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Black Holes.

by a.ratnaparkhi
Tags: black, holes
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a.ratnaparkhi
#1
Nov3-10, 10:32 AM
P: 30
How we can prove the existence of black holes?
And how they are located in in the space as they can absorb light too?
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NobodySpecial
#2
Nov3-10, 10:41 AM
P: 474
Stuff falling into black holes is heated to very high temperatures as it collides with other stuff also being pulled into the blackhole - we don't see the blackhole directly but we do see a huge amount of x-rays and other radiation from this material.

We can calculate the size of the object at the center of this and if it is small enough we know it is a blackhole.
94JZA80
#3
Nov3-10, 01:24 PM
P: 121
some would say that, b/c we have yet to literally "see" the shadow of a BH (the event horizon) against the backdrop of stars and illuminated gas and dust, we do not have definitive proof of the existence of black holes. but if you ask me, the abundance of evidence - not proof, but evidence - of the existence of black holes is too convincing to possibly suggest that they don't exist.

we've observed and measured the motions of stars extremely close to the galactic center, and found that they orbit so fast within a radius so small, that the only way to explain their motions is if a mass of ~2.6 million suns is present. we know that any mass that causes a star to orbit it lies completely interior to the orbital path of the star itself. thus, the volume occupied by these ~2.6 million solar masses can be no larger than the volume contained within the radii of the orbits of the stars going around it. this is strong evidence that there may be a BH at the center of our galaxy...

...however this information alone is not enough to say definitively that the object is definitely a black hole (and not a huge conglomeration of burned-out white dwarfs or other massive, hardly-radiating objects). but we've studied the motions of other objects (gas, dust, etc.) much closer to the center of our galaxy than the actual stars that orbit it...this is what NobodySpecial is talking about. as gas and dust accretes around the BH and sink closer to the event horizon, its angular velocity (the speed at which it orbits) increases dramatically to appreciable fractions of the speed of light, and copious amounts of energy are released via compression and friction as the material orbits the BH. not only can we detect the high-energy radiation coming from this accretion process, but we can use spectroscopy to calculate red shifts and blue shifts caused by the motions of this orbiting gas and dust. if we were to view a BH edge-on or close to it (kind of like we view the possible BH at the center of our own galaxy), then the orbiting material would be more or less traveling away from us on one side of the BH, and toward us on the other side. we've found using spectroscopy that the red shifts and blue shifts induced by the gas and dust orbiting closest to the central object are significant, indicating that the velocities of the orbiting material is relativistic. this is more strong evidence that the central object is a black hole (and not a white dwarf or neutron star).

these are just a few of the many pieces of evidence suggesting the existence of black holes. the relativistic jets of active galactic nuclei that stretch anywhere from thousands to millions of light years into space and are as straight as an arrow also suggest the existence of black holes.

i haven't even scratched the surface now that i think about it. if you're seriously interested in learning about the all the evidence suggesting the existence of black holes, but don't want to deal with lots of technical jargon and math formulas, check out "The Black Hole At The Center Of Our Galaxy" and "The Edge Of Infinity: Super Massive Black Holes In The Universe," both by Fulvio Melia...they're both chalked full of information that is easily retained, yet aren't boring at all, and are quite easily understood by the layman.

abhishek k
#4
Nov3-10, 02:12 PM
P: 3
Black Holes.

i was reading a brief history of time and i answers very clearly the very question you asked. Though black holes absorb all light(one way to detect anything) it does have a observable gravitational effect through which the have been proved to exist. they also release x-rays which they don't suck in(this i couldn't understand why) which is another way to detect them
George Jones
#5
Nov3-10, 02:33 PM
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P: 6,242
Quote Quote by 94JZA80 View Post
some would say that, b/c we have yet to literally "see" the shadow of a BH (the event horizon) against the backdrop of stars and illuminated gas and dust, we do not have definitive proof of the existence of black holes.
There is a good chance that, within a decade or so, we should be able to "image" in this way the astrophysical black hole at the centre of our galaxy. See

http://www.scientificamerican.com/ar...f-a-black-hole

http://arxiv.org/abs/astro-ph/0607279.
94JZA80
#6
Nov3-10, 02:36 PM
P: 121
Quote Quote by abhishek k View Post
i was reading a brief history of time and i answers very clearly the very question you asked. Though black holes absorb all light(one way to detect anything) it does have a observable gravitational effect through which the have been proved to exist. they also release x-rays which they don't suck in(this i couldn't understand why) which is another way to detect them
You've got a bit of a contradiction in your statement - X-rays are a form of light/electromagnetic radiation, so a BH can't absorb all light and still emit X-rays. I'm actually currently reading Hawking's "A Brief of Time," and while I don't recall the exact part you're referrencing, i'd have to imagine that the X-ray emission you'rereferring to is coming from the radiation given off by the accretion disk of material orbiting and falling into the BH, and not from the BH itself...
abhishek k
#7
Nov5-10, 02:49 AM
P: 3
Quote Quote by 94JZA80 View Post
You've got a bit of a contradiction in your statement - X-rays are a form of light/electromagnetic radiation, so a BH can't absorb all light and still emit X-rays. I'm actually currently reading Hawking's "A Brief of Time," and while I don't recall the exact part you're referrencing, i'd have to imagine that the X-ray emission you'rereferring to is coming from the radiation given off by the accretion disk of material orbiting and falling into the BH, and not from the BH itself...
yup! you are right i read the part again after writing the previous reply. The x-rays are not emmited by the black hole itself but the matter at the event horizon. This is the reason why x-rays aren't absorbed by the black hole
Driftwood1
#8
Nov5-10, 04:40 AM
P: 66
Quantum uncertainty allows for one of the particle pairs to escape at the event horizon boundary

You can view it as Quantum mechanics allowing the speed of one of the particle pairs to exceed the speed of light and therefore escape

Some Big Relativistic Rule seems to be violated here
94JZA80
#9
Nov5-10, 09:20 AM
P: 121
Quote Quote by abhishek k View Post
yup! you are right i read the part again after writing the previous reply. The x-rays are not emmited by the black hole itself but the matter at the event horizon. This is the reason why x-rays aren't absorbed by the black hole
keep in mind that other forms of radiation will escape in the same manner. the accretion disk emits less and less energetic radiation as as you make your way from the inner edge of the disk to the outer edge, that is, as you progressively get further away from the event horizon. i don't know enough about high-energy astrophysics to say for sure, but if X-rays are being emitted by the accretion disk very close to the event horizon, then i would imagine that UV and visible light are emitted further from the event horizon, and so on and so forth all the way down through radio emission. perhaps even gamma rays are being produced in a region closer to the event horizon than where the X-rays are being produced. now whether we can see all this radiation being emitted is another story, since often times vast amounts of interstellar dust blocks our direct line of sight with the BH. again, i don't specialize in this kind of thing at all, so i'm merely postulating, but i would also imagine that its tough for us to see UV and visible light due to their range of wavelengths being similar in size to the dust particles they're trying to penetrate as they travel to us. infrared, microwave, and radio emission however i would think is easier to see b/c the range of wavelengths that these energies correspond to are much larger than the diameters of the dust particles they're traversing through. so keep in mind that it isn't just X-rays that are emitted from the general vicinity of a BH, regardless of whether that's all we can see or not...


Quote Quote by Driftwood1 View Post
Quantum uncertainty allows for one of the particle pairs to escape at the event horizon boundary

You can view it as Quantum mechanics allowing the speed of one of the particle pairs to exceed the speed of light and therefore escape

Some Big Relativistic Rule seems to be violated here
i don't know if its correct to view one particle of a spontaneous pair as exceeding the speed of light, as i've never heard of that approach before. perhaps that is the violation of relativity? that being said, there is an explanation for it. Hawking Radiation makes the black hole "appear" as though its emitting radiation. but as we know, mass and energy can only cross the event horizon in one direction, and can never cross back over. from what i've read, the energy that "appears" to be coming directly from the black hole is actually coming from the energy in the BH's magnetic field exterior to its event horizon. minor fluctuations in these fields lead to the quantum uncertainty and the brief existence of particle-particle pairs. if one crosses over the event horizon before the pair disappears altogether, then the other particle of the pair appears to have been emitted by the BH itself b/c the creation of the pair happened so close to the event horizon in the first place. but again, the source of energy that creates the pair in the first place comes from the BH's magnetic field exterior to the event horizon.
Chimps
#10
Nov8-10, 01:20 PM
P: 70
I am not convinced by the mathematics of Hawking radiation. I believe it may be flawed.
Vagn
#11
Nov8-10, 01:41 PM
P: 295
Quote Quote by Chimps View Post
I am not convinced by the mathematics of Hawking radiation. I believe it may be flawed.
Why? What do you disagree with about it, and what makes you think your opinion is worth more than Hawking's? We may have actually observed Hawking radiation using an 'analogue black hole', where the refractive index increases as ligh is added to the substance, such that the light gets trapped inside the black, yet radiation was still detected coming out of the 'black hole'.

Source
The Actual Paper
94JZA80
#12
Nov9-10, 10:26 AM
P: 121
Quote Quote by Chimps View Post
I am not convinced by the mathematics of Hawking radiation. I believe it may be flawed.
i'm also skeptical about Hawking Radiation, but only b/c i admittedly know little about the topic. i mean i think i have a grasp on the basic concepts and generalizations, but i have not specifically studied it. i don't have a problem with the existence of Hawking Radiation, but i'm not quite sold on the idea that it always implies a net loss of mass/energy of a black hole, that is, i'm not sold on the whole idea of BH evaporation.

we know that quantum fluctuations occur in the magnetic field of a BH, and that those minute amounts of energy are used to bring particle-antiparticle pairs into existence for infinitesimal time intervals. now i can understand that, if this occurs very near the BH's event horizon, it is entirely possible that one of the particles plunges through the event horizon never to return, while the other escapes to infinity, giving us the impression that the BH is radiating. i also understand that each particle must carry a fraction of the total energy that went into creating them. and so if the total energy taken from the BH's magnetic field in order to create a particle-antiparticle pair is 10, then the sum of the particles' energies is 10...that is, each particle must carry an energy less than the total energy 10 (i left units out for simplicity's sake). so if a BHs' magnetic field gives up a specific amount of energy in creating a particle-antiparticle pair, and the BH swallows one of the particles before the two can annihilate and return the energy to the vacuum, then the BH will have reacquired some, but not all, of the energy that went into creating the particle-antiparticle pair. hence the result is a net loss of mass/energy for the BH.

the things that make me unsure about the above scenario are the assumptions that have to be made in order for things to work out that way. what if we made some other initial assumptions? for instance, what if BOTH particles are swallowed by the BH before they can annihilate? then wouldn't the BH have reacquired ALL of the energy that its magnetic field gave up to create the pair in the first place? it would seem to me that this result has no net gain or loss of mass/energy for the BH, and therefore no evaporation (BH radiation) in this case. also, what about the instance in which the particle-antiparticle pair is produced far enough from the event horizon that neither particle falls in? in this case, it would seem to me that the BH loses the entire amount of energy it put into making the pair in the first place, and is evaporating (radiating) at an increased rate.

considering the above scenarios (and not knowing if all of them are even possible in reality), it would seem to me that in order for a BH to truly evaporate over time, the instances in which both particles of the particle-antiparticle pairs escape the BH would have to occur at approx. the same rate at which the instances in which both particles of the particle-antiparticle pairs fall into the BH occur, thus canceling each other's net effects of mass/energy gain and loss. this leaves only one other type of instance - the one in which one particle of the particle-antiparticle pair falls into the BH, and the other escapes, which yields a net loss of BH mass/energy each time it occurs, and hence results in a BH that truly evaporates over time.

perhaps i'm over-analyzing things - as i said somewhere above, i don't even know if its possible for both particles of an particle-antiparticle pair to fall into a BH, or for both of them to escape it altogether for that matter. again, those were assumptions i made for a theoretical/mental exercise. if the scenarios i suggested are realistically impossible, then the only scenario left is the one in which one particle fall into the BH, and one escapes it, again yielding a net mass/energy loss for the BH. but i would need someone knowledgeable to show me that the alternative instances which i suggested either do not exist in reality, or do exist, but cancel each other's effects in order to convince me that Hawking Radiation in fact results in the evaporation of a BH over time...
stevebd1
#13
Jan7-11, 03:00 AM
P: 608
Quote Quote by a.ratnaparkhi View Post
How we can prove the existence of black holes?
And how they are located in in the space as they can absorb light too?
You might also find this web page of interest-

http://maxim.gsfc.nasa.gov/
Cigarshaped
#14
Jan12-11, 06:43 PM
P: 4
One thing's for sure we cannot PROVE the existence of black holes.
Quote Quote by a.ratnaparkhi View Post
How we can prove the existence of black holes?
And how they are located in in the space as they can absorb light too?
We can construct theoretical proofs and adapt our cosmology..
Quote Quote by nabeshin View Post
..But strictly speaking, a black hole is the mathematical solution to Einstein's Equations corresponding to a delta-function mass distribution.


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