Sky and Telescope - May 2016 - 10

Learn more about the discovery and watch
videos explaining gravitational waves at
http://is.gd/gravitationalwaves.

News Notes

PHYSICS I Gravitational Waves Detected,
Discovery Heralds New Era of Astronomy
On February 11th LIGO scientists announced the direct
detection of gravitational waves, a discovery that will
smash wide open a new door on the cosmos.
Gravitational waves are ripples in the fabric of spacetime, predicted by Einstein's general theory of relativity
(S&T: Dec. 2015, p. 26). They're created by accelerating
masses, much as paddles sweeping through water spur
vibrations in a pond. Previously, astronomers only had
indirect evidence of their existence. But at 9:50:45 Universal Time on September 14, 2015, LIGO's two L-shaped
labs - one near Hanford, Washington, and the other in
Livingston, Louisiana - caught the gravitational-wave
signature of two colliding black holes.
LIGO detected the signal shortly after both its facilities
turned back on following ﬁve years of intensive upgrades.
The instruments shoot infrared lasers through 4-kilometer-long arms of near-perfect vacuum. The laser light
reﬂects oﬀ ultrapure, superpolished, and seismically
isolated quartz mirrors. The passing gravitational waves
slightly altered the path lengths in the arms of both
detectors, by about 1/1000 the width of a proton. That slight
change created an interference pattern in the laser light,
which LIGO scientists detected as an "event." They've
dubbed the event GW150914.

Vibrations as Gravitational Waves Passed

Fractional change in
detector's length (10 −21)

Inspiral

Merger

Ringdown

1.0
0.5

0
−0.5
−1.0
0.30

0.35

0.40

0.45

Time (seconds)
This reconstructed signal from LIGO shows the last few orbits
of the black hole duo's dance of death, the merger itself, and the
"ringdown" as the newly formed black hole settled down.
S&T: GREGG DINDERMAN / SOURCE: LIGO SCIENTIFIC COLLABORATION AND VIRGO COLLABORATION /
PHYSICAL REVIEW LETTERS 2016 (DOI10.1103/PHYSREVLETT.116.061102)

May 2016 sky & telescope

Based on the gravitational waves' frequency, team
members estimate that the colliding black holes had
the masses of about 36 and 29 Suns, respectively. LIGO
watched all three predicted phases of the collision: the
black holes' death spiral, the ensuing merger, and the
"ringing" of the merged object as it settled into its new
form. The newly created black hole contains about 62
solar masses; the gravitational waves carried away three
solar masses' worth of energy, just as predicted.
With only two detectors, LIGO can't pinpoint the
source's exact location - it could come from anywhere
within about 600 square degrees of sky near the Large
Magellanic Cloud in the Southern Hemisphere sky. The
source has a redshift between 0.05 and 0.12, meaning
the waves traveled between 700 million and 1.5 billion
years before reaching us.
The team claims a 5.1-sigma detection, meaning the
odds of the signal occurring by chance are about one in
3.5 million. The result appears in Physical Review Letters.
The direct detection of gravitational waves opens up
an entirely new spectrum that doesn't involve any form
of light. As LIGO member Eric Katsavounidis (MIT) puts
it, "This is the end of the silent-movie era in astronomy."
Gravitational waves are incredibly diﬃcult to detect,
for three main reasons: (1) gravity is the weakest of the
four known forces of nature, (2) the strength of the
waves falls oﬀ sharply as they traverse space, and (3)
matter barely feels their presence. "The gravitational
waves from a distant galaxy that are detectable to LIGO
are squeezing and stretching the Milky Way Galaxy by
the width of your thumb," says science team member
Chad Hanna (Penn State University).
LIGO began its ﬁrst "advanced" observing run last
fall, but improvements continue and future runs will
have at least twice the sensitivity and enable LIGO to
survey 10 times the volume of space. Theorists predict
Advanced LIGO should catch an additional ﬁve binary
black hole mergers in its next observing run. They also
expect roughly 40 binary neutron star mergers every
year it runs, and an unknown number of black holeneutron star mergers and supernovae.
Although the detected merger went entirely according
to predictions, scientists hope to eventually see discrepancies that could provide vital clues to new physics,
potentially reconciling contradictions between relativity
and quantum theory.
■ ROBERT NAEYE

http://www.is.gd/gravitationalwaves

Sky and Telescope - May 2016

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