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


http://www.skyandtelescope.com

Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
Sky and Telescope - February 2017 - 4
Sky and Telescope - February 2017 - 5
Sky and Telescope - February 2017 - 6
Sky and Telescope - February 2017 - 7
Sky and Telescope - February 2017 - 8
Sky and Telescope - February 2017 - 9
Sky and Telescope - February 2017 - 10
Sky and Telescope - February 2017 - 11
Sky and Telescope - February 2017 - 12
Sky and Telescope - February 2017 - 13
Sky and Telescope - February 2017 - 14
Sky and Telescope - February 2017 - 15
Sky and Telescope - February 2017 - 16
Sky and Telescope - February 2017 - 17
Sky and Telescope - February 2017 - 18
Sky and Telescope - February 2017 - 19
Sky and Telescope - February 2017 - 20
Sky and Telescope - February 2017 - 21
Sky and Telescope - February 2017 - 22
Sky and Telescope - February 2017 - 23
Sky and Telescope - February 2017 - 24
Sky and Telescope - February 2017 - 25
Sky and Telescope - February 2017 - 26
Sky and Telescope - February 2017 - 27
Sky and Telescope - February 2017 - 28
Sky and Telescope - February 2017 - 29
Sky and Telescope - February 2017 - 30
Sky and Telescope - February 2017 - 31
Sky and Telescope - February 2017 - 32
Sky and Telescope - February 2017 - 33
Sky and Telescope - February 2017 - 34
Sky and Telescope - February 2017 - 35
Sky and Telescope - February 2017 - 36
Sky and Telescope - February 2017 - 37
Sky and Telescope - February 2017 - 38
Sky and Telescope - February 2017 - 39
Sky and Telescope - February 2017 - 40
Sky and Telescope - February 2017 - 41
Sky and Telescope - February 2017 - 42
Sky and Telescope - February 2017 - 43
Sky and Telescope - February 2017 - 44
Sky and Telescope - February 2017 - 45
Sky and Telescope - February 2017 - 46
Sky and Telescope - February 2017 - 47
Sky and Telescope - February 2017 - 48
Sky and Telescope - February 2017 - 49
Sky and Telescope - February 2017 - 50
Sky and Telescope - February 2017 - 51
Sky and Telescope - February 2017 - 52
Sky and Telescope - February 2017 - 53
Sky and Telescope - February 2017 - 54
Sky and Telescope - February 2017 - 55
Sky and Telescope - February 2017 - 56
Sky and Telescope - February 2017 - 57
Sky and Telescope - February 2017 - 58
Sky and Telescope - February 2017 - 59
Sky and Telescope - February 2017 - 60
Sky and Telescope - February 2017 - 61
Sky and Telescope - February 2017 - 62
Sky and Telescope - February 2017 - 63
Sky and Telescope - February 2017 - 64
Sky and Telescope - February 2017 - 65
Sky and Telescope - February 2017 - 66
Sky and Telescope - February 2017 - 67
Sky and Telescope - February 2017 - 68
Sky and Telescope - February 2017 - 69
Sky and Telescope - February 2017 - 70
Sky and Telescope - February 2017 - 71
Sky and Telescope - February 2017 - 72
Sky and Telescope - February 2017 - 73
Sky and Telescope - February 2017 - 74
Sky and Telescope - February 2017 - 75
Sky and Telescope - February 2017 - 76
Sky and Telescope - February 2017 - 77
Sky and Telescope - February 2017 - 78
Sky and Telescope - February 2017 - 79
Sky and Telescope - February 2017 - 80
Sky and Telescope - February 2017 - 81
Sky and Telescope - February 2017 - 82
Sky and Telescope - February 2017 - 83
Sky and Telescope - February 2017 - 84
Sky and Telescope - February 2017 - Cover3
Sky and Telescope - February 2017 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/newtrack/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com