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LIGOand the
Search for Gravitational Waves
Barry Barish
Stanford Colloquium15-Jan-01
15-Jan-02 Stanford Colloquium 2
Sir Isaac NewtonUniversal Gravitation
Three laws of motion and law of gravitation (centripetal force) disparate phenomena» eccentric orbits of comets» cause of tides and their variations» the precession of the earth’s axis» the perturbation of the motion of the
moon by gravity of the sun
Solved most known problems of astronomy and terrestrial physics» Work of Galileo, Copernicus and Kepler
unified.
15-Jan-02 Stanford Colloquium 3
Einstein’s Theory of Gravitation
Newton’s Theory“instantaneous action at a distance”
Einstein’s Theoryinformation carried by gravitational radiation at the speed of light
15-Jan-02 Stanford Colloquium 4
General Relativity the essential idea
Overthrew the 19th-century concepts of absolute space and time
Einstein: gravity is not a force, but a property of space & time» Spacetime = 3 spatial dimensions + time» Perception of space or time is relative
Concentrations of mass or energy distort (warp) spacetime
Objects follow the shortest path through this warped spacetime; path is the same for all objects
15-Jan-02 Stanford Colloquium 5
General Relativity
Einstein theorized that smaller masses travel toward larger masses, not because they are "attracted" by a mysterious force, but because the smaller objects travel through space that is warped by the larger object
Imagine space as a stretched rubber sheet.
A mass on the surface will cause a deformation.
Another mass dropped onto the sheet will roll toward that mass.
15-Jan-02 Stanford Colloquium 6
Einstein’s Theory of Gravitationgravitational waves
• a necessary consequence of Special Relativity with its finite speed for information transfer
• time dependent gravitational fields come from the acceleration of masses and propagate away from their sources as a space-time warpage at the speed of
light
gravitational radiationbinary inspiral of compact objects
15-Jan-02 Stanford Colloquium 7
Einstein’s Theory of Gravitationgravitational waves
0)1
(2
2
22
htc
• Using Minkowski metric, the information about space-time curvature is contained in the metric as an added term, h. In the
weak field limit, the equation can be described with linear equations. If the choice of gauge is the transverse traceless gauge the formulation becomes a familiar wave equation
• The strain h takes the form of a plane
wave propagating at the speed of light (c).
• Since gravity is spin 2, the waves have two components, but rotated by 450 instead of 900 from each other. )/()/( czthczthh x
15-Jan-02 Stanford Colloquium 8
Gravitational Waves the evidence
Neutron Binary SystemPSR 1913 + 16 -- Timing of pulsars
17 / sec
~ 8 hr
Neutron Binary System• separated by 106 miles• m1 = 1.4m; m2 = 1.36m; = 0.617
Prediction from general relativity• spiral in by 3 mm/orbit• rate of change orbital period
15-Jan-02 Stanford Colloquium 9
Hulse and Taylorresults
due to loss of orbital energy period speeds up 25 sec from 1975-98 measured to ~50 msec accuracy deviation grows quadratically with time
emission of
gravitational waves
15-Jan-02 Stanford Colloquium 10
Direct DetectionLaboratory Experiment
ExperimentalGeneration and Detection
of Gravitational Waves
gedanken experiment
a la Hertz
15-Jan-02 Stanford Colloquium 11
Radiation of Gravitational Waves
Radiation of Gravitational Wavesfrom binary inspiral
system
LISA
Waves propagates at the speed of lightTwo polarizations at 45 deg (spin 2)
15-Jan-02 Stanford Colloquium 12
Interferometers space
The Laser Interferometer
Space Antenna (LISA)
• The center of the triangle formation will be in the ecliptic plane • 1 AU from the Sun and 20 degrees
behind the Earth.
15-Jan-02 Stanford Colloquium 13
Astrophysics Sourcesfrequency range
EM waves are studied over ~20 orders of magnitude» (ULF radio HE -rays)
Gravitational Waves over ~10 orders of magnitude
» (terrestrial + space)
Audio band
15-Jan-02 Stanford Colloquium 14
Suspended mass Michelson-type interferometerson earth’s surface detect distant astrophysical sources
International network (LIGO, Virgo, GEO, TAMA) enable locating sources and decomposing polarization of gravitational waves.
Interferometersterrestrial
15-Jan-02 Stanford Colloquium 15
Laser
Beam Splitter
End Mirror End Mirror
ScreenViewing
Michelson Interferometer
15-Jan-02 Stanford Colloquium 16
Recycling Mirror
Optical
Cavity
4 km or
2-1/2 m
iles
Beam Splitter
Laser
Photodetector
Fabry-Perot-Michelson with Power Recycling
Suspended
Test Masses
15-Jan-02 Stanford Colloquium 17
Sensing a Gravitational Wave
Laser
signal
Gravitational wave changes arm lengths and amount of light in signal
Change in arm length is ~10-18 meters
h = L/L ~ 10-21
4 km
15-Jan-02 Stanford Colloquium 18
How Small is 10-18 Meter?
Wavelength of light, about 1 micron100
One meter, about 40 inches
Human hair, about 100 microns000,10
LIGO sensitivity, 10-18 meter000,1
Nuclear diameter, 10-15 meter000,100
Atomic diameter, 10-10 meter000,10
15-Jan-02 Stanford Colloquium 19
What Limits Sensitivityof Interferometers?
• Seismic noise & vibration limit at low frequencies
• Atomic vibrations (Thermal Noise) inside components limit at mid frequencies
• Quantum nature of light (Shot Noise) limits at high frequencies
• Myriad details of the lasers, electronics, etc., can make problems above these levels
15-Jan-02 Stanford Colloquium 20
Noise Floor40 m prototype
• displacement sensitivityin 40 m prototype. • comparison to predicted contributions from various noise sources
sensitivity demonstration
15-Jan-02 Stanford Colloquium 21
Phase Noisesplitting the fringe
• spectral sensitivity of MIT phase noise interferometer
• above 500 Hz shot noise limited near LIGO I goal
• additional features are from 60 Hz powerline harmonics, wire resonances (600 Hz), mount resonances, etc
expected signal 10-10 radians phase shift
demonstration experiment
15-Jan-02 Stanford Colloquium 22
Interferometer Data40 m prototype
Real interferometer data is UGLY!!!(Gliches - known and unknown)
LOCKING
RINGING
NORMAL
ROCKING
15-Jan-02 Stanford Colloquium 23
The Problem
How much does real data degrade complicate the data analysis and degrade the sensitivity ??
Test with real data by setting an upper limit on galactic neutron star inspiral rate using 40 m data
15-Jan-02 Stanford Colloquium 24
“Clean up” data stream
Effect of removing sinusoidal artifacts using multi-taper methods
Non stationary noise Non gaussian tails
15-Jan-02 Stanford Colloquium 25
Inspiral ‘Chirp’ Signal
Template Waveforms
“matched filtering”687 filters
44.8 hrs of data39.9 hrs arms locked25.0 hrs good data
sensitivity to our galaxyh ~ 3.5 10-19 mHz-1/2
expected rate ~10-6/yr
15-Jan-02 Stanford Colloquium 26
Optimal Signal DetectionWant to “lock-on” to one of a set of known signals
Requires:• source modeling• efficient algorithm• many computers
15-Jan-02 Stanford Colloquium 27
Detection Efficiency
• Simulated inspiral events provide end to end test of analysis and simulation code for reconstruction efficiency
• Errors in distance measurements from presence of noise are consistent with SNR fluctuations
15-Jan-02 Stanford Colloquium 28
Results from 40m Prototype
Loudest event usedto set upper-limit onrate in our Galaxy:
R90% < 0.5 / hour
15-Jan-02 Stanford Colloquium 29
Setting a limit
Upper limit on event rate can be determined from SNR of ‘loudest’ event
Limit on rate:R < 0.5/hour with 90% CL = 0.33 = detection efficiency
An ideal detector would set a limit:R < 0.16/hour
15-Jan-02 Stanford Colloquium 30
LIGOastrophysical sources
LIGO I (2002-2005)
LIGO II (2007- )
Advanced LIGO
15-Jan-02 Stanford Colloquium 31
Interferometersinternational network
LIGO
Simultaneously detect signal (within msec)
detection confidence locate the sources
decompose the polarization of gravitational waves
GEO VirgoTAMA
AIGO
15-Jan-02 Stanford Colloquium 32
Astrophysical Signaturesdata analysis
Compact binary inspiral: “chirps”» NS-NS waveforms are well described» BH-BH need better waveforms » search technique: matched templates
Supernovae / GRBs: “bursts” » burst signals in coincidence with signals in electromagnetic radiation » prompt alarm (~ one hour) with neutrino detectors
Pulsars in our galaxy: “periodic”» search for observed neutron stars (frequency, doppler shift)» all sky search (computing challenge)» r-modes
Cosmological Signals “stochastic background”
15-Jan-02 Stanford Colloquium 33
“Chirp Signal”binary inspiral
•distance from the earth r•masses of the two bodies•orbital eccentricity e and orbital inclination i
determine
15-Jan-02 Stanford Colloquium 34
Binary Inspiralssignatures and sensitivity
Compact binary mergers
LIGO sensitivity to coalescing binaries
15-Jan-02 Stanford Colloquium 35
Signals in Coincidence
Hanford Observatory
LivingstonObservatory
15-Jan-02 Stanford Colloquium 36
Detection Strategycoincidences
Two Sites - Three Interferometers» Single Interferometer non-gaussian level ~50/hr
» Hanford (Doubles) correlated rate (x1000) ~1/day
» Hanford + Livingston uncorrelated (x5000) <0.1/yr
Data Recording (time series)» gravitational wave signal (0.2 MB/sec)
» total data (16 MB/s)
» on-line filters, diagnostics, data compression
» off line data analysis, archive etc
Signal Extraction» signal from noise (vetoes, noise analysis)
» templates, wavelets, etc
15-Jan-02 Stanford Colloquium 37
gravitational waves
’s
light
“Burst Signal”supernova
15-Jan-02 Stanford Colloquium 38
Supernovaegravitational waves
Non axisymmetric collapse ‘burst’ signal
Rate1/50 yr - our galaxy3/yr - Virgo cluster
15-Jan-02 Stanford Colloquium 39
pulsar proper motions
Velocities - young SNR(pulsars?) > 500 km/sec
Burrows et al
recoil velocity of matter and neutrinos
Supernovaeasymmetric collapse?
15-Jan-02 Stanford Colloquium 40
Supernovaesignatures and sensitivity
15-Jan-02 Stanford Colloquium 41
“Periodic Signals”pulsars sensitivity
Pulsars in our galaxy»non axisymmetric: 10-4 < < 10-6»science: neutron star precession; interiors»narrow band searches best
15-Jan-02 Stanford Colloquium 42
“Stochastic Background”cosmological signals
‘Murmurs’ from the Big Bangsignals from the early universe
Cosmic microwave background
15-Jan-02 Stanford Colloquium 43
LIGO Sites
Hanford Observatory
LivingstonObservatory
15-Jan-02 Stanford Colloquium 44
LIGO Livingston Observatory
15-Jan-02 Stanford Colloquium 45
LIGO Hanford Observatory
15-Jan-02 Stanford Colloquium 46
LIGO Plansschedule
1996 Construction Underway (mostly civil)
1997 Facility Construction (vacuum system)
1998 Interferometer Construction (complete facilities)
1999 Construction Complete (interferometers in vacuum)
2000 Detector Installation (commissioning subsystems)
2001 Commission Interferometers (first coincidences)
2002 Sensitivity studies (initiate LIGOI Science Run)
2003+ LIGO I data run (one year integrated data at h ~ 10-21)
2006 Begin LIGO II installation
15-Jan-02 Stanford Colloquium 47
LIGO Facilitiesbeam tube enclosure
• minimal enclosure
• reinforced concrete
• no services
15-Jan-02 Stanford Colloquium 48
LIGObeam tube
LIGO beam tube under construction in January 1998
65 ft spiral welded sections
girth welded in portable clean room in the field
1.2 m diameter - 3mm stainless50 km of weld
NO LEAKS !!
15-Jan-02 Stanford Colloquium 49
LIGO I the noise floor
Interferometry is limited by three fundamental noise sources
seismic noise at the lowest frequencies thermal noise at intermediate frequencies shot noise at high frequencies
Many other noise sources lurk underneath and must be controlled as the instrument is improved
15-Jan-02 Stanford Colloquium 50
Beam Tube bakeout
• I = 2000 amps for ~ 1 week
• no leaks !!
• final vacuum at level where not limiting noise, even for future detectors
15-Jan-02 Stanford Colloquium 51
LIGOvacuum equipment
15-Jan-02 Stanford Colloquium 52
Vacuum Chambersvibration isolation systems
» Reduce in-band seismic motion by 4 - 6 orders of magnitude» Compensate for microseism at 0.15 Hz by a factor of ten» Compensate (partially) for Earth tides
15-Jan-02 Stanford Colloquium 53
Seismic Isolation springs and masses
damped springcross section
15-Jan-02 Stanford Colloquium 54
Seismic Isolationsuspension system
• support structure is welded tubular stainless steel • suspension wire is 0.31 mm diameter steel music wire
• fundamental violin mode frequency of 340 Hz
suspension assembly for a core optic
15-Jan-02 Stanford Colloquium 55
Thermal Noise ~ kBT/mode
Strategy: Compress energy into narrow resonance outside band of interest require high mechanical Q, low friction
15-Jan-02 Stanford Colloquium 56
LIGO Noise Curvesmodeled
wire resonances
15-Jan-02 Stanford Colloquium 57
Core Opticsfused silica
Caltech data CSIRO data
Surface uniformity < 1 nm rms Scatter < 50 ppm Absorption < 2 ppm ROC matched < 3% Internal mode Q’s > 2 x 106
15-Jan-02 Stanford Colloquium 58
Core Optics installation and alignment
15-Jan-02 Stanford Colloquium 59
ITMx Internal Mode Ringdowns
14.3737 kHz; Q = 1.2e+79.675 kHz; Q ~ 6e+5
15-Jan-02 Stanford Colloquium 60
LIGO laser
Nd:YAG
1.064 m
Output power > 8W in TEM00 mode
15-Jan-02 Stanford Colloquium 61
Commissioning configurations
Mode cleaner and Pre-Stabilized Laser 2km one-arm cavity short Michelson interferometer studies
Lock entire Michelson Fabry-Perot interferometer
“First Lock”
15-Jan-02 Stanford Colloquium 62
Why is Locking Difficult?
One meter, about 40 inches
Human hair, about 100 microns000,10
Wavelength of light, about 1 micron100
LIGO sensitivity, 10-18 meter000,1
Nuclear diameter, 10-15 meter000,100
Atomic diameter, 10-10 meter000,10
Earthtides, about 100 microns
Microseismic motion, about 1 micron
Precision required to lock, about 10-10 meter
15-Jan-02 Stanford Colloquium 63
Laserstabilization
IO
10-WattLaser
PSL Interferometer
15m4 km
Tidal Wideband
Deliver pre-stabilized laser light to the 15-m mode cleaner• Frequency fluctuations• In-band power fluctuations• Power fluctuations at 25 MHz
Provide actuator inputs for further stabilization• Wideband
• Tidal
10-1 Hz/Hz1/2 10-4 Hz/ Hz1/2 10-7 Hz/ Hz1/2
15-Jan-02 Stanford Colloquium 64
Prestabalized Laser performance
> 18,000 hours continuous operation
Frequency and lock very robust
TEM00 power > 8 watts
Non-TEM00 power < 10%
15-Jan-02 Stanford Colloquium 65
LIGO“first lock”
signal
LaserX Arm
Y Arm
Composite Video
15-Jan-02 Stanford Colloquium 66
Watching the Interferometer Lock
signal
X Arm
Y Arm
Laser
X arm
Anti-symmetricport
Y arm
Reflected light
2 min
15-Jan-02 Stanford Colloquium 67
Lock Acquisition
15-Jan-02 Stanford Colloquium 68
2km Fabry-Perot cavity 15 minute locked stretch
15-Jan-02 Stanford Colloquium 69
An earthquake occurred, starting at UTC 17:38.
The plot shows the band limited rms output in counts over the 0.1- 0.3Hz band for four seismometer channels. We turned off lock acquisition and are waiting for the ground motion to calm down.
From electronic logbook 2-Jan-02
Engineering Rundetecting earthquakes
15-Jan-02 Stanford Colloquium 70
17:03:03
01/02/2002
=========================================================================
Seismo-Watch
Earthquake Alert Bulletin No. 02-64441
=========================================================================
Preliminary data indicates a significant earthquake has occurred:
Regional Location: VANUATU ISLANDS
Magnitude: 7.3M
Greenwich Mean Date: 2002/01/02
Greenwich Mean Time: 17:22:50
Latitude: 17.78S
Longitude: 167.83E
Focal depth: 33.0km
Analysis Quality: A
Source: National Earthquake Information Center (USGS-NEIC)
Seismo-Watch, Your Source for Earthquake News and Information.
Visit http://www.seismo-watch.com
=========================================================================
All data are preliminary and subject to change.
Analysis Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration),
=========================================================================
15-Jan-02 Stanford Colloquium 71
Detecting the Earth Tides Sun and Moon
15-Jan-02 Stanford Colloquium 72
LIGOconclusions
15-Jan-02 Stanford Colloquium 73
Noise Spectrum: 2K Recycled
Factor of 200 improvement(over E2 spectrum)
Recycling Reduction of electronics
noise Partial implementation of
alignment control
15-Jan-02 Stanford Colloquium 74
Initial LIGO Sensitivity
Frequency noise Improve PSL Table layout
(done) Tailor MC loop (done) Implement common-mode
feedback from arms
Electronics noise Non-linearities? Filters? Alignment? Others?
15-Jan-02 Stanford Colloquium 75
TAMAperformance
15-Jan-02 Stanford Colloquium 76
LIGOconclusions
LIGO construction complete
LIGO commissioning and testing ‘on track’
“First Lock” officially established 20 Oct 00
Engineering test runs begin now, during period when emphasis is
on commissioning, detector sensitivity and reliability
First Science Run will begin during 2003
Significant improvements in sensitivity anticipated to begin about
2006
15-Jan-02 Stanford Colloquium 77
15-Jan-02 Stanford Colloquium 78
TAMA1000 hour run 86% duty cycle
15-Jan-02 Stanford Colloquium 79
TAMAconclusions
15-Jan-02 Stanford Colloquium 80
TAMAinterferometer stability
• Signal to Noise Ratio
• Binary Inspirals at 10 Kpc
15-Jan-02 Stanford Colloquium 81
How LIGO Works LIGO is an interferometric detector
» A laser is used to measure the relative lengths of two orthogonal cavities (or arms)
As a wave passes, the arm lengths change
in different ways….
…causing the interference pattern
to change at the photodiode
• Arms in LIGO are 4km» Current technology then allows one to
measure h = L/L ~ 10-21 which turns out to be an interesting target
15-Jan-02 Stanford Colloquium 82
Compact binary inspiral
Neutron star binaries» Equation of state» Size of stars?» Thro tidal disruption
Black hole binaries» Spins» Only way to see them
15-Jan-02 Stanford Colloquium 83
Spinning neutron stars
Isolated neutron stars with deformed crust
Newborn neutron stars with r-modes
X-ray binaries may be limited by gravitational waves
15-Jan-02 Stanford Colloquium 84
Short duration bursts
Supernova hangup Core collapse Other routes
BBH merger phase» Short duration, high SNR
15-Jan-02 Stanford Colloquium 85
Physical Effects of the Waves As gravitational waves pass, they change the distance between
neighboring bodies
• Fractional change in distance is the strain given by
h = L / L
15-Jan-02 Stanford Colloquium 86
Configuration of LIGO Observatories
2-km & 4-km laser interferometers @ Hanford
Single 4-km laser interferometer @ Livingston
15-Jan-02 Stanford Colloquium 87
Energy Loss Caused By Gravitational Radiation Confirmed
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.
15-Jan-02 Stanford Colloquium 88
Spacetime is Stiff!
=> Wave can carry huge energy with miniscule amplitude!
h ~ (G/c4) (ENS/r)
15-Jan-02 Stanford Colloquium 89
Interferometer Control System
•Multiple Input / Multiple Output
•Three tightly coupled cavities
•Ill-conditioned (off-diagonal) plant matrix
•Highly nonlinear response over most of phase space
•Transition to stable, linear regime takes plant through singularity
•Requires adaptive control system that evaluates plant evolution and reconfigures feedback paths and gains during lock acquisition
•But it works!
15-Jan-02 Stanford Colloquium 90
Digital Interferometer Sensing & Control System
15-Jan-02 Stanford Colloquium 91
When Will It Work?Status of LIGO in Spring 2001
Initial detectors are being commissioned, with first Science Runs commencing in 2002.
Advanced detector R&D underway, planning for upgrade near end of 2006» Active seismic isolation systems» Single-crystal sapphire mirrors» 1 megawatt of laser power circulating in arms» Tunable frequency response at the quantum limit
Quantum Non Demolition / Cryogenic detectors in future? Laser Interferometer Space Antenna (LISA) in planning and
design stage (2015 launch?)