Sky and Telescope - February 2017 - 17

The ergosphere
A spinning black hole is more like a whirlpool. The swirling water
in this analogy is spacetime itself. It's pulled around as the black
hole rotates and falls down the maw of its drain. This region of
twisted spacetime is called the ergosphere. It is impossible to
stand still in this region.
A spinning black hole has kinetic energy bound up in its
spin, in the same way that a spinning top is more energetic
than a top lying down. That energy can be tapped into and
transferred to other things in the black hole's environment.
(The term ergosphere comes from the Greek word for "work.")
Astronomers think that a black hole powers its jets with energy
from its spin.

Spinning Black Hole
(as it would look in 'convenient' coordinates)

How fast does a black hole spin?
When astronomers measure a black hole's spin, they report
the value as a fraction of the maximum allowed spin (which
would be 1). For example, the bigger member of the black
hole binary in the quasar OJ 287 has a spin, labeled a, of
0.313, or 31.3% of its max. But what does that mean?
Event horizon

This number is related to the angular momentum; it's
not a fraction of the speed of light. But we can turn it into
a fraction of the speed of light. S&T readers are often
no strangers to math, so here you go: the rotational
velocity at the event horizon of a black hole is given by

A Spinning Black Hole's
Effect on Spacetime

ൌ

Ergosphere

ͳ൅

ͳ

ʹ

where c is the speed of light. (The derivation is a bit
messy; this equation comes thanks to Scott Hughes at
MIT.) So for OJ 287 and its spin of a = 0.313, the black
hole is spinning at 0.16c, or 16% the speed of light.
Do black holes really exist?
We're pretty darn sure. Stars and gas at the centers
of many galaxies orbit around invisible but incredibly
massive objects, and we can tell how massive the object
is based on these orbits: millions to billions of Suns'
worth of mass. No observers have detected surfaces for
these objects. In addition, brilliant beacons called active
galactic nuclei (the galactic-center kind of black hole) and
X-ray binaries (the stellar kind of black hole) put out so much
radiation - notably as jets - that the only explanation we've
found that works is that these powerhouses are fueled by
black holes.
Plus, the two gravitational-wave events recently detected by
LIGO (S&T: Oct. 2016, p. 10) came from the merger of objects
that look just like the black holes predicted by Einstein's theory
of gravity. In fact, black holes are an inevitable outcome of
Einstein's theory, a fact the famed physicist wasn't happy about
and actually tried - and failed - to disprove.

CREDIT X X X X X X X X X X X X X X X

Event horizon

(Scientists don't agree
what happens inside a
real spinning black hole)

?

Could it be that we don't really understand gravity, and
something else explains all these phenomena? Yes. But so far
no other ideas have worked out. And black holes work out really
well. As bizarre as they are, my money is on their existence.
-Camille M. Carlisle
s k y a n d t e l e s c o p e .c o m

* FEBRUARY 2017

17
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Sky and Telescope - February 2017

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