Gravitational Waves

Gravitational Waves

Danielle Kumpulanian
March 24, 2005

World Year of Physics 2005
 International celebration of
physics highlighting the
importance of physics in the
past, present, and future – in
both technology and society
 100th anniversary of Einstein’s
“miracle year” – published
three papers that have had a
tremendous impact on the
world
 Worldwide events, programs,
and speakers to inform and
inspire

General Relativity
 Einstein predicted that objects cause the fabric of
space-time around them to curve.
 Moving objects should therefore create ripples in
space-time.
 Einstein predicted that the more massive the
object, the larger the gravitational waves it would
create.

Tests of General Relativity
 Einstein’s formulas
explained Mercury’s
perihelion changes.
 Observations of starlight
passing the sun during a
solar eclipse in 1919
confirmed that light is
bent by the curved space-
time surrounding the sun.
 Observations of light from
white dwarfs verified that
in a gravitational field,
spectral lines of
substances are shifted
toward the red.

Cassini Test
 Test took place in September
2002, when the sun was
between the Cassini spacecraft
and Earth
 Cassini confirmed the theory
with 50x greater precision than
previous tests.
 Researchers observed a
frequency shift of radio waves
traveling to and from Cassini
when the waves passed near
the sun.
 The extra distance that the
radio waves traveled was
measured by the time they were
delayed in reaching Earth. http://saturn.jpl.nasa.gov

What exactly are gravitational waves?
 Ripples or oscillations in space-time itself, unlike
electromagnetic radiation, which passes through
space-time
 They travel at the speed of light.
 Their strength weakens proportionally to the
distance traveled from the source.
 By the time the waves reach Earth, they are
weak and difficult to detect – comparable to
detecting a change the size of an atom in the
distance between the sun and the earth.

 Neutron star: an extremely dense burnt-out core
left behind after a star explodes
 Can have as much mass as the sun in a smaller
space (a few miles wide) – larger density
 Imagine two neutron stars orbiting each other.
Their motion causes space time to be “stirred”
and gravitational waves are generated and sent
outward from the stars.

 In 1974, Joseph Taylor and Russell Hulse found
a pair of neutron stars (PSR 1913+16) in the
Milky Way.
 One of the stars was found to be a pulsar.
 The radio pulses coming from the pulsar can be
used to measure the orbits of the two stars.
 After 20 years of measuring these pulses, the
shift in their timing indicated that the pulsar’s
orbital period decreased by 75 µs per year – the
stars are spiraling in towards each other.
 Taylor and Hulse won the Nobel Prize in 1993
for their work.

 The difference in energy was just the amount
predicted if the system was radiating
gravitational waves!
 Another binary pulsar system, PSR 1534+12
was discovered in 1991 and will provide more
proof of gravitational waves once enough data is
collected.
 More binary pulsar systems have since been
detected.

What else causes
gravitational waves?
 Supernovae and stars’ collapse into neutron
stars
 Two black holes colliding or orbiting each other
 Neutron star orbiting a black hole
 Rotating neutron stars – continuous source of
waves
 Colliding galaxies
 Stochastic background of gravitational waves
emitted in the early stages of the universe –
comparable to microwave background
 Other new and exciting objects

http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html
http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html
Two neutron stars colliding

http://www.ligo-wa.caltech.edu/ligo_overview/ligo_overview.html
Gravitational-Wave Observatory
LIGO: Laser Interferometer
How will we detect

 LIGO is a collaboration between the California
Institute of Technology (Caltech) and the
Massachusetts Institute of Technology (MIT).
 It is funded by the National Science Foundation.
 It will function as a national resource for both

physics and astrophysics, and universities and
institutions around the world will be involved.
 Two locations:

Goals of LIGO
 Prove the existence of gravitational waves by
direct measurements
 Confirm that gravitational waves travel at the
speed of light
 Verify that gravitational waves cause
disturbances of predicted amounts in the matter
they pass through.
 Learn more about black holes by proving their
existence and study their behavior.
 Gain other knowledge about the universe,
including more information about supernovae
and the big bang.

How it works:
•Michelson Interferometer
•Each arm is 2.5 miles (4 km) long.
•The laser light is allowed to bounce
back and forth multiple times.
•If the path is the same length for the
laser in both arms, the light will be
directed back toward the laser.
•If any difference in path is detected,
the photodetector will produce a
signal.


 The distance measured by a light beam to
changes as the gravitational wave passes by.
 The photodetector then produces a signal which
shows how the light changes over time.
 Basically, the laser interferometer converts
gravitational waves into electrical signals.
 LlGO requires at least two detectors, in different
locations, operated in unison, to confirm the
results.
 LlGO must detect deviations in distance as small
as one thousandth the diameter of a proton.
 Interferometers used in LIGO are the world’s
largest precision optical instruments.

 This requires very precise instruments, including
the vacuum tubes, lasers, mirrors, and
mechanical systems involved in the setup.
 LIGO’s vacuum system is one of the largest, with
a volume of about 300,000 cubic feet.
 Pressure inside the tubes must be one trillionth of
an atmosphere so there are minimal gases to
interfere with the laser beams.


 Tubes are made of steel with a very low
dissolved hydrogen content.
 Solid-state lasers are regulated so that in 0.01
seconds, the frequency varies by less than a few
millionths of a cycle.
 Mirrors are suspended and shielded from
vibrations – so isolated that they can detect the
random motion of atoms in the mirror itself.
 30+ control systems involved in keeping mirrors
and lasers aligned without human intervention.
 State of the art electronics and software

So what happens now?
 Gravitational waves originating in different
sources will have different, unique interference
patterns.
 Where the source is known, scientists can match
the source with the pattern.
 Eventually, they will be able to build a catalogue
of these patterns and know what the sources of
the waves are and the properties of the sources.
(huge challenge)

Distorted black hole
•“Ringing mode”, or normal mode
•Ringing mode frequencies depend on mass and spin
of black hole
•LIGO detection would allow the mass to be
determined

Distorted, rotating black hole
•Similar to non-rotating case, but frequencies are different
•LIGO detection would allow mass and spin to be determined

Colliding black holes
•Two equal mass black holes colliding head-on
•They form a larger, distorted black hole that ends up
as a spherical black hole
•LIGO detection would allow mass and spin of the
final hole to be determined
http://archive.ncsa.uiuc.edu/Cyberia/NumRel/LIGO.html

Other detectors
(laser interferometric)

 GEO600 in Germany – two 600 m arms
 VIRGO in Italy – two 3 km arms
 TAMA300 in Japan – two 3 km arms
 Australian International Gravitational
Observatory (AIGO) in Australia – two 80 m
arms
 Laser Interferometer Space Antenna (LISA)
mission – three spacecraft flying about 5 million
km apart in equilateral triangle (2011)

Einstein@home
A distributed computing project which relies on
computer users worldwide to donate some of
their computer’s time.
 Automatically downloads portions of data
collected by LIGO and GEO600 to analyze and
sends results back during computer’s idle time.
 Screen saver showing constellations, known
pulsars and supernovae, detector locations, and
current search position.
 Leading developer is Bruce Allen of University of
Wisconsin-Milwaukee’s LIGO Scientific
Collaboration (LSC) group.
 http://einstein.phys.uwm.edu/

Other Detectors
(resonant detectors)

 ALLEGRO (US)
 AURIGA (Italy)
 EXPLORER (Italy)
 NIOBE (Australia)
 MiniGRAIL (Denmark)
 GRAVITON (Brazil)

Why should we care about
 Learning about gravitational waves will expand
our knowledge of the universe.
 They are thought to remain unchanged by
passing through material – can carry unaltered
information about their source.
 Could gain insight into why the universe is the
way it is and what it’s fate will be.
 Can accurately determine cosmological
distances
 Searching for existence of gravitational waves
may uncover new phenomena.

 Scientists can detect a black hole using gravitational
waves – and how big and how fast the black hole is
spinning.
 The gravitational waves emitted from each binary system
– inspiral waves – have characteristic frequencies and
amplitudes.
 These characteristics depend on properties of the
system (mass, orbital period, etc.).
 When waves emitted during the merging of two neutron
stars are detected, we will be able to learn more about
their structure and equation of state.
 Eventually, we will be able to use the information from
inspiral waves to perform more precise tests of general
relativity, measure the Hubble constant, and understand
the geometry of the space-time around black holes and
other objects.

 http://www.ligo-
wa.caltech.edu/ligo_overview/ligo_overview.
html
 http://www.ligo-
wa.caltech.edu/teachers_corner/BrockTchrG
uidev2.pdf
 http://spaceplace.nasa.gov/en/kids/lisa_fact2
.shtml
 http://lisa.jpl.nasa.gov/index.html
 http://www.astronomy.org.nz/aas/Journal/Oct
2003/EinsteinsTheory.asp
 http://archive.ncsa.uiuc.edu/Cyberia/NumRel/
GravWaves.html
 http://archive.ncsa.uiuc.edu/Cyberia/NumRel/
LIGO.html
 http://www.laserfantasy.com/abcs.asp
 http://pcl.physics.uwo.ca/pclhtml/gravitywave
s.html
 http://www.virgo.infn.it/
 http://www.geo600.uni-
hannover.de/gwlinks.html
 http://gravity.phys.lsu.edu/
 http://www.auriga.lnl.infn.it/
 http://saturn.jpl.nasa.gov

AURIGA – Resonant detector
 The mechanical resonator is an aluminum cylinder (bar):
3m long, 60cm in diameter, and a mass of 2.3 tons.
 Bar is suspended in a vacuum and cooled to
temperatures close to absolute zero to reduce vibrations
from noise and thermal motion.
 Gravitational waves are detected when the bar is
squeezed or stretched.
 A second mechanical resonator, a resonant transducer,
is attached to one end of the bar and picks up the
vibrations but with a larger amplitude.
 Vibrations are converted into oscillations in an electric
current and are analyzed.
 Even for huge events such as black hole collisions, the
vibrations in the bar are very small.

http://www.auriga.lnl.infn.it/

Laser Interferometer Space
Antenna (LISA)
•Joint mission between NASA
and ESA
•LISA will make observations in
a low-frequency band (space-
time swells) – complementary
to ground-based detectors
(space-time ripples)
•Three freely flying spacecraft,
5 million km apart in a triangle Courtesy NASA/JPL-Caltech

•Laser beams connect the
spacecraft – any movement
due to a passing gravitational
wave can be detected

•5-year lifetime, about 163 gigabytes of data for analysis
(could be extended to 10 years)
• 20 degrees behind
Earth’s orbit

• 1 AU from the Sun,
with an incline of 60
degrees to the ecliptic
plane

Courtesy NASA/JPL-Caltech

Animation courtesy of Jet Propulsion Laboratory.

Cassini
 Experiment could not have been conducted in the past
due to noise on the radio link induced by the solar
corona
 Cassini was fitted with multiple links at different
frequencies.
 This allowed scientists to remove noise caused by solar
and interplanetary interference.
 Noise from Earth’s atmosphere was reduced by a new
34-meter diameter antenna and other special equipment
installed at the Goldstone complex.

Goldstone Complex

http://www.jpl.nasa.gov

Gravitational Waves

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