How long does it take for the black hole at the center of our galaxy to make 1 full rotation?
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what is your axis? have you read this https://en.wikipedia.org/wiki/Rotating_black_hole ? – anna v Oct 07 '17 at 04:04
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This , for another black hole, https://en.wikipedia.org/wiki/GRS_1915%2B105 gives revolutions of 1.150 per second. This for the central one in the galaxy, gives no estimate https://en.wikipedia.org/wiki/Sagittarius_A*#Central_black_hole – anna v Oct 07 '17 at 04:13
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There is no evidence that Sagittarius A is a black hole. – safesphere Oct 07 '17 at 06:30
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@safesphere none whatsoever? I wonder why so many seem to believe it is – Asher Oct 08 '17 at 02:20
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@Asher All we know it's a massive object and not necessarily a single one. Why do we say it's a black hole? It's popular and brings more funding for research projects. – safesphere Oct 08 '17 at 02:31
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@safesphere Sagittarius A* contains four million solar masses in a space smaller than Mercury's orbit. If you can figure out a way to do that without having (or very rapidly forming) a black hole, you should have the results published. I'd love to read it. – Asher Oct 08 '17 at 02:46
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@Asher "We don't know what else it could be" is not a scientific argument, but unfortunately a common trend in the modern culture. You are welcome to believe whatever assumption you prefer, but when you obtain a direct observational evidence of what it actually is, I'd be equslly happy to read your honest publication of the facts rather that hypothesis. Best of luck :) – safesphere Oct 08 '17 at 03:00
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@safesphere on the other hand, disagreeing with a conclusion doesn't allow one to accurately say there is "no evidence" for the conclusion. – Asher Oct 08 '17 at 03:02
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@Asher A "conclusion" based on assumptions is not a fact. – safesphere Oct 08 '17 at 04:51
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@safesphere no, but if you prefer to treat GR as an assumption, you may fit in better at the philosophy SE. – Asher Oct 08 '17 at 05:24
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@Asher Perhaps you should stop the harassment and leave me alone. – safesphere Oct 08 '17 at 06:14
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@safesphere To wildly throw assertions like "there is no evidence that Sagittarius A is a black hole. " is totally unscientific and is patently untrue. But your assertion is worse than this; because it is clear that you do understand that such evidence exists but choose to ignore it in favour of your own contrarian doctrine. As Asher has said: if you have an alternative theoretical idea for how you squeeze (a dark) 4 million solar masses into a space smaller than Mercury's orbit, then explain/publish it (or point to peer-reviewed papers that do). It would be more remarkable than a black hole. – ProfRob Oct 09 '17 at 13:05
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@RobJeffries You guys seem to have a hard time seeing a difference between facts and conclusions. Your "scientific" logic is so full of holes, not even funny. But most remarkable is the level of intolerance on this forum to different opinions. I've made a comment and you can give it a thought or not, up to you. I'm not trying to convince anyone, just hopefully prompt some logical thinking. Unsuccessful, fine. No need to try to convince me "the Earth is flat". Thank you. No need to keep beating this dead horse. But most of all, no need for personal attacks. Best regards. – safesphere Oct 09 '17 at 17:10
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@RobJeffries I didn't mean you regarding personal attacks, sorry, I should've made this more clear. Also, I don't want to provide an alternative explanation. I only pointed out that facts are not the same as conclusions. I disagree with the sensorship that pointing out conviniently ignored holes in "conventional" logic must be banned. If this was the scientific practice, the Earth would be still flat. I appreciate your offer to help. It is much nicer than a closed minded suggestion for me to leave this forum. Not sure I need help right now, but I sure cont on your expertise in the future :) – safesphere Oct 09 '17 at 18:15
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@safesphere This is not the way science works. Someone makes an observation; I say it can be explained by a supermassive black hole. You (or anyone else) is free to (I) falsify that theory by pointing to evidence that disagrees with it and then (II) come up with a better hypothesis that explains all observations. Until you can do either of these then you are contributing noise. I have never seen an electron but I have no doubt that the hypothesis of a point-like negative particle explains all current observations. I have posted a Q to provide you or anybody else the opportunity. – ProfRob Oct 09 '17 at 18:33
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@RobJeffries This is really good news, because not too long ago science worked by burning those alive who disagreed with the consensus. Seriously, how can you possibly still be missing that I was not trying to falcify this theory or provide a different one? I've made a comment indicating that this case is not closed. Take it or leave it. No need to silence alternative opinions. And on the electron, oh man :) If you posted "a point-like negative particle explains all current observations" as a question, you'd be heavily down voted on this forum by the "no particles" field theorists. – safesphere Oct 09 '17 at 19:08
2 Answers
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The spin rate of the black hole at the centre of our Galaxy has not yet been established.
However, X-ray observations of gas falling into the supermassive black holes in many other galaxies appears to have established that most rotate at close to the maximum possible (see below, from here).
If we assume the 4 million solar mass black hole in our Galaxy is not unusual, then it too will have an event horizon moving at more than 60% of the speed of light.
As the spin increases, the event horizon shrinks, becoming half the Schwarzschild radius at the maximal spin. So for a 4 million solar mass black hole, the event horizon will be perhaps a little bigger than $GM/c^2 \simeq 6$ million km. If rotating near the speed of light, then an answer to your question would be a bit more than 2 minutes.
ProfRob
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+1 Very informative, helpful, and specific down to estimated numbers. – safesphere Oct 09 '17 at 18:29
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Measuring the spin of a black hole is a tough problem. There is a useful discussion of the problem in the answers to Coupling between galaxy spin and central black hole spin. It has been done, for example see this article in Nature, but only approximately and only in favourable circumstances.
Sadly Sagittarius A$^*$ doesn't lend itself to this sort of observation, so at the moment we have no idea how fast it is spinning.
John Rennie
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