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The Astrophysics Spectator

March 15, 2006

This issue of The Astrophysics Spectator adds a new page to the “General Relativity” topic path that describes the “spinning” black hole. In addition to this new page, this issue includes an update to the page entitled “Falling Through the Event Horizon” to include the appearance of the sky at the event horizon of a black hole.

Every object in astrophysics carries some angular momentum. This reflects the absence of physical mechanisms that efficiently transfer angular momentum between the planets, the stars, and the galaxies. Inevitably the black holes that form from massive stars and from the accumulation of stars and gas at the centers of galaxies are born with some angular momentum. The black hole candidates we see in binary star systems and at the centers of active galaxies are surrounded by gas disks that drop gas, and with it angular momentum, onto the black holes.

The gravitational field of a Kerr black hole is the vacuum gravitational field for an object with both mass and angular momentum. This is the black hole that astrophysicists encounter in their work.

What makes the Kerr black hole interesting is that its gravitational field drags objects around an axis of rotation. For instance, if we dropped an object onto a Kerr black hole, we would not see it fall radially to the event horizon; instead, we would see the object spiral down to the event horizon. If we placed two objects in circular orbits in the black hole's equatorial plane, each at the same radius, but one orbiting clockwise, the other counterclockwise around the spin axis, we would find that the orbits have different periods. Most dramatically, if we tried to place an accelerating observer near the event horizon, we would find that the observer could never accelerate fast enough to maintain a stationary position relative to the stars.

Whether or not we could ever recognize these characteristics in the known black hole candidates is still unsettled. The effects see far from the black hole's event horizon, where the field is weak, should appear in any body that is spinning. This raises the hope of seeing these effects in Earth's gravitational field.

Next Issue: The next issue is scheduled for March 29.

Jim Brainerd

General Relativity

Black Holes. The black hole encountered in astrophysics, the Kerr black hole, carries angular momentum. Like the Schwarzschild black hole, the Kerr black hole has an event horizon. The peculiarity of the Kerr black hole is that objects dropped onto it spiral around it's spin axis. This dragging property is so strong close to the black hole's event horizon that objects cannot accelerate fast enough to remain fixed relative to the distant stars. (continue)


Falling through the Event Horizon. The page describing the fall of an object through an event horizon has been extended to describe the sky's appearance both to an observer hovering over the event horizon and to the object falling through the event horizon. A hovering observer sees himself as at the bottom of a black urn, with the stars visible in a small opening overhead. An object free-falling through the black hole's event horizon sees itself above the event horizon, with the edge of the sky below him. (continue)

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