APOD: The Black Hole in the Milky Way (2012 Nov 02)

[BDanielMayfield]

But, what do YOU, think Bruce? Convincing evidence (a large number of stars very rapidly orbiting something that’s so dim we can’t see it) leads this Bruce to believe in the existence of the Milky Way’s central Super-Massive Black Hole...

Hi, other Bruce.

I think it's likely that there's something compact and massive there, but black holes (in my naive opinion) have always seems like a 'default' explanation. The argument seems to go something like: we know what 'holds up' regular matter but sometimes the pressure is too high and it collapses into white dwarf matter; we know what holds that up but sometimes the pressure is too high and it collapses into neutron star matter; we know what holds that up but sometimes the pressure is too high and... we don't know of anything that could hold that up, so we'll assume that nothing can.
A singularity has lots of problems, though.

I once attended a lecture by the head of a university's astronomy department. He said that astronomy was a very exciting field because massive changes in beliefs were relatively frequent. I kept quiet my opinion that that might be because astronomers (and cosmologists in particular) seems relatively happy to jump to conclusions!

Thank you for that excellent answer Bruce! I’d go a little farther and say that we absolutely know that the mass is there, but you’re right to question the form this mass may take. I like your line of reasoning as to what “holds up” the various known states of matter. Why couldn’t there be another level hiding behind the event horizon?

It’s often said that in black holes the laws of physics breakdown. But, that’s not really true though. It would be more accurate to say that in BH’s the KNOWN laws of physics breakdown. But BH’s obey the laws of physics just like everything else in the Universe; it’s just that in this case we haven’t learned all the rules yet.

Bruce D. Mayfield

[DavidLeodis]

"over 100 million degrees Celsius". Wow, that's hot! It won't take long to grill a burger (at least well done). That's so hot it's .

[Chris Peterson]

You may be right that cosmologists are happy to jump to conclusions, at least sometimes, but my personal opinion is that cosmologists have often been too happy to "anthropomorphize". That is, cosmologists, like most of the rest of us, have been happy to think that the universe should make sense in human terms. And it should, preferably, put ourselves in the center of it all.

Everybody tries to view the world in a way that makes sense. We're built to do that. Cosmologists are no different from other scientists, and I don't think there's anything to suggest that cosmologists are more likely to "anthropomorphize" as you put it. In fact, I think that scientists who have experience with so many unobvious or nonintuitive things, like cosmologists or quantum physicists, are more likely to develop non "anthropomorphic" hypotheses.

However, those cosmologists of yore did stick to the idea that the Earth is the center of the universe. It took a long time to discard that idea.

Not really. In fact, this was all occurring during the birth of modern science. There weren't more than a handful of scientists in the world at the time. Most educated people were inside the Church, and applied nothing like modern reasoning to their thinking. Those who were adopting modern scientific thinking very quickly recognized the correct structure of the Solar System.

Your other examples simply reflect science in operation. There were different hypotheses (which I prefer to "beliefs") and as evidence was acquired, those narrowed down into solid theories. That's how all science works, not simply astronomy.

What can make astronomy different, and can make early hypotheses seem quite radical, is that so much is not open to experimentation, or only to limited or very indirect experimentation. That can leave more ideas open to examination for much longer than is often the case in more experimental areas.

[Chris Peterson]

It’s often said that in black holes the laws of physics breakdown. But, that’s not really true though. It would be more accurate to say that in BH’s the KNOWN laws of physics breakdown. But BH’s obey the laws of physics just like everything else in the Universe; it’s just that in this case we haven’t learned all the rules yet.

Of course. And no physicist believes that anything is breaking down inside a black hole. When a scientist says that laws break down, they always mean laws in the sense of our models of reality. Not reality itself!

[emc]

Black Holes are fascinating. It’s one of the things I don’t feel stupid about not understanding.

[Tetrodehead]

1) If I compare the before and after pics, they seem to be identical...
2) Both of them show a dumbell shape. Could It be a pair of black holes in a binary system?

[BDanielMayfield]

Not impossible perhaps but it seems to me that if there was a binary SMBH here then this entire galactic core area would be much more active than what is presently observed.

[spif]

I've heard a couple speculations regarding black holes have a lot of ring of common sense about them... And whether or not these ideas are true, they may help to ground some of the wild imaginations people tend to launch into when pondering black holes.

Singularities and Degeneracy Pressure
One notion addresses the singularity concept that so many people, including scientists, are uncomfortable with and which was hinted at by Bruce... That is, neutron stars are a form of matter that hold up due to degeneracy pressure. But degeneracy pressure has a limit impost by relativity... the individual neutrons in the center cannot exceed the speed of light and so there is a cap to how much pressure they can provide to hold up the star. Once gravity exceeds that maximum pressure, the neutron star collapses farther.

The question is, does it stop collapsing when another more fundamental degeneracy is achieved or does it collapse infinitely into a "singularity". The singularity idea seems like nonsense to me... somehow I think the universe must be a bit more rational than that. One idea I've heard is that perhaps strings can exist in a kind of ground state that can't get any more fundamental. So perhaps the matter in a black hole is a kind of degenerate ball of strings. String theory posits objects that have a very tiny but finite dimension. But they are possibly so compact that any clump of strings in this state is always smaller than its own Schwartzchild radius?

I don't know enough about the physics to challenge the idea, but the concept of there being a compact form of matter with finite dimensions seems more appealing to me than the infinite singularity, because there is hope of it being understandable. Infinities are sort of inherently irrational.

Could such a string ball have a degeneracy pressure limit due to special relativistic speed limit? I guess the question there is, how massive does the object have to be to achieve that limit? Perhaps that number is way bigger than is practical to form... many many galaxies worth of mass compressed into a single black hole? More mass than the known mass of the universe perhaps? That would be a practical "can never happen" kind of limit.

Crossing the Event Horizon ... no big deal.
The other notion I've heard is that the event horizon of a black hole is not really an object ... you could theoretically pass through an event horizon and the laws of physics "as we know them" continue to work just fine as always. I believe the surface gravity of super massive black holes (at the event horizon) is tolerable. There is some funniness with time relative to outside observers and the bending of light. But relative to the traveller, time always flows normally. Once inside the event horizon, I believe the observer would continue to be able to see out, perhaps with some gravitational lensing effects, but otherwise the traveller would not experience anything too jarring while crossing the horizon.

As I understand it, not all back holes will crush you or shred you right away until you get REALLY close to them... I believe the supermassive ones have event horizons well outside the radius where gravitational tidal forces start to become a problem for normal matter.

Anyway, if anyone understands the physics in more detail, I'd like to hear more about the plausibility of these speculations. I heard both from established astrophysicists but I don't recall precisely who or in what context I read these notions.

-s