Gravitational Waves:
A new window to the universe
Presented by:
John S. Jacob
Sr. Research Fellow,
School of Physics, University of Western Australia
Project Engineer,
Australian International Gravitational Research Center
Gravitational Waves:
A new window to the universe
Acknowledgements
ACIGA: Australian Consortium for Interferometric Gravitational Astronomy:
David McClelland (ANU), Jesper Munch (U. Adelaide), Tony Lun (Monash).
Gabriella Gonzales for some slides
Prof. David Blair for some slides and inspired leadership of the center
Dr. Ju Li for help with some slides
All members of the UWA Gravity Waves Group
LIGO Scientific Collaboration
International Advisory Committee, High Optical Power Project
The VIRGO Project, The TAMA Project and the GEO Collaboration
Looking Out at the Universe
Gamma Rays
Bursts (BATSE)
Infrared
Microwave
Background
(COBE)
These are all parts of the same
Electromagnetic Spectrum.
Gamma Rays
Bursts (BATSE)
Infrared
Microwave
Background
(COBE)
The Electromagnetic Spectrum?
What’s That?
In 1820, Hans Christian Oersted discovered
that electric currents (moving electric charges)
create magnetic fields.
In 1831, Michael Farraday discovered that
varying magnetic fields create electric currents.
In 1864, James Clerk Maxwell put two and two
together.
• Oscillating electric fields create oscillating
magnetic fields.
• Oscillating magnetic fields create oscillating
electric fields.
• Together, electromagnetic waves propagate
through empty space at a speed Maxwell
calculated to be 3 x 108 meters per second.
In 1886, Heinrich Hertz experimentally
demonstrated creation and propagation of
electromagnetic waves at the speed of light.
It was eventually realized that:
• Radio
• Microwave
• Infrared Light
• Visible Light
• Ultraviolet Light
• X-Ray Radiation
• Gamma Ray Radiation
are all forms of Electromagnetic Waves.
It was eventually realized that:
• Radio – 100 km to 1 meter
• Microwave – 10 cm to 1 mm
• Infrared Light - 1/1000 mm
• Visible Light – 500 micron
• Ultraviolet Light – 100 nm to 10 nm
• X-Ray Radiation - 10 nm to 0.01 nm
• Gamma Ray Radiation – 0.01 nm to ??
are all forms of Electromagnetic Waves.
In 1915, Albert Einstein presented his theory of
General Relativity.
• Matter changes the shape of space.
• Space changes the path of matter.
• Waves in the continuum of space can
propagate forward at the speed of light.
“Gravity waves.”
Gravitational Waves:
a different kind of waves
They were predicted by Einstein:
all moving masses change space time around them!
Matter tells space how to curve.
Space tells matter how to move.
Not light waves!! But they also have different wavelengths…
Gravitational waves: a new window
GWs are produced by accelerated masses.
They represent an entirely new spectrum.
Gravity wave detectors are the ears that will allow us to listen to the sounds of
the universe.
Gravitational wave sources:
• periodic sources: binary systems,
• rotating stars…
• burst sources: supernovae, collisions,
black hole formations, gamma ray bursts?…
• stochastic sources: clutter of signals from
the entire universe, early moments of the big
bang, cosmic strings?...
But they are very weak….
We’ve been looking for them with detectors sensitive to changes in
distance A BILLION TIMES smaller than an atomic diameter… and
nothing yet!?
Resonant bar detectors:
University of Western
Australia :NIOBE
Lousiana State
University (USA)
ALLEGRO
Astronomers are not surprised: most strong sources are VERY far away!
Gravitational waves produce larger effects if the detectors are
VERY long. We also want to try different wavelengths!
What do we know
about gravitational waves?
That they exist!…
Nobel Prize
Physics 1993
Hulse & Taylor
There is indirect evidence for
the existence of Gravity Waves.
Nobel Prize
Physics 1993
Hulse & Taylor
However, no one has yet been able
to observe them directly.
The ability to do so is important as a
test of General Relativity and as a
totally new kind of astronomy!
Gravitational waves:
an international dream
Resonant
Mass
Detectors
Gravitational waves:
an international dream
GEO600 (British-German)
Hannover, Germany
TAMA (Japan)
Mitaka
LIGO (USA)
Hanford, WA and Livingston, LA
AIGO (Australia),
Wallingup Plain, 85km north of Perth
VIRGO (French-Italian)
Cascina, Italy
Gravitational Waves:
what else do we know?
•Gravitational Waves are ripples in space-time.
•They cause distortions of distances.
•Their strength is measured by strain, the fractional change in
distance, called h. Typical value of h is ~10-21.
•The size of the waves is tiny because space is very “stiff”!
•Because gravity only attracts - never repels, gravity waves have
“quadrupole polarizations” pictured above.
But how can we detect Gravity
Waves?
What measurable
effect
How to
docould
it? Gravity
Waves have?
What kind of instrument could
directly observe them?
Gravitational Waves:
what else do we know?
•Gravitational Waves are ripples in space-time.
•They cause distortions of distances.
•Their strength is measured by strain, the fractional change in
distance, called h.
•The size of the waves is tiny because space is very “stiff”!
•A NS binary, oscillating at ~100 Hz, ~100 Mpc away, produces
h~10-21
Simply measure the distance between two
objects floating in space.
If it’s that easy, why hasn’t it already been
done?
Problems:
• The marks on the ruler would have to be a
billion times smaller and closer together
than the atoms the ruler is made of.
• The objects might be drifting away from
each other. We need a way to control them
without constraining them too much.
• The length of our ruler will also be affected
by the gravity waves.
Make the distance between
controlled objects very large.
The greater the distance, the
greater the effect of gravity waves.
Use a laser beam to measure the
distance.
For a
change in distance
due to gravity waves of 1 mm,
the objects only have to be about one
million billion kilometers apart.
Problem: It would take more
than one hundred years just to
make one measurement.
The distance must also be
smaller than the wavelength
we are trying to measure
Solution: It’s much easier to measure
differences between two large
distances than it is to measure the
large distances themselves.
A device called an Interferometer
does this with utterly ridiculous
accuracy.
An interferometer compares the
distances traveled by two laser
beams. It is sensitive to changes in
length smaller than the wavelength
of its light.
Example of a simple Fabry—Perot Cavity
Interferometer
Fabry-Perot
cavities
Laser
beamsplitter
photo detector
Are not both arms of the interferometer
affected equally by gravity waves?
Fabry-Perot
cavities
Laser
beamsplitter
photo detector
Answer: Yes. However, due to the “quadrupole polarization” of
gravity waves, the effects do not happen to both sides at the same
time!
Fabry-Perot
cavities
Laser
beamsplitter
photo detector
By having its arms at right angles, an interferometer’s sensitivity
to gravity waves is effectively doubled!
Problem: Noise
Many sources of “noise” reduce the
sensitivity of an interferometer:
• Laser fluctuations
• Photon noise effects
• Thermal vibrations of mirrors
• Seismic noise
Noise Solutions
Laser Stabilization
Frequency stabilization
PBS
Isolat or PC
Laser
Laser geometry fluctuation stabilization
RF
Oscillator
T o laser
interferometer
PBS
Mixer
Reference
Cavit y
 
Mode cleaner—long optical
cavity
Photon Noise Effects:
• Statistical sampling of photons:
precision of the phase measurement
increases as N1/2.
• Radiation pressure of photons exerts
random forces on mirrors, also
increasing as N1/2.
Solution:
• Use an optimum number of photons.
• Present detectors use 100 times too few
photons.
• Use a very powerful laser (100watts and
build it up by resonance to 1 Megawatt).
Problems with High Light Power
• Powerful lasers cause Thermal
Lensing.
• The radiation pressure forces push
the mirrors apart and create stability
problems (before they cause photon
noise.)
Solutions:
• Develop thermal lensing compensation
techniques.
• Develop better control systems.
AIGO High Optical Power Test Facility
Injection bench
Injection locked
100W laser
ACIGA
-
Pre-stabilisation
cavity
Beam expander
10m Mode cleaner
Power recycling
Detection bench
Sapphire
Input
Mirror
ACIGA arm
80m high power test cavity
Sapphire
end mirror
Problems:
• Seismic noise is a 1012 times stronger
than gravity waves.
• Ocean waves, people, cars and
kangaroos!
• AIGO site is 1000 times better than
UWA.
Full vibration isolation system
thin fibre
pendulum link
simple wire
pivot
magnets
copper
Eddy
current
viscous
coupling
2-d gimbal
pivot
concentric
with wire
pivot Eddy
current
damped
rocker
vertical
Euler
springs
to next
stage
Self damped pendulum
LIGO: Now testing, planning upgrade
Estimated noise sources
Australian International Gravitational
Observatory
AIGO (opened in 2000) and Wallingup
Plain
Gravity Discovery Centre
SCCC: opened in 2001
Conclusion
• AIGO is developing vital technology for the
upgrade of detectors to reach a sensitivity where
known sources are detectable.
• It will be an essential element in the world array of
detectors.
• It offers opportunities to promote science for the
benefit of all West Australians