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
meep? (Score:1, Insightful)
Hollow Earth theory? (Score:1, Insightful)
I'm not sure if this article presents a convincing enough argument in favor of the black hole argument.
Thoughts?
From the article: (Score:3, Insightful)
So, does that mean that in time, the blackhole will swallow up the star?
-Cyc
Re:They're talking about... (Score:2, Insightful)
Actually, it is completely knowable. It's just impossible to relay that information outside the event horizon.
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...?
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.
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.
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.
I think your math is off (Score:2, Insightful)
Re:Diameter of a Black Hole (Score:1, Insightful)
While that is sort-of true, it doesn't quite mean what you think it does.
Think of, for instance, the Earth: when you think of its density, you would measure the solid matter (and, if you want, the gaseous shell). However, the radius at which an object of size "x" and vector "y" will be captured by the planet is much greater than the area that you just measured the density of.
Nope, not possible -- the overall density in your cluster could be higher than the "density" of the "event horizon" enclosed area, but the mass is still spread over a wide area, thus not generating the extreme curve in space that a "black hole" would.
Remember, the density of the actual "black hole" is mass over area -- mass doesn't matter, because the area is zero.
mass/area is thus infinite density, which you cannot obtain with your star cluster. Sorry.
-iv
Can anyone do the math? (Score:2, Insightful)
Our sun is about 10^30x2 kg while the earth is 10^25x0.6 kg That makes the sun about 10^6 times heavier than the earth([1] [vendian.org])
This black hole now is about 2.6 to 3.7 times that heavy when compared to a sun of the size of our own.
Our planet roates around the Sun at about 150,000,000 km at a speed of ca. 29.658 km / s if my math isn't wrong.
That other sun rotates around the black hole at about 17 light hours at a speed of 240.652 km / s (if I am not mistaken here either).
The speed of that sun is more than 8x the speed of the Earth, generating a significantly higher centrifugal force.
Now, that sun is 127.5 times further away from the black hole's event horizon than Earth is from Sun.
At the same time, the increased distance should provide a significantly lower gravitational pull than the 3.6 x relative weight of the sun could provide.
As this sounds completely bogus to me, I'd be happy if someone could enlighten me how this is supposed to work.
Re:Event Horizon (Score:5, Insightful)
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: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:Event Horizon (Score:2, Insightful)
Space time expanded like a rubber sheet, with every point moving away from every other point.
It's actually still doing this. My astronomy professor back in the day described the universe as a loaf of raisin bread, with matter being the raisins, all moving away from each other as it baked.
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.
The "size" of a black hole.... (Score:1, Insightful)
FYI, the event horizon is the point at which the black hole's gravitational force becomes greater than that of the original star (and exponentially so as one moves closer to the hole). Outside the event horizon, the gravitational pull is the same as the orignal star's.
Also FYI, the largest known star is about one billion times the volume of our Sun.
Finally, who cares about density? Mass means mass, not density. An "object" millions of times as massive as the Sun is indeed super-massive, regardless of its density. Density is merely the ratio of mass to volume. Low density does not mean low mass--a chunk of aerogel the size of the Sun would not be very dense, but would still be massive.
Re:Black Hole 1 + Black Hole 2 = ? (Score:2, Insightful)
gravitational radiation in the process, and eventually settle down into a nice Kerr hole.
There is some hope that gravitational wave observatories like LIGO II and LISA will see the signature of these events (although they are expected to be rare - neutron star/neutron star, neutron star/black hole collisions are more frequent. Most people think these are the gamma ray bursts).
People are trying to figure out the expected waveform of the emitted radiation with numerical simulations, which are notoriously difficult.
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: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.