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There's a Hole in the Middle of It All
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.
the scientists in the article seem to assert that this is CONCLUSIVE proof of a black hole's existence. but i remember reading a few months ago about a schism in the physics community - a sizable segment of the community is disputing the theoretical existence of black holes! i wonder how this discovery will affect that debate....
smd4985
Current theories in no way preclude the formation of a singularity. In fact it is pretty much the required outcome when a sufficiently massive start reaches the end of it's life. There is some discussion that when quantum theory and gravity are unified quantum effects may smear the singularity out of existence, but at this point it is all hand waving. Perhaps what you're thinking of is a naked singularity. A naked singularity is a singularity that is not cloaked by an event horizon, and is extraordinarily unlikely to occur.
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" :)
Jon Erikson, IT guru
Theoretcially (we'll likely never have building materials struturally sound enough to test this) light should behave in almost exactly this manner close to a black hole. For example, say you've built a circular torus space station around a black hole. If you're within a certain radius to the singularity, but still outside the event horizon, light will bend towards the blackhole, allowing your vision to see 360 degrees around the torus. You can stand in one point and see your back an apparent distance equal to the circumfurence of your imaginary torus away. Closer than that radius means that the torus would appear to bend the wrong way.
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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.
Jon Erikson, IT guru
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
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?
Don't Bogart the fish sticks
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...
Jon Erikson, IT guru
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
I'm assuming the modern theories that your referring to are the string theories and more recent m-theory. These look promising, and would result in the behavior that you describe. Their predictions about how the formation of singularities are affected by quantum gravity is the discussion that I'm referring to. But unfortunately they have not been able to make a single prediction that can be tested as of yet.
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.
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?
0x or or snor perron?!
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.
> 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.
A friend and I worked out a few calculations on the black hole...
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.
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