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.
size (Score:5, Informative)
black holes ARE a point.... (Score:2, Informative)
With a black hole this big, you can actually cross the event horizon, and not be torn apart because the change in gravity over a certain distance (6 feet or so for your height) isn't great enough. Smaller black holes will rip you apart quicker though
Super-Massive Black Holes (Score:5, Informative)
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:I'm no astrophysicist... (Score:5, Informative)
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]
Size quoted is the orbit of a star (Score:5, Informative)
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.
density of blackhole (Score:2, Informative)
Re:They're talking about... (Score:2, Informative)
You could go in and find out, but due to time dilation, you would see the rest of time flash before your eyes, and then witness the end of the universe. You wouldn't be able to tell anybody though, because no signals can escape unless they travel faster than the speed of light (which is of course impossible). You would also be dead, but that's another story.
Better article (Score:5, Informative)
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.
I'm an astrophysicist... (Score:2, Informative)
Black holes are good candidates for causing a galaxy to accumulate. It can be kind of hard to explain what causes galaxies to form.
I'm getting off-topic, but I don't care...
One of the favorite explanation comes from irregularities in mass distribution as evidenced by perturbations in the cosmic microwave background. That's one of the reasons that the CMB became such a hot topic, it offers insight into the origin of large scale order in the universe.
Also of interest to
Like anything else in cosmology, it's all rather speculative (at least as compared to many other physical models).
Find the link on your own (/. might've even covered this topic).
Once again life imitates art... (Score:2, Informative)
It's been a few years since I've read those, but I do remember that the fact that that planet was at the "center" was a pivital plot points in one of the later books.
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
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.
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.
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.
The math doesn't match the description! (Score:5, Informative)
So much for that idea!
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: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?
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
but is it? (Score:2, Informative)
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 . . .
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:To clarify... (Score:2, Informative)
Assuming the force is proportional to 1/r^2, scaling constants and units are ignored to simplify things.
Let your toes be closer to the hole, and at distance r. Force_t = 1/r^2.
Let your head be distance eps further away, at distance r+eps. Force_h = 1/(r+eps)^2.
Difference in forces = 1/r^2 - 1/(r+eps)^2
= (2*eps*r + eps^2)
-----------------
r^2(r+eps)^2
Now we wave our hands and say that eps is negligible compared to r, and when added to r leaves you with basically r still.
Difference in force = 2*eps/r^3
Now if you think the force grows quickly as r decreases, having a growth 1/r^2, then the _difference_ in force grows like 1/r^3 which grows far quicker as r decreases.
Now the difference in force will be felt as a stretching force on your body. You will be pulled to bits as r decreases.
THL.
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.
Re:size (Score:2, Informative)
The "size" of a black hole is, in fact, the size of the Schwarzchild Radius, which is the distance at which neither light nor matter can escape. The black hole itself, the singularity, is indeed a point of infinite density.
Um, not quite. The Schwarzschild Radius defines the volume of a sphere into which you need to pack matter to make the black hole. It's related to the mass of the matter involved: r = 2Gm/c^2, so more mass, larger radius. But the radius is still really, really small even when m is large, so black holes require mass on solar scales, low initial density (gaseous), and enormous force (supernova).
When matter is compressed to a volume defined by the Schwarzschild Radius, it exerts a gravitional field so strong that the escape velocity is > c (i.e. light can't escape). At this point it collapses and the math gets a lot more complicated. :-)
Paul
Re:Diameter of a Black Hole (Score:2, Informative)
"Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull."
How big is a black hole?
"The more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius (which means the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that supermassive black hole has a radius only about four times bigger than the Sun."
Doesn't sound like a point to me...
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.
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)
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:Event Horizon (Score:2, Informative)
If general relativity is correct, there is no other radius that can be measured by an external observer (assuming some rather general conditions concerning the mass distribution).
So as long as you are using general relativity as a framework, from a scientific standpoint there is not much use in speculating about something that you can never measure.
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. :-)
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.
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:Black hole v. singularity (Score:4, Informative)
String Theory... (Score:5, Informative)
BlackGriffen
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.
Re:Event Horizon (Score:3, Informative)
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.
Re:Event Horizon (Score:2, Informative)
This limitation excludes any other explanations, such as a dense cluster of stars or a cloud of stellar material. That much mass in that little space would inevitable collapse and become a black hole.
"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)
Re:Event Horizon (Score:2, Informative)
As with most of Gribbin's books, it's written towards smart non-physics people, conveying the main concepts and a good bit of physics history without going into much the math.
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
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:I'm very confused (Score:2, Informative)
(sorry, I'm British):
size of event horizon (36 light second)
= 100 million football fields
closest approach distance of star to hole
(17 light hours)
= 184 billions football fields
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.)
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
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]
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.
What a coincidence (Score:1, Informative)
http://www.esa.int/export/esaMI/Integral/
3 hours until launch.. now i have to install realplayer *shrug*
Live rm feed at http://esa.capcave.nl/esa/integral/info.html
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!
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.
Some Related News Stories (Score:2, Informative)
The BBC [bbc.co.uk] have a similar story [bbc.co.uk]
Enjoy
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