Sky and Telescope - September 2017 - 29

t SEEING BLIND LIGO is often called a
"blind" detector in the sense that it can't tell
exactly where a signal is coming from, as
demonstrated by this sky map of the origins
of the four signals seen during the first two
observing runs. With only two detectors,
LIGO cannot easily triangulate position; the
sources of GW150914 (blue), LVT151012
(green), GW151226 (orange), and GW170104
(magenta) lie somewhere in the linear fields
marked on the map. The areas of these fields
are 600, 1,600, 1,000, and 1,200 square
degrees, respectively. Additional data from
the Virgo detector in Italy should significantly
improve our ability to pinpoint future sources.

made clear by my initial reaction on September 14th.
The ﬁrst LIGO discovery, on the other hand, provided solid
observational evidence that black holes more than 25 to 30
times the mass of the Sun form in nature. Mergers of these
heavy black holes can create even more massive beasts, with
as much as 50 or 60 times the Sun's mass. The newest detection in January 2017 supports this result. These discoveries
challenge our understanding of massive stars' evolution.
Coupled with X-ray observations, LIGO's discoveries also
indicate that black holes have a wide range of masses. Understanding this population, especially the binaries' origins and
evolutionary histories, is now one of the most exciting investigations in modern astrophysics.
Finally, for experimentalists and observers, gravitationalwave detections are of historic signiﬁcance. Since LIGO's

inception in the early 1970s, numerous physicists and astrophysicists doubted this measurement would ever be possible.
When I joined the collaboration as a young postdoctoral
researcher about two decades ago, for example, the majority
of my astronomy mentors advised me against it.
Yet today LIGO's results represent the most precise measurement ever achieved by humans in any ﬁeld of science and
engineering: LIGO detects changes on the order of one part
in one thousand billion billion (10 -21). And with Advanced
LIGO's sensitivity in 2015, we can "hear" black hole mergers out to more than 8 billion light-years. By the time the
Advanced LIGO detectors reach their design sensitivity in a
few years, we'll gain yet another factor of three in reach.
These discoveries represent a completely new way of
observing the cosmos, distinct from the electromagnetic

Black
Holes
ofKnown
Known
Mass
Black
Holes of
Mass
GW150914

70
60
LVT151012

LIGO
Solar Masses

M ASS CH A RT: LIGO / CA LTECH / SONO M A STATE (AURORE SIM ONNE T );
TIME CH A RT: LIGO / CA LTECH / MIT / UNIV ERSIT Y OF CHICAGO (BEN
FA RR); M A P: CON TOURS: LIGO / CA LTECH / LEO SING ER, MILK Y WAY:
A X EL MELING ER

50
GW150914

GW151226

40
30
20

GW170104

X-Ray Studies
GW170104

LVT151012

GW151226

Time observable by LIGO (seconds)

p MASSIVE DISCOVERIES Left: LIGO's detections have uncovered a population of massive, merging black holes that X-ray observations hadn't
revealed. Historically, black holes have made themselves known by the X-rays they emit as they feed on gas pulled from a companion star. LIGO has
shown that some black holes are far more massive and electromagnetically silent, having long ago swept up any gas and other potentially X-rayemitting material. Right: This chart shows reconstructions of four gravitational wave signals, three of them confident detections and one more
tentative, seen by LIGO's Hanford detector and shown as a function of time. Only the portions of each signal that LIGO was sensitive to (the final
moments leading up to the merger) are shown here.
s k y a n d t e l e s c o p e .c o m

* SEPTEMBER 2017

http://www.skyandtelescope.com

Sky and Telescope - September 2017

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