Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 23/11/2006 06:31:59
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How do we know black holes exist? How can we distinguish between a black hole and a very massive, sometimes non luminiferous object?
And since the strength of gravity varys with distance. If you pushed two particles close enough together, could they form a black hole?
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thebrain13>How do we know black holes exist?
Have you visited the Galactic Centre Research project? There's a movie of stars within 10 light days of GC performing orbits around a 3.6x10^6 Solar mass object that has 'flared' in the infrared a couple of times recently but is otherwise noticeable by its invisibility.
The 'size' of these gravitationally compact objects is related to their mass and spin rate: search for Schwarzschild and Kerr-solution to find the exact details. ed.
http://www.mpe.mpg.de/ir/GC/index.php
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There is a great deal of observational evidence for the existence of black holes.
However, no one has directly observed the event horizon of a hole which would clearly be unequivocal proof.
Most of the evidence, such as that posted by newolder above, excludes almost every possibility other than a black hole so the question really is : What is this massive, compact object if it isn't a black hole ?
No one has any answers to that question so the conclusion drawn from the observational evidence of these bizarre objects is that they are indeed black holes.
But bear in mind that no one has ever seen one.
Yet.
I'm sure it won't be too much longer before we have direct observations of the event horizon although there's a part of me that would find it much more interesting if we didn't find a black hole but something even weirder.
And even weirder than a black hole is something well... really, really weird.
[:D]
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In what sense could we be said to see a black hole?, We could not bounce radar pulse's of it as they would just disappear at the event horizon, we cannot detect the thermal radiation from it as they are very cold they are not called black for no reason.
We can detect their presence by their gravity and the disturbance they cause to nearby bodies but as for "seeing" them no way
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Black holes are actually extremely hot indeed.
Actually it's the matter around them that is hot. Matter falling into the hole gets torn apart by the steepening gravity well this distorts the object and creates heat.
This is a minor source when compared to the heat generated by matter spinning around the event horizon before it eventually falls in.
Matter gets accelerated to huge speeds, stars get ripped apart etc...
All this action causes huge amounts of emission in the X-ray band.
Black holes are hot.
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The only reason the hot part of the black hole does not reach out is because of the extremely high amount of gravity that also keeps the heat from escaping.
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The hot part does reach out.
That's how it's possible to detect them.
By using instruments to detect the hot x-rays etc...
X-rays travel at the speed of light.
They can therefore easily escape from the pull of a black hole as long as they have not crossed the Event Horizon.
They might loop around the hole a few times if they're particularly close but they will always escape if they don't cross.
If they do cross then it's "goodnight Vienna" for the poor old x-ray and so we don't detect that particular x-ray photon.
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Black holes are only hot if there is sufficient material in their vicnity to get hot as it loses angular momentum to fall into the hole and is not too big.
If there is no material around they are very cold indeed. stellar mass black holes are colder than the microwave background radiation and so growing by absorbing that.
Very big black holes (billions of solar masses) have soft enough event horizons to avoid disrupting whole stars as they cross the event horizon and so are quite likely to be less active when material is falling into them.
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Astronomical data seems to indicate that a few number of stars have revolutionary paths around the same point in space yet there is no body located there (presenting the possibility of a black hole's presence that is causing these star paths). [::)]
DocN
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Very big black holes (billions of solar masses) have soft enough event horizons to avoid disrupting whole stars as they cross the event horizon and so are quite likely to be less active when material is falling into them.
So that means that this matter is shredded after, rather than before, it crosses the event horizon, making the results of that act undetectable?
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Pretty much yes.
The technical term for getting ripped apart by gravity in the vicinity of a Black Hole is (and I'm not kidding here) : Spaghettification.
This doesn't happen if you have a really big event horizon radius.
You can cross into the interior of the black hole without even realising it.
In fact, there are some who say that this has already happened.
[:D]
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It is an interesting idea that we are already inside a king sized black hole!, If we were would we be seeing the expansion we observe rather would we not accelerating towards the central singularity?.
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Just think about a few of the possibilities about what goes on inside a black hole. For simplicity let us think of it as a single event and not worry too much about stuff falling in afterwards.
One very simple approach is that it all collapses down towards the middle going faster and faster somehow misses passes through the middle and bounces back again so we have a highly dense and heated material expanding.
Another possibility is that as the material collapses and gets denser so the gravitational field increases further so a second event horizon forms, the first event horizon occurs when the gravity stops light escaping into the universe but the material inside the hole has a higher field than that and the light cant even get to that first event horizon as the material collapses this second event horizon will collapse too and get smaller and more intense it will also emit hawking radiation eventually it will collapse to a size where the energy loss from the hawking radiation balances out the energy in the hole so it might achieve some sort of stable state.
Now neither of these take angular momentum into account and this is probably the most important feature. It is highly unlikely that any black hole will form without it having some rotation so as it collapses it spins faster and faster until the energy in the angular momentum balances out the gravitational energy in the hole. What might happen then I will save to another post as I have to leave the discussion now.
but in all three cases we have things happening that will turn the collapse into an expansion or a radiating object.
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The technical term for getting ripped apart by gravity in the vicinity of a Black Hole is (and I'm not kidding here) : Spaghettification.
Does it mean it's better to stay out of italian culture? [;)]
In response to the OP, according to Sthephen Hawking's theory, a black hole should radiate a certain amount of EM radiatiom from the events horizon, so, looking at it carefully, it should appear as a black hole inside a sort of bright "crown".
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The actual radiation from black holes is so tiny that only the very smallest holes radiate significantly see
http://xaonon.dyndns.org/hawking/
one that is at about the same temperature as the cosmic microwave background is only .007 cms across. and bigger ones are colder the sun would make one with a diameter of about 2 miles
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what about my second question. Why couldnt you push two particles close enough together, so spacetime would bend so much that light could not escape?
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You can.
The problem is getting them close enough together.
The particles are very small to start with and the event horizon would be amaaaazingly small so getting enough energy into the particles to get them close enough together is a major challenge.
It is possible that the LHC will start to produce mini black holes when it comes on line next year.
If you do manage to shove the particles together to create the hole it would evaporate in a burst of Hawking Radiation in a fraction of a second. It is this characteristic radiation signature that will show that black holes have been generated in the LHC.
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I have a theory that explains black holes, slight variations in time detected by atomic clocks and how galaxies are formed. This theory also explains why some galaxies spin in different directions and why some stars spin in different directions in relation to us towards the center of our own Milky way. It also works quite well with the observations of infinity, quantum mechanics and hologram theory. However I am not a Physicist and I'd like to know the best way to publish this idea and receive some small credit before real physicists get the pleasure of going, wow, we can get rid of all that made up dark matter nonsense and have a party.
I am happy to write a short paper on it but I'd like to know the 10 places I should be putting it. I mention it here as I think you'll find it interesting. Reply to me and not the thread might be best. ;-)
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RBB - you can post in the New Theories sub-forum linked here (http://www.thenakedscientists.com/forum/index.php?board=18)
We do get theories which overturn modern physics a couple of times a week tho! So you might not get the acclamation or immediate acceptance that you expect ;D
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After watching a special on National Geographic, a question came to mind about the blackness, so to speak, about the nature of black holes. If, as this special contends, time stops for material crossing the event horizon and represents itself to an outside observer as frozen upon it's surface, then how can the black hole be truly black? And considering the volume of material that would be involved, gases, planets, and also brightly shining stars, wouldn't these still be seen clustered around the parameter of the black hole? This being the case, the volume of luminous materials involved, being frozen in time, should be quite bright. If on the other hand, black holes are truly black, then the idea that an obsever should still see every object frozen in time surrounding the event horizon is in error. These two concepts can't co-exist.............................L_N
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Yes they can and you are quite familiar with them in everyday life when similar things happen.
Consider a conventional filament lamp, for example, a car headlamp bulb it is turned on and emits a bright light. You turn it off and the light does not go out instantly it gradually fades and if it is dark and you have not looked at the bright like you can still see it glowing a couple of seconds after you switched off the bulb if you used an infra red camera you might still be able to detect it after ten or fifteen seconds and if you used very sensitive resistance measurements on the filament you could detect that it had been turned on and was still cooling maybe fifteen minutes after it had been turned off in theory it will be cooling off for ever because that's what the equations say, but the difference eventually becomes too small to detect.
When objects fall into black holes if they are hot and emitting light and you can see them this light is red shifted continuously becoming infra red microwave and eventually radio waves just like the filament cooling. In theory this goes on for ever but you would not be able to see this or even detect it after a brief period of decay.
For some stupid reason the "gee whizz" side of science publicity tends to latch on to this and get normal people totally confused. It really annoys me and I wish science publicists would tell people things properly as they are in ways that they would be familiar with and not try to give what are perfectly normal things spurious air of mysticism.
OK a luminous object would not instantly vanish the moment it crosses the even horizon it would just fade out after a brief period if you watched it.
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OK a luminous object would not instantly vanish the moment it crosses the even horizon it would just fade out after a brief period if you watched it.
OK,.............So what you're saying is that the frozen image of the falling object would, basically, remain forever at the horizon but it's visual representation would fade out rather quickly, if I correctly understand your answer???
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Yes they can and you are quite familiar with them in everyday life when similar things happen.
Consider a conventional filament lamp, for example, a car headlamp bulb it is turned on and emits a bright light. You turn it off and the light does not go out instantly it gradually fades and if it is dark and you have not looked at the bright like you can still see it glowing a couple of seconds after you switched off the bulb if you used an infra red camera you might still be able to detect it after ten or fifteen seconds and if you used very sensitive resistance measurements on the filament you could detect that it had been turned on and was still cooling maybe fifteen minutes after it had been turned off in theory it will be cooling off for ever because that's what the equations say, but the difference eventually becomes too small to detect.
When objects fall into black holes if they are hot and emitting light and you can see them this light is red shifted continuously becoming infra red microwave and eventually radio waves just like the filament cooling. In theory this goes on for ever but you would not be able to see this or even detect it after a brief period of decay.
For some stupid reason the "gee whizz" side of science publicity tends to latch on to this and get normal people totally confused. It really annoys me and I wish science publicists would tell people things properly as they are in ways that they would be familiar with and not try to give what are perfectly normal things spurious air of mysticism.
OK a luminous object would not instantly vanish the moment it crosses the even horizon it would just fade out after a brief period if you watched it.
SS - I think you are in danger of over simplifying - from an outside accelerated reference frame the object does not appear to cross the EH (or at least not observably); although it never stops it gets slower and slower in its passage toward the EH. the difference in gravitational potential means that time dilation is extreme and two consequences of this are the increasing red-shifting (as you explain) and the slowing of the object from the perspective of the observer. From the frame of the object then nothing weird occurs at all on the event horizon
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Some Black holes are form with the help of two Dwarf planet right?But what happens to dwarf are they fused into one Because of the gravity?
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Not correct the smallest black holes observed have a mass of 5-10 solar masses
From Wiki
"This suggests that there must be a lower limit for the mass of black holes. Theoretically, this boundary is expected to lie around the Planck mass (mP = √ħc/G ≈ 1.2×1019 GeV/c2 ≈ 2.2×10−8 kg), where quantum effects are expected to invalidate the predictions of general relativity"
There seems to be no way that low mass black holes can be created in the pesent universe and any created at the bigbang would have long since evaporated
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I agree imatfaal We are always talking about what an outside observer would see and not what the object at a distance would experience. This I believe is the biggest problem because most people do not believe that what they see as an outside observer is very different from what the traveller that they are observing would experience. The traveller always thinks things are perfectly normal locally with the exception of local temperatures and gravity gradients of course. Distant objects of course are distorted for them though.
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Slowed time of star reduces brightness of the star relatively of our time. :P
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I agree imatfaal We are always talking about what an outside observer would see and not what the object at a distance would experience. This I believe is the biggest problem because most people do not believe that what they see as an outside observer is very different from what the traveller that they are observing would experience. The traveller always thinks things are perfectly normal locally with the exception of local temperatures and gravity gradients of course. Distant objects of course are distorted for them though.
QFT - Exactly.
"This I believe is the biggest problem because most people do not believe that what they see as an outside observer is very different from what the traveller that they are observing would experience." I might have to steal that paragraph :-)
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"Not correct the smallest black holes observed have a mass of 5-10 solar masses"
Hoisted by my own petard, astronomers can be forgiven for using their own special units such as Solar masses for out of this world things but I wince when they go on about Ergs or Angstroms when there are proper well understood S.I units.
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syhprum - Soulsurfer, or JP could confirm but I believe that once you get into physics properly systems of units and measurement become a very movable feast, with different sub-disciplines using different units and setting different constants equal to one, ignoring others etc. As long as you are consistent - you should be safe, the danger lies when you start mixing n matching
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fblogs.scienceforums.net%2Fswansont%2Ffiles%2F2012%2F04%2Frocket-science.jpg&hash=60975df1964ae6819e5f55f39b3d4458)
from here http://blogs.scienceforums.net/swansont/archives/11498
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I'm pretty sure they must exist, and that they must be really, really, weird phenomena considering that they can frame drag the space around them at the (almost) speed of light according to some ideas. That as they seem to spin that fast. there should be all kinds of weird phenomena due to that spin.
Both math and observations agree here.
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I think that the super massive black holes at the centre of each galaxy are 3D electromagnetic, and this central magnetic force field holds all the stars in that galaxy in positions so that they can spin together without interfering with each others orbits. If you cant see it then it has no mass; right; but magnetism has no mass either!
CliveS
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When a "Black Hole" evaporates, do the enormous polar jets of gamma rays, plasma and particles also vanish? Is it possible to have a "Black Hole" without the gamma ray jets? Hmmm.
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If they can get it to work, the LIGO gravity wave observatory should be able to detect black holes and other supermassive compact objects as they enter a death-spiral around each other, radiating away huge amounts of energy as gravitational waves.
By looking at the final orbits of the objects, they should be able to identify some objects which are definitely too compact and too massive to be anything but a black hole. Beyond a certain density, the "escape velocity" will exceed the speed of light, and it will become a black hole, no matter what it is made of...
It's a question of how much they can improve the sensitivity of the instrument, and what is the probability of a massive-enough event occurring close enough to our part of the galaxy...
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Robro, I think the black hole would lose its gamma ray jets when it lost its magnetic field. moving charged particles can bend in a magnetic field, so if its strong enough the only places where it can escape are directly on the north and south sides. If the black hole loses its mass, it probably loses its magnetic field and thus its ability to bend as well.
And Yor On, my original thinking (and it's only a hunch) is that something is going to stop gravity from growing stronger and stronger indefinitely. What I think probably happens is that gravity weakens gravity. That is gravity doesn't grow linearly with each additional piece of mass but it gets weakened as the amount of mass in an area grows. eventually the strength of gravity will reach a maximum that peaks under the escape velocity of light allowing us to avoid all of this "going back in time/wormholes/whatever" type nonsense and conclude that black holes are just really dense objects with a ton of mass.