Gravitational Wave Observatories Gravitational Wave Observatories

By: Matthew FournierBy: Matthew Fournier

OutlineOutline

Intro to Gravitational WavesIntro to Gravitational WavesWhat are theyWhat are theyWhere do they come fromWhere do they come fromWhy are they interestingWhy are they interesting

Detection TechniquesDetection TechniquesTechniques UsedTechniques UsedPast, present and future detection attemptsPast, present and future detection attempts

Outline

• Problems with detection– Noise sources– Solutions

Introduction To Gravitational Waves

Gravitational WavesGravitational Waves

A consequence of General RelativityA consequence of General RelativityEinstein’s equation admits wave solutionsEinstein’s equation admits wave solutionsMoving masses create a propagating Moving masses create a propagating

disturbance in the space-time fabricdisturbance in the space-time fabricThis disturbance stretches space on one This disturbance stretches space on one

axis, squeezes it on the otheraxis, squeezes it on the otherThis distortion does not move other This distortion does not move other

masses, but changes the distance in masses, but changes the distance in between thembetween them

Mysterious, Enigmatic . . . Ripples Mysterious, Enigmatic . . . Ripples in Space-Timein Space-Time

Up until now, these waves have escaped direct Up until now, these waves have escaped direct detectiondetection

They can carry an enormous amount of energy, They can carry an enormous amount of energy, but do not couple strongly with matterbut do not couple strongly with matter

Theory predicts that a typical source would only Theory predicts that a typical source would only displace masses by about 10E(-20) m displace masses by about 10E(-20) m

This makes them interesting because of the This makes them interesting because of the information they could carryinformation they could carry

Evidence existsEvidence exists

Sources of Gravitational Waves

• Come from lots of places:

– Binary Stars– Super Massive Black

Holes– Supernovae– Inflation

Detection

DetectionDetection

Very challenging problem, to say the leastVery challenging problem, to say the least No exaggeration in saying that this is one No exaggeration in saying that this is one

of the most challenging problems in of the most challenging problems in physics todayphysics today

Two Main Techniques: Two Main Techniques:

• Resonant BarResonant Bar• Laser InterferometerLaser Interferometer

There are some other interesting methodsThere are some other interesting methods

Resonant Mass Bar DetectorsResonant Mass Bar Detectors Efforts Started with Joseph Weber, resonant mass Efforts Started with Joseph Weber, resonant mass

detectors in the 1960sdetectors in the 1960s These are a large bars of metal, cooled to reduce These are a large bars of metal, cooled to reduce

thermal noise, with stress sensors attachedthermal noise, with stress sensors attached The bars are designed so that their resonant frequency The bars are designed so that their resonant frequency

is in the range of the gravitational waves that are being is in the range of the gravitational waves that are being targetedtargeted

The tidal force, or displacement, caused by a periodic The tidal force, or displacement, caused by a periodic gravitational wave should resonate with the bar, causing gravitational wave should resonate with the bar, causing a vibration that should be picked up by the stress a vibration that should be picked up by the stress sensorssensors

Resonant Mass detectors aren’t working, only sensitive Resonant Mass detectors aren’t working, only sensitive to around one part in 10E(-18)to around one part in 10E(-18)

Interferometers

• Nowadays, huge interferometers are most promising

• Easy to realize how these things work: a gravitational wave changes the length of the arms, and interferometers are really good at detecting this sort of change, as you all know

InterferometersInterferometers

Several projects throughout the world, Several projects throughout the world, includingincludingLIGOLIGOVIRGOVIRGOGEO 600GEO 600TAMA 300TAMA 300AIGOAIGO

Space missions are in the works, LISASpace missions are in the works, LISA

Problems With Detection

There are some big onesThere are some big ones

Main problem in detection is the nature of Main problem in detection is the nature of gravitational waves: they produce tiny gravitational waves: they produce tiny displacements. displacements.

This is why bar detectors aren’t working so This is why bar detectors aren’t working so wellwell

I’ll concentrate on interferometersI’ll concentrate on interferometers

Noise in InterferometersNoise in Interferometers

Two Main types of noise:Two Main types of noise:

Displacement: unwanted disturbances causing actual Displacement: unwanted disturbances causing actual displacement of test massesdisplacement of test masses

Sensing: Noise in sensing apparatus, electronics, etc, Sensing: Noise in sensing apparatus, electronics, etc, like shot noise that was seen in another presentationlike shot noise that was seen in another presentation

I’ll concentrate on displacement noise due to I’ll concentrate on displacement noise due to time constraintstime constraints

Displacement NoiseDisplacement Noise

Sources of noise include everything, but in Sources of noise include everything, but in particular: particular:

Seismic NoiseSeismic Noise Machinery and AutomobilesMachinery and Automobiles Laser NoiseLaser Noise Thermal Noise and resonance problemsThermal Noise and resonance problems Rain and windRain and wind Tidal ForcesTidal Forces Mirror surface chargeMirror surface charge

Solutions? Or is it Hopeless?

• I think it’s hopeless.

• Put it in Space!

• More solutions . . .

SolutionsSolutions

Isolation stacks:Isolation stacks: Reduce seismic, industrial, and tidal noiseReduce seismic, industrial, and tidal noise Mirror is hung on a pendulum inside an isolation stackMirror is hung on a pendulum inside an isolation stack Isolation stacks are a large stack of masses on Isolation stacks are a large stack of masses on

springs, each with a different resonant frequency springs, each with a different resonant frequency The different masses absorb a different frequency The different masses absorb a different frequency

range of noiserange of noise Cuts Noise by a factor of 100 in the pendulum and a Cuts Noise by a factor of 100 in the pendulum and a

million in the stacksmillion in the stacks

SolutionsSolutions

Mirror Heating:Mirror Heating:The laser beam can heat the mirror to a point The laser beam can heat the mirror to a point

where it warps and does not reflect the laser where it warps and does not reflect the laser light evenlylight evenly

This causes unwanted effectsThis causes unwanted effectsOne solution is to heat the outside of the One solution is to heat the outside of the

mirror very slightly, so that it is all at the same mirror very slightly, so that it is all at the same temperature and does not warptemperature and does not warp

SolutionsSolutions Mirror Surface Charge Mirror Surface Charge

The mirrors in the interferometer can pick up a The mirrors in the interferometer can pick up a surface charge by interacting with the laser.surface charge by interacting with the laser.

Originates from Compton Effect, Photoelectric Effect, Originates from Compton Effect, Photoelectric Effect, and ionizationand ionization

This is a problem because it introduces uncontrolled This is a problem because it introduces uncontrolled electric fields into the system. electric fields into the system.

These fields vary uncontrollably due to thermally These fields vary uncontrollably due to thermally driven mechanismsdriven mechanisms

The effect is tiny, but increases with time as the The effect is tiny, but increases with time as the charge builds upcharge builds up

The solution is simple: coat the mirror with a The solution is simple: coat the mirror with a conducting, transparent thin filmconducting, transparent thin film

SolutionsSolutions

Laser Noise:Laser Noise: No laser is perfect, and small fluctuations in the No laser is perfect, and small fluctuations in the

number of photons being emitted can disturb the number of photons being emitted can disturb the mirrormirror

This is because photons carry momentum, and can This is because photons carry momentum, and can kick the mirror aroundkick the mirror around

One could reduce this problem by turning down the One could reduce this problem by turning down the intensity of the laser, but this makes detection more intensity of the laser, but this makes detection more difficultdifficult

Must tune laser as well as possible, and strike Must tune laser as well as possible, and strike balance between noise and detectionbalance between noise and detection

SolutionsSolutions

There are some sources of noise which There are some sources of noise which need to be actively damped, such as lunar need to be actively damped, such as lunar tidestides

This is achieved by a system of This is achieved by a system of electromagnets attached to each mirror, electromagnets attached to each mirror, that provides a force to cancel out an that provides a force to cancel out an unwanted periodic displacementunwanted periodic displacement

References• http://www.ligo.org/• Scientific American• http://hubblesite.org• Serway, Moses, Moyer “Modern

Physics”• http://www.astrophysicsspectator.com• http://archive.ncsa.uiuc.edu/Cyberia/N

umRel/

• Braginsky et al: “Notes about Noise in Gravitational Wave Antennae Created by Cosmic Rays”

• R. Weiss “Note on Electrostatics in the LIGO suspensions”