassfos02.ucsd.edu/public/tutorial/GR.html

Death and the Sun
Friday, November 25th, 2005, 05:00 AM
The equations of general relativity have an interesting mathematical property: they are symmetric in time. That means that you can take any solution to the equations and imagine that time flows backwards rather than forwards, and you'll get another valid solution to the equations.

:scratch:


I've always thought that "time" was a concept that was considered irrelevant, if not actually nonexistent, in physics.

Zyklop
Friday, November 25th, 2005, 05:06 AM
:scratch:


I've always thought that "time" was a concept that was considered irrelevant, if not actually nonexistent, in physics.How so? :scratch:

Death and the Sun
Friday, November 25th, 2005, 05:15 AM
How so? :scratch:

I mean that "time" is a human concept. Energy, matter and physical laws don't care if something takes seconds or millenia.

You can't reverse "the flow of time", because it is something that exists only in the human mind. We have a reasonable reliable way of measuring the "time" it takes for physical phenomena to happen, in the sense the that these measurements seem reliable and consistent to us, but still such measurements are an artificial construct.

lei.talk
Friday, November 25th, 2005, 08:26 AM
You can't reverse "the flow of time"...years ago,
a friend gave me a bound version
of richard feynman (http://en.wikipedia.org/wiki/Richard_Feynman)'s famous lectures (http://en.wikipedia.org/wiki/The_Feynman_Lectures_on_Physics).

in one of the ancillary diagrams (http://en.wikipedia.org/wiki/Feynman_diagram)
i was disturbed to find
a photon (http://www.vega.org.uk/video/programme/45) with a negative time-vector.

i was unconvinced.

GreenHeart
Saturday, December 31st, 2005, 02:41 AM
I've always been fascinated by black holes too. Along with stars, constellations, planets and the universe in general.

Cole Nidray
Sunday, September 3rd, 2006, 07:58 AM
20/20 report on black suns and the apocalypse.

http://abcnews.com/2020/Science/story?id=2365372&page=1




http://a.abcnews.com/images/Technology/abc_2020_blackhole_060823_sp.jpg


Aug. 28, 2006 — Imagine a black hole swallowing Earth, ending life in an instant. It's not only the stuff of pulp sci-fi novels but, scientists say, a looming possibility.


"It would be a bad day for the solar system if we got visited by a black hole," says Neil deGrasse Tyson, director of New York's Hayden Planetarium at the American Museum of Natural History.

...<cont'd>


http://a.abcnews.com/images/site/icon_video_transparent.gif (http://a.abcnews.com/Video/playerIndex?id=2381076) Last Days: Asteroid Collision (http://a.abcnews.com/Video/playerIndex?id=2381076)
'Last Days on Earth' (http://a.abcnews.com/2020/story?id=2319986)
When an Asteroid Hits -- a Killer Crash (http://a.abcnews.com/2020/Science/story?id=2321761)
http://a.abcnews.com/images/site/icon_video_transparent.gif (http://a.abcnews.com/Video/playerIndex?id=2381125) Last Days: Nuclear War (http://a.abcnews.com/Video/playerIndex?id=2381125)
Planets Added to Solar System, Why Now? (http://a.abcnews.com/Technology/wireStory?id=2318597)
http://a.abcnews.com/images/site/icon_vote.gif (http://a.abcnews.com/2020/popup?id=2369750) What Will End Our Days on Earth? (http://a.abcnews.com/2020/popup?id=2369750)
http://a.abcnews.com/images/site/icon_video_transparent.gif (http://a.abcnews.com/Video/playerIndex?id=2381100) Last Days: Intelligent Machines (http://a.abcnews.com/Video/playerIndex?id=2381100)---

From the Poetic Edda, Völuspá :


Now death is the portion of doomed men,
Red with blood the buildings of gods,
The sun turns black in the summer after,
Winds whine. Well, would know more?

Earth sinks in the sea, the sun turns black,
Cast down from Heaven are the hot stars,
Fumes reek, into flames burst,
The sky itself is scorched with fire.

I see Earth rising a second time
Out of the foam, fair and green;
Down from the fells fish to capture,
Wings the eagle; waters flow.

SuuT
Monday, September 3rd, 2007, 06:35 PM
When superstring theory arrived in physics in 1984 as a potential theory of the universe, it was considered by mainstream physicists as little better than religion in terms of constituting a viable, testable theory. In string theory, the fundamental particles were string-like, rather than point particles; the universe had 10 or 11 dimensions, rather than four; and the theory itself existed at an energy so far from earthly energies that it took a leap of enormous faith to imagine the day when an experiment could ever test it. Quite simply, string theory seemed an excessively esoteric pursuit, which it still is. But the last five years have seen the theory undergo a series of major breakthroughs–theoretical ones, at least–while simultaneously entering the mainstream of the field. Last summer's string theory conference in Santa Barbara was attended by 350 physicists. And universities have taken to holding bidding wars to recruit the best string theorists–with Harvard, Stanford, and Princeton lately leading the way.
Among the hottest physicists driving the string theory revolution is Harvard University's Andrew Strominger (http://forums.skadi.net/redirector.php?url=http%3A%2F%2Fwww.phys ics.harvard.edu%2Ffac_staff%2Ftheoretica l.html), who over the past five years has led the way in merging the study of quantum-mechanical black holes–a pursuit popularized by Cambridge University's Stephen Hawking–with that of string theory to advance understanding in both fields. During the spring and summer of last year, Strominger's 1996 paper in Physics Letters B, "Microscopic origins of the Bekenstein-Hawking entropy," written with Harvard colleague Cumrun Vafa, routinely appeared in Science Watch’s Physics Top Ten, taking the top spot in the May/June 1998 (http://forums.skadi.net/redirector.php?url=http%3A%2F%2Fwww.scie ncewatch.com%2Fmay-june98%2Fsw-may-june98_page1.htm) issue. That paper has now been cited nearly 500 times (see table on the next page, paper #1 (http://forums.skadi.net/redirector.php?url=http%3A%2F%2Fwww.scie ncewatch.com%2Fmay-june99%2Fsw_may-june99_page4.htm%23table)). In a field in which many researchers have taken to publishing their papers only electronically, Strominger has published more than 25 papers that have each attracted more than 100 citations, while his 1985 paper on "Vacuum configurations for superstrings, " written with Phil Candelas, Gary Horowitz, and Ed Witten, has garnered well over 1,500 citations (see table, paper #2 (http://forums.skadi.net/redirector.php?url=http%3A%2F%2Fwww.scie ncewatch.com%2Fmay-june99%2Fsw_may-june99_page4.htm%23table)).
Strominger, 43, graduated from Harvard University in 1977. He completed a Master's degree at the University of California at Berkeley before earning his doctorate at the Massachusetts Institute of Technology in 1982 with Roman Jackiw. Strominger spent the next five years at the Institute for Advanced Study in Princeton before joining the faculty of the University of California at Santa Barbara. In 1997, Strominger moved back to Harvard University, where he is now a professor of physics...


http://www.sciencewatch.com/may-june99/sw_may-june99_page3.htm (http://forums.skadi.net/redirector.php?url=http%3A%2F%2Fwww.scie ncewatch.com%2Fmay-june99%2Fsw_may-june99_page3.htm)

Feyn
Thursday, November 10th, 2011, 04:48 AM
most people have a completely wrong idea how black holes actually work and what they really are. So i would like to clear up the most common misconceptions and explain how they are created etc. First things first :
A black hole is created when a very massive star (roughly 10+ times the mass of our sun) is at the end of its life and becomes a hypernova. To really explain that let me first explain how stars in general work.
You see in every star we have a fragile balance between 2 forces. A star is made of gas as we all should know. The gravity of the star compresses this gas more and more. From physics classes we know if you compress gas it gets warm (for example if you pump up your bicycle wheel the pump gets warm, try it out). But in a star it gets much warmer, so warm that the hydrogen starts a nuclear reaction that turns it into helium, which is responsible for the light and warmth a start gives of (the reaction, not the helium ^^). So the gravity is the first force, what is the second ? Well the gas is under pressure, and pressurized gas expands outwards. Those 2 forces, the pressure of the gas pressing outwards, and the gravity pressing it inwards form at some point a balance.
That works as long as there is still hydrogen to form into helium, but when the hydrogen runs out the star has a problem. Gravity still compresses inwards, but the outwards pressure is missing, so the star starts to collapse. It gets again hotter and the next level starts, the helium starts turning into carbon. This face happens much faster, so the helium is soon burned out, and the next phase starts and so on.
Now while the hydrogen to helium phase needs ,millions of years the next phases happen very rapidly (i am talking hours-->minutes-->seconds) which turns the star into a nova, a supernova or even a hypernova in which the star literally explodes and all the energy of this nuclear process is freed at once.
Such a supernova can be so bright it outshines the whole galaxy its in for a few hours to days, and a galaxy is made of billions of stars (some have several hundred billion stars, for example the milky way has 200 billion to 400 billion stars astronomers estimate, and its a rather average galaxy, but there are also dwarf galaxies with only a few million stars)
Now back to this balance : the heavier a star is, the more it compresses its gas till this balance is working. That also means its much hotter in the inside, and thus it burns up its fuel much faster (while a typical star like the sun burns 1 billion years or longer a really massive star can burn out in just a few million years). So completely counter intuitive, the bigger a star, the faster it burns out, the fewer life time it has !

So, now we know how a star works in general, lets get to the death. I have already explained it explodes in a nova, supernova or even hypernova, but what happens to the rest ? That depends on how heavy the rest is, so on how much of its mass it blew into space during the nova and how heavy it was to begin with. Most stars become a white dwarf (like our sun will become, and with it roughly 97% of all stars). A white dwarf is the core of the star. It contains the heavy erlements thzge star had to begin with, and some of the heavy elements that where püroduced during the nova. Here the wiki on white dwarfs :

http://en.wikipedia.org/wiki/White_dwarf

As you see its basically extreme dense matter, but still normal matter, only extremely compressed, since no gas pressure can counter the gravity any longer ^^ A typical white dwarf has the size of the earth, while it has the mass of the sun (the sun in our solar system has 99.9% of the mass of solar system, and all the planets, meteorites, comets etc the rest 0.1%)^^

But what happens when there is even more matter left ? In that case the rest forms what we call a neutron star. From the physics class we know an atom is mostly vacuum. To give you an idea if the atom is a football stadium, the core is a peanut laying in the middle. But in a neutron star the atoms get so compressed that we have the atomic cores directly next to each other and the electrons are absorbed by the protons in the core, thus building neutrons (hence the name neutron star). Here the wiki on neutron stars :

http://en.wikipedia.org/wiki/Neutron_star

A typical neutron star has between 1.3 and 2 times the mass of the sun, but compressed to a ball of 10-20 km diameter. Imagine how dense that is, its nearly unimaginable !

If it has even more mass it collapses to a black hole, perhaps the strangest object in the cosmos. A black hole is defined by its so called event horizon. That is a thought line in space and once you have passed that line even light is too slow to ever get out again. If earth would collapse to a black hole the event horizon would be a few millimeter (afaik 5mm, but don´t quote me on that, i am too lazy right now to do the math ^^)
We don´t really know whats inside the event horizon, since our formulas simply give meaningless results past this line. I know most physicists claim there would be a singularity in the middle of it, where all the mass is concentrated in a mathematical point, but since our formulas don´t make sense i would say that´s pure guesswork.

A few more facts about black holes : many people think they are this all eating monsters. Well if you get very close to it that´s correct, BUT a little further away the black hole reacts exactly like the star it once was. Lets say for the sake of the argument the sun would collapse to a black hole, then nothing would happen to the planets. They would do exactly what they did before, orbit the black hole like they did orbit the sun before, nothing has changed. The mass is exactly thew same, so the gravity is exactly the same. This changes only once we get inside the old radius of the sun, and not earlier, and even then it changes slowly. You have to get quite close to really see a difference.
Another interesting fact is that there are giant black holes at the core of each galaxy, the one at the core of our galaxy has 4.1 million times the mass of the sun, and our galaxy revolves around it. The biggest ones known even have several hundred million times the mass of the sun ^^

Now if you need more info just ask ^^ I hope i could clear up most misconceptions, and let you really understand at least on a basic level what they are and what they do.

GroeneWolf
Thursday, November 10th, 2011, 05:53 AM
Very interesting read, and more expanded then that a black hole is a large star who had imploded and whose gravity is so strong that not even light can escape from it. But I am not sure about this part of your piece :


(for example if you pump up your bicycle wheel the pump gets warm, try it out)

Is that not more the result of friction then of compression?

Feyn
Thursday, November 10th, 2011, 06:03 AM
Thank you very much guys, always happy to help. Physics is a hobby of mine now, and i once studied it, so i know a thing or two and i am happy to share said knowledge ^^

Its both, but more the compression then the friction. You should also know the counterpart of this effect, that gas cools of when it expands, for example when you refill a lighter. If you let the gas flow out it becomes ice cold very quick. Ice spray works also like this (the one dentists use to test if your tooth is still alive for example)
btw a fridge works with the same 2 principles. Inside the fridge the cooling agent expands and thus cools down, and at the back of the fridge its compressed again, and thus gets warm. Ever felt the backside of a fridge ? It usually gets quite warm ^^


btw what i forgot to mention, all the heavier elements stem from such novas. After the big bang ONLY helium and hydrogen existed (about 24% helium, the rest hydrogen). All the other elements came from the first generation of stars. I am always fascinated that our whole planet and all the stuff we are made of was born in the death of a star, that shined for a while brighter then a whole galaxy and gave of more energy in a few days then the sun in its whole existence. When the beatles sang:"we are star dust" you can really take that literally. That really blows ones mind, doesn´t it ? Since i am at it :
Most of the time this reaction only gets up to iron and then stops, only in rare cases the nova is strong enough to also produce heavier elements then iron. That´s the reason all the elements heavier then iron (like gold) are so much more seldom then anything below iron.
In the year 1054 astronomers around the planet saw a new star at the sky, that was so bright you could even see it by day. Modern day astronomers searched for the source, and found it, its today known as the crab nebula. That thing is 6500 light years away, and was bright enough to be seen by day, and yet it was just a small supernova compared to what else the sky has to offer ^^ Here the wikiarticle on the crab nebula, i find it quite interesting :

http://en.wikipedia.org/wiki/Crab_Nebula


and again, any questions feel free to ask, no matter what it is. Remember, there are no stupid question, the only thing stupid is not to ask at all ;)