Transcript Gravitational Waves

Gravitational Waves
The Basics
• According to Newton the force of
gravity is instantaneous
• Einstein says nothing can travel
faster than light
• Remember from General Relativity:
gravity is space-time which has
been warped by matter
– More mass = More warping
• So how does gravity travel ‘through’
space?
Mechanics of These Waves
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Like light Gravity travels as a wave
As electro-magnetic waves propagate
they affect the two planes
perpendicular to their line of motion
– The electric and magnetic fields
gravity propagates does the same
thing
– Compression and expansions
Moving charge - light
Moving mass - gravity
Gravitational waves also exhibit a
particle nature
– Light has the photon
– Gravity has the graviton
Neither particle has mass
Therefore gravitational waves can
travel at the speed of light
They travel the same speed through
any medium
"Electromagnetic waves are
oscillations of the electromagnetic
field that propagate through
spacetime; gravitational waves are
oscillations of the 'fabric' of
spacetime itself." (Kip Thorne)
Indirect Evidence
• According to General Relativity a binary system of
stars should lose energy to gravitational radiation
• This means the stars should slow down
• In 1993 the Nobel Prize was awarded to Hulse and
Taylor for proving this
• In 1974 they found a neutron star binary
• Over twenty years of observations they found a
shift in the in the orbits which was consistent with
the predictions of General Relativity
Strength of These Waves
• If every moving mass causes a
gravitational wave why aren’t we all
stretching and contracting now?
• Compared to the other 3 forces gravity is
very weak.
• A black hole binary, each with the mass of
10 suns, 1 billion light years away, would
displace the oceans by only 10 times the
diameter of an atomic nucleus
• What can we use to detect such a small
affect
INTERFEROMETRY
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Capable of detecting a path
difference smaller than the
diameter of the nucleus of an
atom
Light passed through beam
splitter
Travels down either arm and is
reflected back and out for
observation
If both paths are the same there
should be constructive
interference
If the path difference changes
the interference pattern will
shift which can be detected
This is used to detect
abnormalities in a surface, find
the refractive index of an optical
material, the change in
refractive index etc.
Interferometers to Detect
Gravitational Waves
• LIGO - Laser
Interferometer
Gravitational-wave
Observatory
• LISA – Laser
Interferometer Space
Antenna
LIGO
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In operation since 2002
Two active observatories
in Livingston, Louisiana
and Richland, Washington
Have not detected
anything significant
Each arm is 4000m, these
are the largest
interferometers ever built
to detect gravitational
waves
LIGO uses two
interferometers to rule out
any discrepancies
LlGO detects movements
as small as one
thousandth the diameter
of a proton
LIGO – more specs
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The vacuum tubes housing
the interferometer have a
volume of nearly 300,000
cubic feet
They are evacuated to a
pressure one-trillionth of an
atmosphere
The suspended mirrors are
even shielded from vibrations
occurring due to the random
motion of the atoms within the
mirrors
Over 30 control systems are
used to hold the lasers and
mirrors in place
VIRGO is another ground
based interferometer
LISA
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If approved development will begin in 2004
Launch set in 2011 for a duration of five years
Will consist of three unconnected spacecraft
five million km apart, in an equilateral triangle
50 million kilometers above Earth
Each spacecraft will contain two test masses
floating freely space craft will follow the masses
with an accuracy of 10^-9 m
Noise due to:
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Solar pressure
Solar magnetic field
Earths gravity
Micro-thrusters will attempt to move LISA out of
such noise if detected
Separation between spacecraft will be
maintained with an accuracy of 10^-12 m
Main craft fires laser to second which fires laser
in phase to third which fires laser in phase back
the main craft
This goes in both directions
Each can craft calculates the path deviation
By measuring the relative positions of the test
masses
LISA will not be ‘better’ than LIGO or VIRGO but
will be able to detect different frequencies of
waves
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LIGO detects high frequencies
LISA will detect low frequencies
Promising Sources
• Binary Systems
– Stars
– Neutron Stars
– Black Holes
• Non-Spherical Rotating
Objects
– Super nova
– Vibrating Black Holes
• Galaxy Formation
• Dark Matter
• Big Bang ???
MOVIE TIME
Low Frequency
waves due to a
rotating binary
system
High Frequency
waves due to a
rotating binary
system
Types of Signals
• Chirps
– Binary Systems
• Burps
– Collapse of Supernova
– Black Hole eating a star
• Periodic
– Non-Spherical Objects rotating
• Stochastic
– Big Bang
• Need much improvement i.e. another observatory
Summary
• Gravitational waves warp spacetime as they
travel
• Indirect Evidence exists
• Direct evidence places its hope mostly on
interferometers
• Since they can travel through anything
gravitational waves offer a new and exciting view
of the universe
– Comparable to the first use of x-ray etc. astronomy
My sources
• LIGO Laboratory Homepage
– http://www.ligo.caltech.edu/
• Laser Interferometer Space Antenna
– lisa.jpl.nasa.gov/
• Jillian’s Guide to Gravitational Waves
– http://www.gothosenterprises.com/gravitational_waves/
• Ripples in Space Time
– http://archive.ncsa.uiuc.edu/Cyberia/NumRel/GravWaves.html
• List of Gravitational Wave Detectors
– www.johnstonsarchive.net/relativity/gwdtable.html
• Gravitational Wave Sources
– http://faculty.fortlewis.edu/tyler_c/classes/454/w03/andrew/sources_of_g
w.htm
• Propagation of Gravity Waves
– http://www.physics.uiuc.edu/Research/CTA/movies/Gw-ns/gw/