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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.
RB
...once you cross this, there's no coming back, and our physics stops at the edge.
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.
The article is referring to a determination of the orbit of the star closest to the galactic center. The periasteron (closest point in the orbit) is 17 light hours from the galactic center. This implies that the mass necessary to create that orbit is concentrated within that radius. The only thing in our current cosmic zoo that fits 3 million solar masses inside of 17 light hours is a black hole. The event horizon itself should be smaller than that, but not by much.
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.
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.
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
a Theory can be proved right meaning it can be fully explained by just reasoning, and in relation to a set of axioms.
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.