Presumably guards walked a patrol around the Black Hole of Calcutta on a regular basis.
Any other black hole is too far away to get to to circumnavigate.
There are only three things that a black hole can give as information: its spin, its mass, and something else which I cannot remember. The infamous line, “Black holes have no hair” was coined by the man who was part of the discovery process of this fact.
If a star that is spinning collapses into a black hole, the black hole exhibits the same spin. Though there is theoretically a point mass inside a black hole, I believe that the spinning action refers to the event horizon itself, and the resultant frame-dragging which the hole exhibits (due to motion along a geodesic or due to rotation). In fact, one of the ways to detect a black hole is to catch glimpses of inward spiralling matter which would create jet streams along the poles; these streams would be far easier to detect than radiation emitted by infalling matter from a non-spinning black hole.
I am almost 100% positive that a spinning black hole will not have a perfectly spherical event horizon, and so I was wondering about whether the distortion caused from conservation of momentum would, in fact, require orbital adjustment.
However, now that I think about it, the shape of the ellipsoid should correspond highly to the shape of the original star (disregarding bumps) so perhaps the center of mass wouldn’t move anyway, regardless of the resulting distortion.
The spin does decrease as the black hole ages due to infalling matter.
IIRC, it’s the electric charge.
First: we have lots of evidence that there’s a big ol’ black hole in the center of the Milky Way. Star velocities near the core is one; they move very quickly, and that movement implies a large mass in a small volume. The big x-ray flare reported yesterday is another.
There is a cloud of antimatter near the core too. See here:
http://antwrp.gsfc.nasa.gov/apod/ap970501.html (I love APOD). There are weird processes generating antimatter near the core, and the stream of this slams into normal matter and produces gamma rays. Gamma rays are extremely high energy, and difficult to produce. Antimatter annhilation is one way.
Second: the black hole itself may be dimensionless, but the event horizon has a finite size. For the Sun’s mass, the event horizon (where the escape velocity becomes the speed of light) is about 3 kilometers in radius. It scales with mass, so the event horizon for the Milky Way black hole (which is 2.6 million solar masses) is 2.6 million x 3 = 8 million kilometers in radius. I am not familiar enough with black hole physics to know how the event horizon changes with spin. I have heard of rapidly rotating black holes forming a torus, which is pretty bizarre even for black holes!
The singularity BH (dot) is a mathmatical representation of what’s inside. In real life we don’t know if it is a point mass or just very condensed matter - perhaps more condensed then a neutron star perhaps not - we just don’t know.
Any angular momentum of the star will cause an infinity rotational ‘speed’ as the radius goes to 0 - not likely but maybe the rules of physics inside a BH allow something else.
If I deciphered my notes correctly, a rotating black hole will indeed fail to have a spherical event horizon, which only makes sense. I’m quite sure that a black hole has only a measured charge, mass, and angular momentum (and also magnetic charge, if magnetic monopoles exist).
And my (semantic) point is that “a black hole is just a mass with a gravitational field” is not a useful definition. ALL masses have a gravitational field. Since I am a mass with a gravitational field, I fit the stated definition of a black hole. Yet I am not a black hole; the definition as given is so imprecise as to be ultimately more or less useless.
This is how I remember it.
At 1.5 Schwarzschild radii a light ray would form a perfect circle around the hole. To a person in a large tube enclosing this circle the tube would appear to be perfectly straight, and he would see infinite images of his back. If he started moving in the tube he would feel no centrifugal force (really centripetal but what the hell) even though he is circling the hole.
If the tube were inside this radius the person would see it curving away from the hole, even though the actual curvature is exactly the opposite. This also means the centrifugal force would tend to throw him inwards toward the singularity.
Bad wording but I just wanted to point out that a BH is nothing magical - it’s a big gravitational field. Planets can orbit it for that matter light can orbit it - there is no reason to think otherwise AFAIK.
Ah. Yes, it’s absolutely true that there’s nothing super mysterious about the concept of a black hole. In strict point of fact, the sun (were it static, not rotating, and perfectly spherical) would have the same gravitational field as a Swarzschild black hole outside of it (by Birkhoff’s theorem, to provide a semi-reference). Inside the sun, of course, the solution would be different, but outside of it, you can’t tell the difference.