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What If We Could Listen to the
Stars?
Fred Raab
LIGO Hanford Observatory
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LIGO’s Mission is to Open a New
Portal on the Universe

In 1609 Galileo viewed the sky through a 20X
telescope and gave birth to modern astronomy
» The boost from “naked-eye” astronomy revolutionized humanity’s
view of the cosmos & astronomers have “looked” into space to
uncover the natural history of our universe


LIGO’s quest is to create a radically new way to
perceive the universe, by directly listening to the
vibrations of space itself
LIGO consists of large, earth-based, detectors that
will act like huge microphones, listening for the most
violent events in the universe
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The Laser Interferometer
Gravitational-Wave Observatory
LIGO (Washington)
LIGO (Louisiana)
Brought to you by the National Science Foundation; operated by Caltech and MIT; the
research focus for more than 500 LIGO Scientific Collaboration members worldwide.
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LIGO Laboratories Are Unique
National Facilities
LHO
MIT
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(+ 998
/10 km
m
s)
CIT
LLO
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
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Observatories at Hanford, WA
(LHO) & Livingston, LA (LLO)
Support Facilities @ Caltech &
MIT campuses
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Part of Future International
Detector Network
Simultaneously detect signal (within msec)
LIGO
GEO
Virgo
TAMA
detection
confidence
locate the
sources
AIGO
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decompose the
polarization of
gravitational
waves
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Big Questions for 21st Century
Science
Images of light from Big Bang imply
95% of the universe is composed of
dark matter and dark energy. What is
this stuff?
The expansion of the universe is
speeding up. Is it blowing apart?
WMAP Image of Relic
Light from Big Bang
There are immense black holes at the
centers of galaxies. How did they form?
What was it like at the birth of space
and time?
Hubble Ultra-Deep Field
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A Slight Problem
Regardless of what you see on Star Trek, the vacuum
of interstellar space does not transmit conventional
sound waves effectively.
Don’t worry, we’ll work around that!
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John Wheeler’s Picture of General
Relativity Theory
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General Relativity: A Picture Worth
a Thousand Words
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The New Wrinkle on Equivalence
Not only the path of
matter, but even the
path of light is affected
by gravity from massive
objects
A massive object shifts apparent
position of a star
Einstein Cross
Photo credit: NASA and ESA
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Gravitational Waves
Gravitational waves
are ripples in space
when it is stirred up
by rapid motions of
large concentrations
of matter or energy
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Rendering of space stirred by
two orbiting black holes:
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Gravitational Collapse and Its
Outcomes Present LIGO Opportunities
fGW > few Hz
accessible from
earth
fGW < several kHz
interesting for
compact objects
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Supernova: Death of a Massive
Star
•Spacequake should preceed optical
display by ½ day
•Leaves behind compact stellar
core, e.g., neutron star, black hole
•Strength of waves depends on
asymmetry in collapse
Credit: Dana Berry, NASA
•Observed neutron star motions
indicate some asymmetry present
•Simulations do not succeed from
initiation to explosions
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The “Undead” Corpses of Stars:
Neutron Stars and Black Holes
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
Neutron stars have a
mass equivalent to 1.4
suns packed into a ball
10 miles in diameter,
enormous magnetic
fields and high spin
rates
Black holes are the
extreme edges of the
space-time fabric
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Artist: Walt Feimer, Space
Telescope Science Institute
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Gravitational-Wave Emission May be the
“Regulator” for Accreting Neutron Stars
•Neutron stars spin up when they
accrete matter from a companion
•Observed neutron star spins “max out”
at ~700 Hz
•Gravitational waves are suspected to
balance angular momentum from
accreting matter
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Credit: Dana Berry, NASA
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Catching Waves
From Black Holes
Sketches courtesy
of Kip Thorne
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Detection of Energy Loss Caused
By Gravitational Radiation
In 1974, J. Taylor and R. Hulse
discovered a pulsar orbiting
a companion neutron star.
This “binary pulsar” provides
some of the best tests of
General Relativity. Theory
predicts the orbital period of
8 hours should change as
energy is carried away by
gravitational waves.
Taylor and Hulse were awarded
the 1993 Nobel Prize for
Physics for this work.
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Sounds of Compact Star Inspirals
Neutron-star binary inspiral:
Black-hole binary inspiral:
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How does LIGO detect spacetime
vibrations?
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Important Signature of
Gravitational Waves
Gravitational waves shrink space along one axis perpendicular
to the wave direction as they stretch space along another axis
perpendicular both to the shrink axis and to the wave direction.
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Sketch of a Michelson
Interferometer
End Mirror
End Mirror
Beam Splitter
Viewing
Screen
Laser
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Core Optics Suspension and
Control
Optics
suspended
as simple
pendulums
Local sensors/actuators provide
damping and control forces
Mirror is balanced on 1/100th inch
diameter wire to 1/100th degree of arc
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How Small is 10-18 Meter?
One meter, about 40 inches
 10,000
100
Human hair, about 100 microns
Wavelength of light, about 1 micron
 10,000
Atomic diameter, 10-10 meter
 100,000
Nuclear diameter, 10-15 meter
 1,000
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LIGO sensitivity, 10-18 meter
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Vacuum Chambers Provide Quiet
Homes for Mirrors
View inside Corner Station
Standing at vertex
beam splitter
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And despite a few difficulties,
science runs started in 2002…
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Binary Neutron Stars:
S1 Range
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Image: R. Powell
Binary Neutron Stars:
S2 Range
S1
Range
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Image: R. Powell
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Binary Neutron Stars:
Initial LIGO Target Range
S2 Range
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Image: R. Powell
What’s next? Advanced LIGO…
Major technological differences between LIGO and Advanced LIGO
40kg
Quadruple pendulum
Sapphire optics
Silica suspension fibers
Initial Interferometers
Active vibration
isolation systems
Open up wider band
Reshape
Noise
Advanced Interferometers
High power laser
(180W)
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Advanced interferometry
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Signal recycling
Binary Neutron Stars:
AdLIGO Range
LIGO Range
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Image: R. Powell
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