Sky and Telescope - September 2017 - 27

September 14th at 09:50:45 UT. Seven milliseconds later, they
hit the LIGO detector in Hanford, Washington. Then they
continued past Earth into the cosmos beyond.
Following a pre-existing plan, my collaborators and I on
the detection committee and across the whole international
scientiﬁc collaboration worked around the clock for weeks,
laboring through a list of more than 170 checks and questions that needed to be addressed before we could conclude
with conﬁdence that the signal was real. We tested the data,
hardware, software, and hundreds of thousands of channels recording extraneous noise. We came up empty-handed.
There was only one possibility left: This was a bona ﬁde signal. We called it GW150914.
A second signal reached us on October 12, 2015, dubbed
LVT151012. It was not nearly as strong - it ended up being
what scientists call a 1.7-sigma detection, with only a probability of about 85% that it is of astrophysical nature; there's
a 15% chance we're just looking at noise.
Then another event arrived on December 26, 2015, named
GW151226, also from a binary black hole but with smaller
members of about 7.5 and 14 solar masses respectively. This
event was strong enough to clearly be a real cosmic signal.
The chirps were here! We had ofﬁcially progressed from
theory into observational astronomy.
Since then, LIGO has continued to ﬁnd merging black
holes. Both detectors detected an event on January 4, 2017,
during the upgraded observatory's second observing run (see
page 10). The component black holes, about 30 and 20 solar
masses respectively, ﬁll in the gap between the GW150914
and LVT151012 events.

Shaking Up Physics & Astronomy
These cosmic chirps have made it clear beyond a doubt: There
is an actual population of merging black holes in the universe. Every chirp that we've detected has lasted about two
seconds or less; yet, even though they're so short, these signals have enormous signiﬁcance for physics and astronomy.
For theoretical physicists, the signals allow us, for the ﬁrst
time, to test general relativity in a regime we've never probed
before. Einstein's theory says that what we experience as gravity

p COMPLICATED COALESCENCE These computer simulations show
the complexity of gravitational-wave patterns, which become especially
tangled near the inspiraling black holes. The red-colored outer sheets
show the expanding wavefront that LIGO ultimately detects.

The "Sound" of Gravitational Waves

N ASA / C. HENZE (3)

◗ Even though we often discuss

gravitational-wave signals as if they
were sounds, it's important to realize
that they're not, in fact, sound waves.
Nevertheless, the analogy is useful
for a couple of reasons.
First, LIGO is sensitive to wave
frequencies of tens to thousands
of hertz. If these frequencies were
sounds, our ears could hear them.

Second, and more importantly, the
LIGO detectors function more like our
ears than our eyes. Eyes - and most
electromagnetic telescopes - have
pointing capabilities. That is, they
can detect a signal as long as they're
pointing in the same direction as the
signal's source. But LIGO doesn't
have strong pointing capabilities.
Instead, it detects gravitational-wave

signals from almost all around us,
just as our ears can hear sounds from
any direction - behind us, below us,
above us, and in front of us. With two
detectors, LIGO can localize sources
to narrow strips of sky that nevertheless cover a lot of area. But with more
"ears," we can triangulate the signal
and better pinpoint its source. That's
why the addition of Virgo data to
LIGO's two detectors is so vital.

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