There's a Hole in the Middle of It All 693
Apparition writes "CNN is reporting that the star at the center of our galaxy is actually a super-massive black hole. The article then claims that it occupies a volume of space about 3 times that of our solar system. If my math is correct, about 230 million suns could fit into that same volume, so it doesn't impress me that the claimed mass of the black hole is only between 2.6 and 3.7 million times that of the sun. So what is up here? Since when do black holes occupy so much space (I thought they were points)? And how can something with a density only 1/100 of our Sun be called super-massive?" I think the article is talking about a maximum possible size of the object, due to limitations on the resolution of our instruments. Nature has a no-registration story about the research. Update: 10/16 23:44 GMT by M : There's an article with more information on space.com, and a press release from the European Southern Observatory.
Event Horizon (Score:5, Insightful)
RB
Re:Event Horizon (Score:5, Informative)
It is not however true that black holes are points. A black hole that became a point gravity source is what is referred to as a singularity. It was a singularity that became the big bang and if the "big crunch" theory is correct, it will probably be a singularity that the universe ends as, but under any other circumstances the creation of a singulairty would require a set of events so astronomically unlikely that it is not believed that any do have or will come into existence during the lifetime of the universe. So in fact black holes DO have a radius, but considering the tremendous size quoted here, I imagine they are in fact referring to the Swartzchild radius.
Re:Event Horizon (Score:5, Interesting)
Re:Event Horizon (Score:5, Informative)
True, but current theories also haven't proven that inside a black hole _is_ a singularity. Although it's been a while, I remember from an Astronomy class I took that due to the rate of spin outside the black hole, and that conservation of momentum would mean it would spin faster inside means that the odds of a true point singularity are relatively low.
But what do I know?
String Theory... (Score:5, Informative)
BlackGriffen
Re:String Theory... (Score:5, Funny)
I know Sting has gone a bit far off the norm recently, but is there really a discipline and a body of scientists dedicated to studying him?
Re:String Theory... (Score:5, Funny)
Maybe Perl can be applied to figure out a really big string
Re:Event Horizon (Score:3, Informative)
Re:Event Horizon (Score:5, Interesting)
Since we have no unified theory, it is not possible to prove anything mathematically with confidence. The current theory of gravitation, Einstein's general relativity, requires a singularity. But GR is presumed not to be valid at quantum scales of distance, and since a singularity is infinitely small in GR, all bets are off.
Re:Event Horizon (Score:5, Insightful)
An event horizon is actually just the boundary between light escaping and light being attracted by mass. It has nothing to do with the star being a singularity or not, only by the attraction force of the mass. That's obvious, right, so if all elements including photons (which have no mass) can no longer escape from the surface of the star, this means that the attraction force is higher than maximum speed of light, c. But does this have to mean that the volume of the mass is close to or equals 0? No. The star can only do 1 thing under extreme pressure: react it's core elements into heavier elements, untill they no longer react or destabilize the star enough to break the cycle, which probably can no longer occur. As the elements react, the star becomes heavier and the density of the volume rises, moving towards a singularity, but there is no reason to assume it _is_ a perfect singularity.
In fact, the black hole is known to radiate Hwaking radiation, which means that the hypothetical perfect singularity black hole model, which can only absorb matter, does not exist. If the said conditions are not perfectly valid for a black hole, then why would it be a perfect singularity, even if this Hawking radiation exists only on a quantum probabilistic level?
Re:Event Horizon (Score:3, Informative)
Maybe you want to verify your knowledge of physics before posting a response?
"Spinor fields describing particles of zero rest mass satisfy the so-called zero rest mass equations. Examples of zero rest mass particles include the neutrino (a fermion ) and the gauge bosons (as long as gauge symmetry is not violated) such as the photon or Higgs boson. "
http://mathworld.wolfram.com/ZeroRestMassEquati
Naked singularities (Score:5, Interesting)
Of course this is all based upon classical arguments, and without a theory of quantum gravity we can't be sure. However it hasn't stopped Hawking and Penrose arguing about "cosmic censorship principles" :)
Re:Naked singularities (Score:3, Informative)
one of the three properties a black hole can have - size, charge and angular momentum
IIRC BH also have entropy.
Re:Event Horizon (Score:5, Informative)
The discussion you refer to is the one about Hawking radiation. Stephen Hawking has demonstrated that Black Holes do actually(counter to intuition) radiate an extroardinarily small amount of energy. There is considerable debate as to whether it is possible for this radiation to ever cause the black hole to dissipate.
How so? (Score:5, Interesting)
Eh? Could you explain what you're talking about here? Because as far as I know, Hawking and Penrose's work has nothing to do with the likelihood of black holes forming. Indeed, one of the things about black hole formation in that no matter how unsymmetrical the initial state the end result is highly symmetrical, possessing no distinguishing features other than mass, charge and angular momentum... the "black holes have no hair" theorem.
Or are you talking about the recent results in M-theory proving Berkentstein's semi-classical formula for black hole entropy? If so, I'm still not sure what that's got to do with black hole formation... it strikes me you've got things confused...
Re:Event Horizon (Score:3, Interesting)
General Relativity on the other hand, has been extensively verified, and has been correct in every test we've set for it. General Relativity predicts that singularities will form from a collapsing star.
I still think that m-theory is handwaving until some testable predictions come out of it. BTW, I think that m-theory or one of it's derivatives will provide us a better description of the universe, but not today.
Re:Event Horizon (Score:5, Funny)
I think you mean the "gnab gib." You, know, a Big Bang backwards? I've seen it before, and it's quite a sight. It plays every night at the Restaurant at the End of the Universe.
Re:Event Horizon (Score:5, Funny)
Re:Event Horizon (Score:5, Funny)
Re:Event Horizon (Score:5, Interesting)
We haven't the foggiest idea what the universe was all the way back to time=0, but starting at at least time = 10^-43 seconds, the universe was a very large, quite possibly infinite, distribution of matter. It was not an explosion away from a point, but an expansion of matter "away". Space time expanded like a rubber sheet, with every point moving away from every other point.
Neat, eh?
Re:Event Horizon (Score:5, Insightful)
Re:Event Horizon (Score:5, Informative)
It's not. Astronomers have known for a while [faqs.org] that the universe was expanding, but didn't know the rate. They recently discovered that the rate was accelerating [faqs.org]!
Cheers
Schwartzchild radius, singularities, etc (Score:5, Informative)
The Chandrasekhar limit gives the size limit for a star to collapse and produce a white dwarf. Most stars end their lives with a gravitational collapse, but electron degeneracy pressure (from the Pauli exclusion principle) prevents further collapse. However, for stars above ~1.2 solar masses, the gravitational collapse will overcome fermion repulsion, and the collapse will continue. Once the star's density has reached a certain point, it will collapse into a singularity. That density times the star's mass determines the Schwartzchild radius.
The event horizon is delineated by those light rays that will neither fall in nor escape from, the black hole. However, just because you cross the event horizon does not necessarily mean you will strike the singularity. Instead, it depends upon the type of black hole you've encountered.
In actual reality, you'll be fried by the blue shifted radiation coming from the accretion disk around the hole, but let's ignore that quibble.
Black holes have mass, spin, and charge. No other properties are discernable behind the event horizon. The fact that the above properties can be determined without a world-line (that is, information also does not propagate faster than light, and hence cannot escape) says something fundamental about those properties.
An uncharged, unspinning black hole is called a Schwartzchild hole. Once you cross the event horizon, you will unavoidably strike the singularity and perish.
In the other types of black holes, such as the Kerr black hole (uncharged, spinning), Reisnner-Nordstrom (charged, zero angular momentum), and the Kerr-Newman black hole (charged, spinning) it is possible to cross the event horizon without striking the singularity. Instead, you can pass into another universe.
Indeed, it's theoretically possible that you will pass through many universes. This is a one-way trip, however. If you try to get back to where you were, you will encounter the singularity and die.
Actual solution of the Einstein field equations for the holes listed above, however, produce perturbations. These perturbations, so far, cancel out the ability to miss the singularity and enter another universe.
Moving on, Hawking demonstrated that black holes evaporate. Hawking radiation is produced when half of a virtual particle pair appears inside the event horizon. Since both particles are no longer available to disappear under the Heisenberg time limit, the remaining particle acquires real energy. This energy comes from the black hole.
Since the rate of evaporation is proportional to surface area/mass, smaller black holes evaporate explosively. Indeed, no black holes smaller than a proton could exist from the big bang.
Finally, recent research shows that the universe is inflating, due to Einstein's cosmological constant (which, he ironically labelled as his "worst mistake"). That is, Hubble's constant is increasing. There will be no Big Crunch. The universe will expand at a faster and faster rate into nothingness.
There are a lot of good books on cosmology. General Relativity is undergoing a renaissance right now because of all of this important, new information.
Re:Event Horizon (Score:3, Interesting)
Re:Event Horizon (Score:3, Funny)
size (Score:5, Informative)
Re:size (Score:5, Informative)
The black hole (of the mass of several million times that of sun) at the center of the Milky Way galaxy, has been suspected for decades. However, as observations keep on shrinking the confinement radius, it keeps on ruling out other potential models.
Re:size (Score:3, Insightful)
Actually, current theories including string theory prevent the infinite point claims, but get to the next best thing (something in the order of 10 to -37 meters if I recall right).
The size reported makes no sense though for a Schwartzchild radius of a black hole with the indicated mass, it's way way too large.
Re:Infinate density (Score:3, Informative)
Infinite density could have finite mass. As you may recall from collegiate math:
lim(x->0) constant/x = infinity.
Re:Infinate density (Score:3, Interesting)
lim(x->0) 1/x -> infinity,
meaning for all "small" numbers d, there exists N such that for all n > N, the nth value of the limit, x < d.
In very few models does lim (x->0) 1/x actually equal zero; this is particularly important in relative comparisons. [See lim x->0 sin(x)/x] The universe may subscribe to other phenomenon with respect to infinity; its fundamental model has changed a few times (eg. Lobachevskian geometry)
Re:Infinate density (Score:4, Funny)
You mean Ellen Feiss?
it was like Beep beep like beep beep
Re:Infinate density (Score:5, Informative)
No. Black holes aren't much different from any other object, at least if you stay outside the event horizon. Things won't be 'pulled in' by a black hole any more than by any other object with the same mass. If the sun suddenly and magically became a black hole (keeping the same mass), the earth would continue on its merry way in the same orbit.
Watch that title (Score:3, Funny)
It's just disgusting!
From the article: (Score:3, Insightful)
So, does that mean that in time, the blackhole will swallow up the star?
-Cyc
Re:From the article: (Score:5, Informative)
Comets can orbit the sun for a really long time; some smack into an object (like the sun, for instance), some escape their orbit, and some just keep orbiting. There's nothing that guarantees the star will get sucked in; it all depends on the orbital path, really. It may experience a slingshot effect and leave the black hole altogether.
Re:From the article: (Score:5, Funny)
-DVK
Visual demonstration of the above (Score:3, Informative)
"This is the coolest this i have seen all week, click
pull off a moon only orbit for maximum kudos"
The physics for object orbits are incredible. This is a great demonstration of the exact effects you describe, and should apply to the questions and comments about orbits around a black hole.
Enjoy!
P.S.: You have no idea what a breath of fresh air it is to be able to visit cool links that aren't being slashdotted to hell and back.
Orbiting a Black Hole (Score:5, Informative)
That said, there is evidence from general relativity that due to graviton radiation (gravity particles), large orbiting bodies slowly move closer to each other. The gravitons leaving such a system take energy out of the system slowly bringing the orbiting bodies together. This effect is (AFAIK) theoretical, although many people are currently working on ways to detect this graviton radiation and show that it is coming from systems like this. So in this case, yes, eventually (think eons) the star and the black hole would slowly move towards each other (the star would move more since it the least mass of the two) and in this type of collision, the black hole wins.
Shrinking orbit effect was observed (Score:3, Informative)
Roger Penrose talks about it in his book `the emperor's new mind', and here is an excellent link [cornell.edu]
Now we know (Score:4, Funny)
Super-Massive Black Holes (Score:5, Informative)
Re:Super-Massive Black Holes (Score:4, Interesting)
As such with a larger black hole (large M, smaller 1/r^2) the difference in gravitational effects over the size of say a person is fairly small because r^2 doesn't change an awful lot. However with a small hole (small M, large 1/r^2) the difference in strength of the gravitational field over the size of a person is a lot larger and so there are tidal forces which tend to cause things to be ripped apart.
academic implications? (Score:3, Interesting)
Re:academic implications? (Score:3, Funny)
Re:academic implications? (Score:3, Funny)
Cygnus X-1 is a black hole, therefore blackholes exist.
Re:academic implications? (Score:3, Informative)
This is not conclusive proof of black hole theory, only conclusive proof of a supermassive object at the center of our galaxy. It does not answer the theoretical question as to whether black holes or gravastars best fit the observations.
Obviously, the scientists making this announcement would be in the black hole segment of the physics community.
Trying to think of something witty and clever to end this post with . . . . ah, screw it . . .
To clarify... (Score:5, Informative)
The "size" of the black hole refers to the size of its event horizon (a.k.a the Schwarzschild Radius), which is R = GM/2c^2. For a huge value of M ("supermassive"), the event horizon is very large: once you cross this, there's no coming back, and our physics stops at the edge. But since R is so large, the tidal forces are small at the event horizon - much smaller than the tidal forces at the event horizon of a smaller black hole. (Chew on it for a second and it makes sense).
The "actual" naked singularity is in fact a point, but we have no way of probing anything inside the event horizon. So calculating the density of a black hole is misleading...
Re:To clarify... (Score:5, Insightful)
I'm not picking on you, others have been saying things like this too. They talk about "there's no coming back", "can't communicate to the outside", and "physics stops at the edge" and such. These are all theories, not facts. I wish people would just be a little more careful in their phrasing, as indeed, black holes themselves are still theories.
Even relativity is only a theory. But I digress.
No, physics doesn't stop at the edge, our understanding of physics breaks down at the edge. We don't know what happens because our physics deals in infinities that make no sense once you cross the event horizon. Physics still exists, it's just undefined to us.
In the same vain, communication from within a blackhole to the outside is impossible, assuming our basic theories of black holes are correct, and assuming that there's no way to communicate faster than the speed of light. Again, relativity is a theory, not a law. It's a theory that has come into question recently as well.
I'm not putting down Einstein or relativity. Amazing stuff, to be sure, but it may not be entirely correct.
Re:To clarify... (Score:5, Insightful)
I wish people had a little bit of training in theory of science, before they started worrying about phrasing in discussions about science.
In day-to-day communication, we use the word "theory" to denote something we are not sure of. Thus in day-to-day communication "just a theory" makes sense.
However, in science, a "theory" is basically what the majority of scientists believe to be the truth. There is no difference between a "natural law" and a theory (In fact, "natural law" is most often viewed as a misnomer, and is simply something we use for historical reasons). And there is no "higher level" something can escape to, when people think it's worthy of a higher status than "just a theory".
If you want a word for what scientists use for the day-to-day usage of "theory", their word is "hypothesis". A hypothesis is nothing but an idea. Most theories start as a hypothesis, and then, after a sufficient number of supporting facts have been found, and experiments have been done, people will then speak of it as a "theory". Sometimes, scientists will also use the word "model" as something in-between, but most often it is used by engineers using well-known theories to model complex phenomena.
As for black holes being "only a theory" (in the meaning of "just a hypothesis". Yes and no! It would be very hard to come up with a cosmological model that fitted our universe, that would not predict the existence of black holes. And it would be very hard to explain some observed phenomena as something else than a black hole. On the other hand, the theories of what goes on inside the hole, how it was created, and how it dies (if ever) is very much up to discussion. As for doubting their existence, well it's possible, but not easy...
As for relativity being "only a theory", again assuming you mean "just a hypothesis". In a word, no! The basic ideas of relativity has predicted a lot of observable things in the universe better than any other model. And it has been verified again, and again through experiments. Is it entirely correct? No, it doesn't fit in with quantum mechanics, and therefore can't explain everything (just like Newtons laws can't explain everything). So it's reasonable to believe that there exists an even more complex theory of everything, that will incorporate both quantum mechanics and relativity. Unfortunately, there haven't been too much success in this area yet.
Re:To clarify... (Score:5, Insightful)
No. You are confusing formal logic with science. Science is a process of falsification, of disproof. Science can only operate by testing to destruction; repeated experiments can lend support to a theory, even overwhelming support as in the case of GR and QED, but no amount of experimentation will truly confirm either.
Re:To clarify... (Score:4, Interesting)
Is this true? Could you/someone explain to me what would prevent me from building a huge strong ring around the event horizon and lowering a probe from that ring through the event horizon? The ring could be stabilized by the gravity of the black hole itself and a counter-weight on the side oppossite to the probe. Would the force on the probe be so strong that no force is strong enough to pull it back? Or is it theoretically impossible to build a probe strong enough to withstand the gravity?
Re:To clarify... (Score:3, Informative)
You could do that, but it would be useless, and for this reason: The force you are applying to the probe counteracts the force of gravity on the probe caused by the black hole, and the *total* force on the probe drops below the amount necessary for it be within the schwarzchild radius. However, you wouldn't be able to probe anything inside the radius. It would just be as if you pushed the event horizon back. Sort of like pushing your hand into a waterbed: your hand is now where the waterbed *used* to be, but you still aren't inside the waterbed. But once you do enter the event horizon we don't know of any way get back.
Re:To clarify... (Score:5, Informative)
The short answer is "relativistic effects".
Near the event horizon, gravity warps space - the conventional notions of "distance" and "time" get fscked up.
What you propose is equivalent to saying "If I'm at the front of a train travelling at 99.999999% the speed of light, and I shoot a bullet forward at 2% of the speed of light, isn't the bullet going to be going faster than light?"
And the answer is, "Well, no. Because space and time are fscked up when you're going very quickly."
From the point of view of a guy standing at the end of the tracks, he'll shine a light down the track, see some X-rays bouncing back from the bullet and the train, before being flattened by both the bullet and the train almost simultaneously.
From the point of view of you (on the train), looking forward, you'll see the entire universe running at about 10000 times normal speed - stars evolving in minutes - and the bullet flying away from you at 2% of the speed of light.
Back to your original question - lowering a probe into the black hole and pulling it out again. Gravity will have a similarly-weird effects.
From the point of view of the guy lowering the probe, the probe will fall towards - but never through - the event horizon. It'll just fall more and more slowly, and if he shines a light at it to observe it, he'll see it get redder and redder, until it vanishes into the infrared. And since the probe never makes it past the event horizon, he never gets any data back from beyond it.
From the point of view of the probe, and looking up, time speeds up dramatically - in a few minutes, he sees the guy lowering him get change shifts, coming back, growing older, dying, the space station being abandoned, stars evolving, billions of years passing, whole galaxies fading into the infrared, and then when he hits the event horizon, he sees nothing avove him, and if he looks down, then it gets real weird. It's quite literally anybody's guess what he sees. But it's quite certain he can't tell anybody above him a word of it.
Relativity's weird like that. The freaky stuff - time dilation and what-not - has all been demonstrated by experiments involving clocks and airplanes and satellites. (The relativistic corrections made to account for a satellite's motion, for instance, are part of why GPS is so accurate.)
Oops. Got one part backwards. (Score:3, Interesting)
> From the point of view of you (on the train), looking forward, you'll see the entire universe running about 10000 times normal speed - stars evolving in minutes - and the bullet flying away from you at 2% of the speed of light.
Argh. The sped-up universe is what a guy on the back of the train looking backwards (and the guy on the black hole probe looking up) sees.
The guy on the front of the train (and you, lowering the probe and observing the probe) sees a universe running at 1/10000th speed - a 2.0 GHz Athlon will look like it's running at 0.2 kilohertz and what-not.
Re:To clarify... (Score:5, Informative)
Moreover, your clarification contains the essential answer to one of the original poster's comments. The mean density within the horizon, assuming the region is spherical, is
M / (4 / 3 pi R^3) = M / [4 / 3 pi (2G M / c^2)^3]
= 3 c^6 / (32 pi G^3 M^2)
The key point being that the mean density within the horizon is inversely proportional to the square of the mass of the black hole. For a black hole of 1 solar mass, the mean density within the horizon works out to be amazingly high : of order 10^16 gm/cm^3! On the other hand, for a billion solar mass black hole, this mean density is much, much smaller : of order
Another key point is that the masses are not directly detected -- the must be inferred by their gravitational influence on surrounding stars and gas. Observers currently do not have the resolution to probe down to the scale of the horizon, so the argument for a black hole is a compelling one, though not absolutely certain. The masses are not directly detected -- the must be inferred by their gravitational influence on surrounding stars and gas. The primary argument in favor of a black hole is the lack of other possible alternatives. One can prove a strict limit on the mass of a neutron star (which is the most compact stable object known to astrophysics) assuming only causality (ie, that whatever is holding up the neutron star has a sound speed less than the speed of light), is around 5 solar masses. Hence, the most tightly packed situation one could possibly imagine, with the same mass as observed, would be a cluster of several hundred thousand to millions of neutron stars. However, even such a situation is dynamically unstable over many orbits : the neutron stars will tend to form tighter and tighter binaries at the core of the cluster until they merge. Even a single merger would likely create a small seed black hole, which swallow up all the surrounding stars until no matter is left to accrete. So even in this extreme situation, the outcome would eventually be a supermassive black hole. For this reason, the argument for a black hole at the center of our galaxy and others is a very strong one -- if it were a legal case, it would likely hold up in a court of law. However, the absolute proof will require a "smoking gun". Perhaps this will consist of a detection of gas emitting from the accretion disk right around the black hole horizon, carrying with it an absolutely unambiguous signature of the horizon. Or perhaps it may come from gravitational waves radiating at very low frequencies (millihertz or below) -- a telltale sign of the slowly oscillating hole. Such waves will be undetectable from the Earth's surface due to ground noise, and will require a spaceborne mission such as ESA's LISA.
Bob
So... (Score:5, Funny)
Just wondering.
-Goran
No... (Score:5, Funny)
That's Florida.
That explains it (Score:5, Funny)
Diameter of a Black Hole (Score:4, Informative)
This would seem to imply that, in theory, a very large black hole could have rather low density inside the event horizon. It seems to me that a black hole could spontaneously form around a particularly dense cluster of stars if it was large enough and they all happened to clump together.
But my head starts to hurt thinking about what happens to physics when a region of normal space suddenly finds itself inside a black hole like that. I am definitely not a physicist, so I can't explain what goes on inside a black hole, or if my globular cluster black hole is even possible.
Crispin
----
Crispin Cowan, Ph.D.
Chief Scientist, WireX Communications, Inc. [wirex.com]
Immunix: [immunix.org] Security Hardened Linux Distribution
Available for purchase [wirex.com]
Globular Cluster Black Holes (Score:3, Informative)
Re:Diameter of a Black Hole (Score:3, Informative)
At some level it will probably always be a mystery. It's a 'world' boundary since information can't get out (can it get in or is information crushed out at some point?). Ultimately it is a physical phenominon, not a mathematical model, so the reality may be quite a bit different than any mathematical model. If you could fly about the galaxy SF style you would probably learn a lot more about the actual structure of the universe from experiments related to this and other black holes.
It's pretty amazing what can be learned this far out. I thought I heard a mention on the NPR report on this about a star headed for the EH. The universe is always running experiments for us if we have the instraments in place to watch closely. Try following the link to the natural nuclear reactors and follow the link under the picture about the constancy of cosmological constants. Very cool instraments ... High res. spectroscopy allows them to look back in time and try to figure out why/how these constants might adjust. The Hubble is cool, but we are going to need an array of flexible instraments above the atmosphere to get at the really interesting questions.
Re:Diameter of a Black Hole (Score:5, Insightful)
Yes, time dialation approaches infinity as you approach the event horizon, so you can never actually enter a black hole, only mosey up to it :-)
This statement is commonly made, but it's not really accurate.
Yes, from the point of view of a distant observer, somebody falling into a black hole takes an infinite amount of time to do it. However, in the frame of reference of the hole-diver, the coordinates used for the far observer are no good. In fact, the Physics shows that in his frame of reference, the hole-diver goes through the hole in a finite amount of time, and that indeed nothing particularly startling happens at the moment of crossing the event horizon. (Other than it is after that that he will inevitably hit the singularity; however, there's no grand event that signifies the moment of crossing.)
Sounds contradictory, so you will ask, which is "really" right? I like to think about it with this thought experiment. Given an arbitrary amount of energy (and technology and ability to withstand tidal forces and etc.), could the far observer, after waiting an arbitrary amount of time, go in and retrive the hole-diver? If the hole-diver really does take an infinite amount of time to cross, then the answer would be "yes". It would be hard, but in principle the far observer could get the hole-diver. However, the coordinates that apply near the event horizon make it clear that the answer is "no". There eventually comes a time when an external observer, if he waits to long, is inable to retrieve the hole-diver.
What the far observer sees is the photons emitted by the hole-diver. As the hole-diver gets closer and closer to the black hole, the photons get further and further apart (time dilation) and longer and longer in redshift (gravitational redshift). The "last" photon is infinitely redshifted and takes an infinite amount of time to get out-- so the far observer never measures the hole-diver to drop through the hole.
-Rob
Re:Diameter of a Black Hole (Score:3, Interesting)
For some reason, I think I remember reading that as matter crosses the event horizon, it's stripped apart at the subatomic level (I suppose due to extreme gravitational forces) and that matter is shot inwards (towards the singulatity), while anti-matter is shot outwards away from the singulatiry. Please do correct me if I'm wrong, but I can look up wherever I read that if you'd like.
You're thinking of Hawking Radiation. It has nothing to do with matter falling through a black hole, but rather with virtual particle/antiparticle pairs that are being created and destroyed everywhere in space all the time. Thanks to Hesienberg's Uncertainty Principle, you can violate the conservation of energy if you do it over a very short period of time. All around us, there's a sort of "froth" in the vacuum made of electrons and antielectrons which spontaneously are created and then annihiliated. It all happens so extremely fast that nobody could observe any violation of conservation of energy.
When this happens right on the event horizon of a black hole, however, you can end up with one particle going into the singularity and the other particle escaping. Now, I don't understand the physics of this process in detail! I should-- I ought to look it up. However, what happens is that if the particle/antiparticle pair is created right at the exact spot, it can happen that rather than annihilating each other, they split and turn into real particles. You can observe the particle coming out, and to keep things balanced, the particle going in then has negative energy. The result is that the black hole loses mass (a very tiny amount, mind you). Over time, therefore, black holes evaporate. (Note that many black holes, esp. those at the center of galaxies, are being fed (and thus growing) much faster than they evaporate due to Hawking radiation.)
The timescale for evaporation of any appreciably sized black hole (solar mass on up to these supermassive black holes) is gigantic-- longer than the age of the universe. Very small ones, though, evaporate pretty quicky. Thus, if there were tiny black holes left over (say) from the Big Bang, we wouldn't expect to find any of them around today.
As for matter falling into a black hole: the tidal forces get larger and larger as you get closer to the black hole. Of course, this happens with any mass. Tidal forces due to being too close to the moon cause the Earth to stretch a bit and its water to slosh around. If you fall into a solar mass black hole, however, even before you got to the event horizon, the difference in the gravitational force on your feet and on your head would tear you apart. This will happen with all black holes, and it's an issue whether you're inside or outside the event horizon. Indeed, the event horizon is largely irrelevant to it, except as a limit of inevitability. For very large black holes, the tidal forces aren't so bad at the event horizon that you ought to be able to drop through it. For solar mass black holes, the tidal forces will kill you long before you can get to the event horizon.If you remember back in 1993 or 1994 when comet Shoemaker-Levy 9 hit Jupiter, it hit in several chunks over the course of several days. The reason was that the comet had been riped apart on a previous pass by Jupiter-- by exactly these same tidal forces. (Comets aren't really held together all that well, as things go, and Jupiter is pretty massive. Tidal forces from Jupiter and the other moons are also what heats up IO and keeps it volcanically active.) My point: tidal forces aren't some mysterious black hole thing, they're something you get with any mass. The only thing about black holes is that you can tend to get a lot closer to that mass than you can with any other form of the smae mass. (E.g., with the Sun, you'd have to be well inside the Sun before the tidal forces got that strong--- but at that point, most of the Sun's mass would be outside your position on the Sun, and therefore you wouldn't be feeling its gravity.)
-Rob
Re:Diameter of a Black Hole (Score:5, Interesting)
Are you posting this AC because you know it's false?
Schwartzschild radius scales as mass; density scales as mass divided by radius cubed; hence the density of black holes scales as 1/mass^2, i.e., as the inverse square of the mass.
Supermassive black holes are indeed quite un-dense. Taking the extreme limit of this relation, in fact, one finds that the observable universe is approximately the size of its own Schwartzschild radius, i.e., perhaps we are all living inside a giant black hole.
Damn! There I go again - it makes my head hurt every time I say that.
-renard
Size quoted is the orbit of a star (Score:5, Informative)
How many "it's the event horizon" posts do we need (Score:3, Insightful)
I can imagine the first few stepping on eachother, but doesn't anyone else bother to see what others have written before posting the same thing... over and over and over...?
Better article (Score:5, Informative)
Discovery Channel makes me think I'm smart (Score:3, Informative)
Interesting stuff -- once they were discovered to be in just about every galaxy, people smarter than me started thinking about how they formed. Conventional wisdom says that they formed after the galaxy took shape, and that stellar matter near the center collided and merged into these monsters. Another theory, however, posits that the SMBH actually triggered stellar formation in a cloud of otherwise unremarkable hydrogen.
The idea is that as the hydrogen gas fell inward and collapsed, the gas in the nearby area would heat up and glow. This is, of course, what we see. However, it goes further to say that this surrounding energetic gas could cause a sort of super-shockwave of energetic particles travelling back out through the surrounding gas, pushing it around and raising the density, causing the whispy bits to compress together to the point of fusion.
Poof! Stars born by black holes at the center of a gas cloud.
Pretty neat, I thought.
GMFTatsujin
Someone Obviously Hasn't Seen Star Trek V (Score:4, Funny)
Black hole size (Score:5, Insightful)
What is an interesting question is where the Roche limit is for these parameters, and how close this star is to that limit. (In other words, how much closer can the star get before it is ripped apart.) I seem to remember that it is possible to set up conditions so that the Roche limit is inside the event horizon. Obviously, the physics around there are very strange.
The math doesn't match the description! (Score:5, Informative)
- r = 2GM/c^2
where G = 6.67e-11 m^3/s^2*kg (the gravitational constant) and c = 3e8 m/s (the speed of light, of course). Plug in 3 million sun-masses (the sun weighs 2e30 kg), and you have- r = 8.9e9 m = 5.5 million miles = 0.06AU
So unfortunately, the event horizon isn't three times as big as the solar system. The earth's orbit is 1AU (that's how the unit is defined). The event horizon barely stretches past the surface of the sun (7e8 meters)!So much for that idea!
I thought... (Score:4, Funny)
The answer is easy (Score:5, Funny)
They're big points.
RMN
~~~
Don't listen to the editorial comment (Score:5, Informative)
I'm sure this editorial comment was well-intentioned, but the article would have been much better off without it. What the article refers to corresponds closely quite nicely to the Schwarzschild radius of a supermassive black hole.
A very massive black hole will necessarily be much less dense than the Sun, and can even be less dense than the Earth.
The simple reason is that (assuming a static, spherically symmetric mass distribution) the mass of an object is directly proportional to its Schwarzschild radius. But density is proportional to mass divided by radius cubed.
So if you double the mass of a black hole, you must necessarily double its radius. By definition this increases its volume eight-fold, and so its density is decreased by a factor of four.
So as you consider larger and larger black holes, you must see that their densities are smaller and smaller.
If you are in the market for a comparatively easy textbook that will teach you more about general relativity, I recommend Exploring Black Holes by Taylor and Wheeler. If you have a firm grasp of calculus and freshman physics, you will be able to handle it. It is more expensive than a normal book, but cheaper than the average textbook.
Comment removed (Score:3, Informative)
If I got this straight... (Score:4, Funny)
Very much like those things you find at a Krispy Kreme shop, but with a lot less frosting...
Does this mean that the voice we will hear at The End of Time will be saying "OOOhhh... donuts..."
Re:If I got this straight... (Score:4, Funny)
Very much like those things you find at a Krispy Kreme shop, but with a lot less frosting...
Does this mean that the voice we will hear at The End of Time will be saying "OOOhhh... donuts..."
Stephen Hawking: "I am intrigued by your theory of a donut-shaped universe, Homer. I may have to steal it."
Cheers,
IT
Black hole v. singularity (Score:3, Informative)
A black hole is just that -- a black hole. It is a region of space from which nothing can escape (approximately; black holes do very slowly radiate heat). In other words, the volume a black hole occupies is defined by the Schwartzchild radius: the point beyond which the escape velocity exceeds c.
A singularity is the "center" of a black hole; it is an infinitely dense point in space, of enormous mass.
Interestingly, black holes may have some useful properties for astronomers. Light heading towards a black hole will be refracted around it and bent; in essence, the black hole acts like a magnifying glass.
Re:Black hole v. singularity (Score:4, Informative)
The Usual Bias (Score:5, Funny)
Jeez.
-Waldo Jaquith
Re:The Usual Bias (Score:4, Funny)
So please, even if You were just a visitor, considering You're posting on slashdot You might as well forget the idea of returning and start living on slashdot. You're confined to the small space of 400 seconds from slashdot.
explanation by a physics geek (Score:5, Informative)
The size issue: the companion star's orbit tells us the maximum possible size of the central object. If the orbit is 17 light hours across, the primary is at most that large. It can be smaller, just as our Sun's diameter is smaller than the orbit of Mercury.
The proof the central object is a black hole is that nothing else can fit millions of solar masses into a sphere 17 light-hours across. The black hole need not fill that volume. More precisely, the event horizon need not fill that volume.
Singularities, point masses, event horizons: the size of a black hole depends what you mean. The singularity is the postulated point of infinite density: outside observers can't see it because it's inside the event horizon. The event horizon is the point of no return; in classical terms, the escape velocity equals the speed of light at the event horizon. The gravitational force is finite at the event horizon, and need not be extreme if the black hole is very, very large. If the universe is closed, we are all inside a black hole now, and will experience singularity at the Big Crunch.
But it isn't useful to think about the inside of a black hole. Different physics might apply -- lots of smart people think so. From the outside, as another poster wrote, all you get to observe is the black hole's total mass, total charge and total angular momentum -- that's plenty to work with in astronomical observations.
As to matter 'spiralling in', or the entire galaxy being sucked in by 'infinite gravity': Earth isn't being sucked into our Sun, is it? Unless you're quite close to one, the gravitational field of a black hole essentially (asymptotically) follows an inverse square law, like the gravity from any object. (When you get close, in units of the Schwarzchild radius, you do indeed 'spiral in' because the field strength increases faster than inverse square. The precession of Mercury's orbit is used to measure the deviation from inverse-square near our Sun, and is one of the 'proofs' of Einstein's General Relativity.)
The other mechanism for 'spiralling in' is loss of orbital energy due to friction, as in the accretion disk around neutron stars, for example.
That is all. Return to your homes and families. :-)
Diameter in story is NOT event horizon (Score:5, Informative)
If you work out the schwartzchild radius of the sun using r=2GM/c^2 it comes out to around 3000 m. For the upper limit of 3.7 million solar masses that would mean that the black hole had a schwartzchild radius of around 1 x 10^10 m. This is about a factor of 14 larger than the radius of the sun which is 7 x 10^8 m.
This is no where near as large as the "volume of space around 3 times larger than the solar system" which is in the article. The poster of the article was also correct that the density was way too low. It is correct that supermassive black holes have large event horizons which are larger than the radii of typical stars like the sun. However, the average density inside of that event horizon is still denser than a neutron star.
I wish I had the 5 moderator points I had last week, I'd go to town on this story...
Re:Diameter in story is NOT event horizon (Score:3, Informative)
wouldn't 3 times the solar system be about 17 light hours ?
Which happen to be the size ot the orbit of the star they were tracking.
not the size of the black hole.
That's the star's closest approach (Score:3, Funny)
This might be a misinterpretation. In the ESO press release [eso.org] they say:
So that puts an upper limit on the scale of the thing, but doesn't imply it takes up all of that space.
"Volume" is not referring to Event Horizon. (Score:5, Informative)
It would seem that the original poster's comment was correct in that this was the _Upper Limit_ of the radius of the supermassive object, and not the Event Horizon radius.
Let me clarify,
The Schwarzschild radius (Or Event Horizon) is given by
r_SCH = 2 G M / c^2
where G is gravitational constant, M is mass of object, and c is speed of light. If we use, as per CNN article (yeah, I know, good source)
M = 3 x 10 ^ 6 * mass of sun
mass of sun = 2 x 10 ^ 30 kg
s.t. M = 6 x 10 ^ 36 kg
and G = 6.67 x 10^ -11 Nm^2/kg^2
and c = 3 x 10^8 m/s^2
then r_SCH = 12 x 10 ^ 36 * 6.67 x 10 ^-11/9 x 10^16
r_SCH ~ 1 x 10^10 meters.
I looked up some values of Pluto's radius, and got about 3000 million miles, or 5 x 10^9 km, or about 5 x 10^12 m.
So this galaxial blackhole seems to have a radius 100-1000 times less than the solar system radius.
And indeed, in the final page of the Schodel paper, there is a mention that the observed radius of the orbiting star is ~ 2000 times the Schwarzschild radius, and not the actual Schwarzschild of the star. i.e. the observed radius of orbit is much much larger than the putative Schwarzchild radius.
This is old news at UCLA (Score:4, Informative)
The Schwarzschild Radius (Score:3, Informative)
Any mass can become a black hole if it collapses down to the Schwarzschild radius
R= 2(MG)/ c^2
Therefore at 3.7 million solar masses...
the Schwarzschild radius is
1.0919401548997975x10^10 M
Which is much smaller than our solar system (the earth orbits at 150,000,000 KM).
But I imgine that they would measure the Acreation Disk.....
The Schwarzschild radius calulation is fun. One can plot density verses radius and it becomes clear that something the size of our galaxy with density of water would be a black hole...
Space is an empty place!
So it's true... (Score:4, Funny)
Some calculations... (Score:3, Interesting)
Assuming it was 3 million solar masses, the diameter of its Schwartzchild limit (effectively the diameter of the black hole) would be 8.8 million kilometers, or about 6-1/3 times the diameter of our sun.
If the Earth were in orbit around this black hole at the same distance we are from the sun (assuming it wouldn't be torn to shreds by tidal stresses), a year would be 5 hours long.
Re:So how long... (Score:4, Funny)
It's nice to see that graduates from the Bob Saget School of Comedy are getting journalism work.
Re:I'm no astrophysicist... (Score:5, Informative)
Re:I'm no astrophysicist... (Score:3, Interesting)
Re:I'm no astrophysicist... (Score:5, Informative)
I am (or rather, was) an astrophysicist. The answer is the rest of the galaxy holds it together, a bit like the gravity of the Earth is what holds the Earth together. The galaxy has the mass of billions of stars - so any stars not at the center are being pulled towards the center.
In answer to the original poster, the 'size' of a black hole is its event horizon radius:
R = 2GM/c^2
where
G = universal gravitational constant
M = mass of the black hole
c = speed of light.
Re:I'm no astrophysicist... (Score:5, Informative)
This is a small misunderstanding. Many people seem to think that a black hole has super gravity or extra strength power just because its a black hole. Actually, it all depends on the mass.
For example, if our sun suddenly turned into a black hole, we wouldn't get sucked in. We'd still orbit our new black hole sun the same way we orbited our old normal sun. Just because it became a black hole doesn't mean its mass changed. And since its mass didn't change, we would still orbit the same.
Ditto for our galaxy. If we didn't have this black hole at the center of the galaxy, but instead 3.7 million suns, everything would orbit just the same
---
A black hole is just God dividing by zero
Re:They're talking about... (Score:3, Informative)
Radius = 2 * "Universal Gravitational Constant" * "mass inside event horizon" / pow("speed of light",2)
For a black hole the mass of our sun the radius is:
Radius = 2 * (6.67 * 10^-11m/kg/s^2) * (2 * 10^30 kg) / (3 * 10^8 m/s)^2 = 2.964 km
When you check my math make sure you get your units right. A black hole three times the size of our solar system would be quite massive, and you should be impressed.
Also, I saw a program on Discovery Channel a while ago (6 months+) which had an interview with an observational astronomer in which he claimed to have observed movement in the center of our Galaxy which was consistant only with a supermassive black hole. I guess he finally published.
Old Physics Joke! (Score:5, Funny)
That or nobody ever gets there and the ride is extremely short.
I can't remember which was the inside observer and which was the outside observer. I think it mixes reference points. The same time reference point is short, and the never arrive takes forever.
Isn't relativity fun?
Light bends in gravity. (Score:5, Interesting)
Re:Down the Drain (Score:5, Interesting)
I've always thought it was obvious that super-massive blackholes lie at the center of galaxies. The intense gravity at the center should create one, and spiral galaxies are all just pinwheeling "down the drain".
Several things wrong in here. First, it's the huge density at the center of the galaxy that would lead you to think a black hole might form there. Yeah, the density is big there because it's way down in a gravitational potential well. But intense gravity doesn't create a black hole-- quite the other way around, in fact.
Second, spiral galaxies are *not* spiralling down the drain. Most of the stars in a spiral galaxy orbit the center approximately circularly; they aren't spiraling in any more than the earth is spiraling into the Sun. So why the spiral shape? Spiral shape can come from a couple of differnet things. In some galaxies, they are density waves. Think of them as a cosmic "traffic jam". In some places, the stars are closer together than other places; in those places, densities are higher, and gas clouds get compressed, and more stars form (which is why spiral arms are bluer). As the wave passes through those stars, they will spread back out. It's similar to sound waves (which are density waves), or, indeed, clumps of cars on freeways (which seem to maintain their identity even though they don't always have the same cars in them-- you pass through them, so for a while you're a part of the clump, but eventually you get past the clump).
Other theories of spiral structure formation are based on the differential rotation; when a big group of stars form, the differential rotation will tend to stretch it out into a little spiral arm segment. These theories are probably more responsible for spiral structure in galaxies where the arms are ratty and choppy. The density wave theory is probably more responsible in "grand design" spirals where you can trace one long arm all the way from the center out to the edge.
One thing spiral galaxies are definitely not however are stars spinning down the drain the way water spins down a drain. It may look obvious, but it's wrong. (Yes, there are ways to get material to sink down to the center of galaxies, but generally it's a whole lot easier with gas and dust than with stars. Gas and dust are viscous fluids, but stars are basically collisionless.)
-Rob
Re:Big Black Holes are Thin (Score:3, Insightful)
If that could be true, how about tiny universes residing in each of our tiny elementary particles?
Think about a proton having a half-life. At that point, all the matter in that little universe has collapsed into little black holes, and it's time for that bugger to move on.
Blah, blah, blah, and so on.