Why? For rotating black holes the event horizon is oblate - fatter at the equator - not spherical.
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Explanation:
Introduction
A black hole in general is a region of space from which nothing, not even light, can escape the gravitational pull (or, to put things in a different way, clearer though perhaps not quite accurate, space-time itself is “falling” inward faster than the speed of light, so one would need to go faster than light merely to stand still something which is impossible for a material object). See further below for much more detailed explanations, and/or see the Wikipedia article on the subject for a start. But for the moment, suffice it to say that of the not-so-many different kinds of black holes in (theoretical) existence, the Schwarzschild black hole is the simplest, non-rotating one, and the Kerr black hole is one which adds angular momentum, i.e., an axis of rotation. While the Schwarzschild black hole is (somewhat deceptively) simple, having as only feature a spherical event horizon which acts as a limit of no-return beyond which all things are inexorably attracted to the central singularity where they are crushed, the Kerr black hole exhibits a tantalizingly complex geometry with two nested horizons and a ring-shaped singularity which act as gates to even more surprising features (travel to different universes, a so-called “negative space”, and a kind of time machine).
These features (although largely theoretical, and presumably absent in real black holes), along with the catchy name wormhole, have made the black hole a popular theme in popular imagination or science-fiction.
Strangely enough, however, it appears that nobody has, hitherto, computed any actual pictures or videos of what one would see, when traveling through such features. (I know that Alain Riazuelo has computed similar videos, e.g., close views of Schwarzschild black holes, traveling through a Reissner-Nordström black hole, and some images of Kerr black holes, but none like the ones presented here.) So the point of this page is to demonstrate what the Kerr black hole and its intriguing features look like. This is a question that had been obsessing me ever since I haad learned about Kerr black holes.
But before I get to the videos themselves, there are a number of things I have to explain if any of this is to make scientific sense (of course, feel free to skip ahead if you just want to see things).
A not-so-short course on black holes
What is a black hole?
A black hole in general is a region of space from which nothing, not even light, can escape the gravitational pull (or, to put things in a different way, clearer though perhaps not quite accurate, space-time itself is “falling” inward faster than the speed of light, so one would need to go faster than light merely to stand still something which is impossible for a material object). The limiting surface beyond which no return is possible is known as an event horizon.
The properties of black holes, to be discussed in more detail below, are described in the general framework of general relativity: mathematically idealized black holes (meaning, in particular, that they are eternal and stationary) are represented by a number of exact solutions of Einstein's equations (i.e., the equations of general relativity) such as the Schwarzschild metric solution and the Kerr metric solution. The Schwarzschild metric is the general relativistic description of any static and spherically symmetric gravitational field outside of the matter creating it, and, in particular, when extended up to and beyond the event horizon it describes a static and spherically symmetric black hole. The Kerr metric adds one parameter of a handful that a black hole can possess, namely angular momentum, and therefore describes a stationary rotating black hole (but with no electric charge, let alone any of the more exotic things like the NUT parameter). This is physically relevant to the geometry of space-time, because the black hole's rotation drags space-time along with it through a process known as the Lense-Thirring effect, so that not only particles coming close to the black hole are gravitationally attracted by it (and once through the event horizon can only fall inward) but they are also made to rotate in the black hole's direction with respect to distant stars (and beyond a surface known as the static limit, they can no longer remain at a fixed angle). Furthermore, the Kerr black hole, in its mathematicall idealized version, exhibits a number of distinctive geometric features, having two concentric horizons, a ring-shaped singularity beyond which lies a “negative space”, and more remarkably, connects an infinite number of universes together (acting as a so-called wormhole).
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