View Full Version : More on Black Holes

E-Mailer

2004-Jan-24, 09:42 AM

I'm not sure if this had been a major issue in the recent past but, I need to raise this matter up. I've read in articles and made several research about blackholes. My findings show that blackholes have the tendency to bend light, take-in matter of no specific mass, or even stretch dimension itself. Is this true? And what more can you say about this?

In my thinking, could there be a probability that if two blackholes collide, they may fuse together and form an even bigger blackhole? Meaning a much powerful gravitational field? what if this comes to be and not only 2 will combine? will the force be massive enough to atleast erase some portions of universal dimensions which according to theory is infinite and empty--not to mention continuously expanding? or is it just moving?

rwald

2004-Jan-24, 10:38 AM

If I understand your questions, you want to know if black holes warp space-time, and what happens if two black holes collide. The short answers are yes, and "you get a bigger black hole."

The long answer (at least for the first one) will require a discuss of what a space-time warp means. Basically, near a black hole, the universe is not Euclidian in the sense most people imagine it. If you measure the angles of a triangle, they will not add up to 180 degrees. If you draw a circle, twice the radius times pi will not be the circumfrence. But even more oddly, time itself will be warped. If you have two clocks at different distances from the black hole, the clock closer to the black hole will tick slower than the one farther away (that is, for every, say 5 ticks of the clock farther away, the closer clock will only have 4 ticks, even if the two ticked identically when they were next to one another). These concepts may not be intuitive, but they've been experimentally tested and found to be true.

For the second question, it really is that simple; if two black holes collide, they'll make a new, bigger black hole. If they don't hit straight on, there's a chance that the new one will be spinning, but that's not immidietly relevant. Of course, there is the matter of the collision emitting gravitational waves, but that doesn't really change anything.

E-Mailer

2004-Jan-24, 11:03 AM

you are definitely making a point but, will there be adverse effects directly on the events taking place on the space being affected by the change?

rwald

2004-Jan-24, 11:06 AM

Well, a black hole causes a distortion in space-time (as do all massive objects), but the actual collision isn't going to cause any permanent effect other than the creation of the new black hole. If people were to come across the new black hole without having seen the collision, they would have no way of knowing that the new black hole had once been two old black holes.

TrAI

2004-Jan-24, 11:19 AM

Hmm, I seem to remember reading that colliding black holes will combine to form a new black hole. This hole will have a mass smaller than the two holes combined mass before the fusing started, as energy is lost as gravity waves, I think it was about 10% of the mass...

rwald

2004-Jan-24, 11:21 AM

The exact amout of mass lost due to gravity waves would depend on the angle which which they collide, whether it's a head-on or side-swipe collision, the initial rotation of the two black holes, etc. But overall, I have no specific reason to doubt your 10% figure.

TrAI

2004-Jan-24, 11:34 AM

The exact amout of mass lost due to gravity waves would depend on the angle which which they collide, whether it's a head-on or side-swipe collision, the initial rotation of the two black holes, etc. But overall, I have no specific reason to doubt your 10% figure.

Yes, there is probably a lot of variables, but its a while since I read the article, so I don't remember that much.. I think it may have been head on collisions that would have that approximate loss of mass....

Taibak

2004-Jan-24, 09:07 PM

I'm not sure if this had been a major issue in the recent past but, I need to raise this matter up. I've read in articles and made several research about blackholes. My findings show that blackholes have the tendency to bend light, take-in matter of no specific mass, or even stretch dimension itself. Is this true? And what more can you say about this?

It's all true. Any gravitational field can bend light. In fact, one of the experiments used to prove general relativity was looking for stars that appeared out of place thanks to the Sun bending the path their light took on route to the Earth. With a black hole, where the effects of gravity is much more pronounced, the bend is more readily apparent. You can even use massive gravity sources, like a black hole or a galaxy, as a lens to help observe objects behind them. And yes, part of that is due to gravity bending space and time. Again, any object can do this it's just that with a black hole the effects become much more pronounced, espescially once your inside the black hole.

I'm not sure what you mean by 'matter of no specific mass.' An object that crosses the event horizon of a black hole can't get out, if that's what you mean. That includes light so neither mass nor speed will change that.

In my thinking, could there be a probability that if two blackholes collide, they may fuse together and form an even bigger blackhole?

Yes. The mass would increase and the radius of the event horizon, the edge of the black hole, is proportional to its mass.

Meaning a much powerful gravitational field? ?

Well, the gravitational field will just be the combined gravity of the two black holes. Good old Newton's laws will still work there.

what if this comes to be and not only 2 will combine? will the force be massive enough to atleast erase some portions of universal dimensions which according to theory is infinite and empty--not to mention continuously expanding? or is it just moving?

I highly doubt it. Sure spacetime might be bent a little more inside the event horizon, but it would not destroy space and time, if that's what you're asking. Also, keep in mind that all of these effects are taking place *inside* the black hole. Outside the black hole, the universe would just continue on as normal. (If it helps to think about it this way, if the Sun were to become a black hole right now, none of the orbits in the Solar System would be affected since its gravitational force wouldn't change). Even inside the black hole, space and time only start acting really weird as you get close to the centre. And even then they're not destroyed.

Mind you, all bets are off at the singularity, the point at the absolute centre of a black hole. As far as I know, nobody has figured out what physics work there.

E-Mailer

2004-Jan-25, 05:42 AM

But I've read in an article (i just can't remember) that blackholes, theoretically have the ability to *suck* objects..while inside, do these objects remain to be what they are or do they change their organization (molecular structures)?

rwald

2004-Jan-25, 07:32 AM

Well, black holes suck stuff in the same sense that the Earth does. When you get closer to them, gravity will start to draw you inwards. Once you're close enough (whether this happens before or after the event horizon depends on the mass of the black hole), tidal forces will cause you to be streached out in the radial direction (towards the singularity) while also squeezing you along the tangential direction (perpendicular to the radial direction). So you would be squeezed into spaghetti. Eventually (in theory), even the molecules in your body would be subjected to these forces. Of course, no one outside the event horizon would see this. I could go into what they would see, but it's a bit complicated.

Taibak

2004-Jan-26, 07:41 PM

Well, black holes suck stuff in the same sense that the Earth does. When you get closer to them, gravity will start to draw you inwards. Once you're close enough (whether this happens before or after the event horizon depends on the mass of the black hole), tidal forces will cause you to be streached out in the radial direction (towards the singularity) while also squeezing you along the tangential direction (perpendicular to the radial direction). So you would be squeezed into spaghetti. Eventually (in theory), even the molecules in your body would be subjected to these forces. Of course, no one outside the event horizon would see this. I could go into what they would see, but it's a bit complicated.

Dead on. Black holes aren't cosmic vacuum cleaners. They're just a dense object with a strong gravitational field - an ultra-condensed star. The only thing that makes their gravity special is that black holes have an event horizon. All that means is that all of the mass in the black hole is within a certain radius, proportional to its mass. So long as you're outside that radius everything works pretty normally. Heck, you could even use Newton's laws and Kepler's laws to get a pretty good approximation of how things move. Its only when objects get *inside* the event horizon that they're stuck in the black hole.

Think about it like this. The classical laws of orbital motion tell us that r = GM/v, where v is the Earth's orbital velocity, G is the universal gravitational constant, M is the mass of the Sun, and r is the average distance from the Earth to the Sun. In other words, the Earth's orbit is determined by the mass of the Sun and the speed of the Earth. Since a black hole is just an ultra-condensed star, if the Sun were to compress itself into a black hole, it's mass would still be the same. Since there'd be nothing changing the Earth's speed, the Earth will keep on orbiting like nothing happened.

Taibak

PS: If you're curious, I got my equation from the Law of Universal Gravitation and the equation for centripetal force.

F = GMm/r^2 = mv^2/r

Since r is not equal to zero,

GM/r = v, therefore r = GM/v

informant

2004-Jan-27, 03:26 PM

Basically, near a black hole, the universe is not Euclidian in the sense most people imagine it. If you measure the angles of a triangle, they will not add up to 180 degrees. If you draw a circle, twice the radius times pi will not be the circumfrence.

How has this part been experimentally tested?

Taibak

2004-Jan-27, 05:20 PM

Basically, near a black hole, the universe is not Euclidian in the sense most people imagine it. If you measure the angles of a triangle, they will not add up to 180 degrees. If you draw a circle, twice the radius times pi will not be the circumfrence.

How has this part been experimentally tested?

Yes and no. We haven't directly measured spacetime around a black hole for all the obvious reasons. But, we can test the geometry here on Earth and in the Solar System since, according to General Relativity, it should be true *everywhere*, not just near a black hole. Under Riemannian geometry, the *surface* of the Earth is a two-dimensional curved surface. As such, if you were to draw a triangle with vertices at the North Pole, the Equator at 0º E, and the Equator at 90º E, you've just drawn a triangle with three right angles. If you then take this geometry, generalize it to four dimensions (space and time) it fits very well with our observations of how gravity works. For instance, it explains the discrepencies in Mercury's orbit (its perihelion advancing) that Newton's and Kepler's Laws cant. Since this geometry seems to work everywhere else in the universe, there's no reason to assume that it won't work outside a black hole.

*Inside* a black hole, well, that's anybody's guess. General relativity makes theoretical predictions of what goes on, but we have no way of proving them.

informant

2004-Jan-27, 05:33 PM

If you then take this geometry, generalize it to four dimensions (space and time) it fits very well with our observations of how gravity works.

So, if I understand correctly, this has to do with the geometric interpretation of gravity in General Relativity...

Tensor

2004-Jan-27, 06:00 PM

If you then take this geometry, generalize it to four dimensions (space and time) it fits very well with our observations of how gravity works.

So, if I understand correctly, this has to do with the geometric interpretation of gravity in General Relativity...

Yep, GR is a geometrical theory. Good example Taibak. The strange one, for me, is the saddle shape, where the angles of a triangle add up to less than 180.

rwald

2004-Jan-27, 09:19 PM

This site (http://cs.unm.edu/~joel/NonEuclid/) has lots of information about hyperbolic geometry, including a Java applet for you to play with to help you get the idea.

Manchurian Taikonaut

2005-Feb-23, 02:31 PM

Well you know that NASA's Chandra has been studying these objects, a swarm of thousands of black holes might be orbiting the Milky Way's supermassive blackhole in the centre, this is what scientists were thinking after NASA's ChandraX-ray latest obseravtions and results

However there is another obseravtion craft up there now, Europe's XMM Newton. Now ESA's X-ray XMM-Newton is looking at these emissions.

QUOTE

Black Holes in a radar trap

23 Feb 2005

Using the X-ray Satellite XMMNewton researchers measure velocities near the speed of light in the vicinity of cosmic mass monsters.

European astronomers succeeded for the first time to confirm the signatures predicted near black holes by Albert Einstein's theory of Relativity in the light of the cosmic X-ray background. The group of scientists led by Günther Hasinger, director at the Max-Planck-Institute for extraterrestrial Physics in Garching near Munich, could identify the spectral fingerprint of iron atoms. They observed a strong, relativistically smeared iron line in the average spectrum of roughly 100 active galaxies, whose X-ray light had been emitted when the Universe was less than half of its current age.

....In a way similar to the radar traps with which the police identify speeding cars, the relativistic speeds of iron atoms circling the black hole can be measured through a shift in the wavelength of their light... From the strength of the signal they deduced the fraction of iron atoms in the accreted matter. Surprisingly, the chemical abundance of iron in the "nutrition" of these relatively young black holes is about three times higher than in our Solar system, which had been created significantly later. The centres of galaxies in the early Universe therefore must have had a particularly efficient method to produce iron, possibly because violent star forming activity "breeds" the chemical elements rather quickly in active galaxies.

http://sci.esa.int/science-e-media/img/da/type-1e2001.jpg

http://sci.esa.int/science-e-media/img/db/lores_36571.jpg

In the immediate vicinity of the black hole, the characteristic spectral fingerprint of iron atoms is smeared out by relativistic effects. The light from those atoms, that move towards the observer appears shifted to shorter wavelengths (blue) and much brighter than that on the receding side of the disk (red).

http://www.esa.int/export/esaSC/SEM14YS1VED_index_0.html

An analysis of 13.5 thousand-million-year-old X-rays, captured by ESA's XMM-Newton satellite, has shown that either the Universe may be older than astronomers had thought or that mysterious, undiscovered 'iron factories' litter the early Universe

http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36570

:)

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