1. Gravitational Wave Observatories By: Matthew Fournier

2. Outline Intro to Gravitational Waves What are they Where do they come from Why are they interesting Detection Techniques Techniques Used Past, present and future detection attempts

3. Outline Problems with detection –Noise sources –Solutions

4. Introduction To Gravitational Waves

5. Gravitational Waves A consequence of General Relativity A consequence of General Relativity Einstein’s equation admits wave solutions Einstein’s equation admits wave solutions Moving masses create a propagating disturbance in the space-time fabric Moving masses create a propagating disturbance in the space-time fabric This disturbance stretches space on one axis, squeezes it on the other This disturbance stretches space on one axis, squeezes it on the other This distortion does not move other masses, but changes the distance in between them This distortion does not move other masses, but changes the distance in between them

6. Mysterious, Enigmatic... Ripples in Space-Time Up until now, these waves have escaped direct detection Up until now, these waves have escaped direct detection They can carry an enormous amount of energy, but do not couple strongly with matter They can carry an enormous amount of energy, but do not couple strongly with matter Theory predicts that a typical source would only displace masses by about 10E(-20) m Theory predicts that a typical source would only displace masses by about 10E(-20) m This makes them interesting because of the information they could carry This makes them interesting because of the information they could carry Evidence exists Evidence exists

7. Sources of Gravitational Waves Come from lots of places: –B–Binary Stars –S–Super Massive Black Holes –S–Supernovae –I–Inflation

8. Detection

9. Detection  Very challenging problem, to say the least  No exaggeration in saying that this is one of the most challenging problems in physics today  Two Main Techniques: Resonant BarResonant Bar Laser InterferometerLaser Interferometer  There are some other interesting methods

10. Resonant Mass Bar Detectors Efforts Started with Joseph Weber, resonant mass detectors in the 1960s Efforts Started with Joseph Weber, resonant mass detectors in the 1960s These are a large bars of metal, cooled to reduce thermal noise, with stress sensors attached These are a large bars of metal, cooled to reduce thermal noise, with stress sensors attached The bars are designed so that their resonant frequency is in the range of the gravitational waves that are being targeted The bars are designed so that their resonant frequency is in the range of the gravitational waves that are being targeted The tidal force, or displacement, caused by a periodic gravitational wave should resonate with the bar, causing a vibration that should be picked up by the stress sensors The tidal force, or displacement, caused by a periodic gravitational wave should resonate with the bar, causing a vibration that should be picked up by the stress sensors Resonant Mass detectors aren’t working, only sensitive to around one part in 10E(-18) Resonant Mass detectors aren’t working, only sensitive to around one part in 10E(-18)

11. 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

12. Interferometers Several projects throughout the world, including Several projects throughout the world, including LIGO LIGO VIRGO VIRGO GEO 600 GEO 600 TAMA 300 TAMA 300 AIGO AIGO Space missions are in the works, LISA Space missions are in the works, LISA

13. Problems With Detection

14. There are some big ones Main problem in detection is the nature of gravitational waves: they produce tiny displacements. Main problem in detection is the nature of gravitational waves: they produce tiny displacements. This is why bar detectors aren’t working so well This is why bar detectors aren’t working so well I’ll concentrate on interferometers I’ll concentrate on interferometers

15. Noise in Interferometers Two Main types of noise: Displacement: unwanted disturbances causing actual displacement of test masses Displacement: unwanted disturbances causing actual displacement of test masses Sensing: Noise in sensing apparatus, electronics, etc, like shot noise that was seen in another presentation Sensing: Noise in sensing apparatus, electronics, etc, like shot noise that was seen in another presentation I’ll concentrate on displacement noise due to time constraints I’ll concentrate on displacement noise due to time constraints

16. Displacement Noise Sources of noise include everything, but in particular: Sources of noise include everything, but in particular: Seismic Noise Seismic Noise Machinery and Automobiles Machinery and Automobiles Laser Noise Laser Noise Thermal Noise and resonance problems Thermal Noise and resonance problems Rain and wind Rain and wind Tidal Forces Tidal Forces Mirror surface charge Mirror surface charge

17. Solutions? Or is it Hopeless? I think it’s hopeless. Put it in Space! More solutions...

18. Solutions Isolation stacks: Isolation stacks: Reduce seismic, industrial, and tidal noise Reduce seismic, industrial, and tidal noise Mirror is hung on a pendulum inside an isolation stack Mirror is hung on a pendulum inside an isolation stack Isolation stacks are a large stack of masses on springs, each with a different resonant frequency Isolation stacks are a large stack of masses on springs, each with a different resonant frequency The different masses absorb a different frequency range of noise The different masses absorb a different frequency range of noise Cuts Noise by a factor of 100 in the pendulum and a million in the stacks Cuts Noise by a factor of 100 in the pendulum and a million in the stacks

19. Solutions Mirror Heating: Mirror Heating: The laser beam can heat the mirror to a point where it warps and does not reflect the laser light evenly The laser beam can heat the mirror to a point where it warps and does not reflect the laser light evenly This causes unwanted effects This causes unwanted effects One solution is to heat the outside of the mirror very slightly, so that it is all at the same temperature and does not warp One solution is to heat the outside of the mirror very slightly, so that it is all at the same temperature and does not warp

20. Solutions Mirror Surface Charge Mirror Surface Charge The mirrors in the interferometer can pick up a surface charge by interacting with the laser. The mirrors in the interferometer can pick up a surface charge by interacting with the laser. Originates from Compton Effect, Photoelectric Effect, and ionization Originates from Compton Effect, Photoelectric Effect, and ionization This is a problem because it introduces uncontrolled electric fields into the system. This is a problem because it introduces uncontrolled electric fields into the system. These fields vary uncontrollably due to thermally driven mechanisms These fields vary uncontrollably due to thermally driven mechanisms The effect is tiny, but increases with time as the charge builds up The effect is tiny, but increases with time as the charge builds up The solution is simple: coat the mirror with a conducting, transparent thin film The solution is simple: coat the mirror with a conducting, transparent thin film

21. Solutions Laser Noise: Laser Noise: No laser is perfect, and small fluctuations in the number of photons being emitted can disturb the mirror No laser is perfect, and small fluctuations in the number of photons being emitted can disturb the mirror This is because photons carry momentum, and can kick the mirror around This is because photons carry momentum, and can kick the mirror around One could reduce this problem by turning down the intensity of the laser, but this makes detection more difficult One could reduce this problem by turning down the intensity of the laser, but this makes detection more difficult Must tune laser as well as possible, and strike balance between noise and detection Must tune laser as well as possible, and strike balance between noise and detection

22. Solutions There are some sources of noise which need to be actively damped, such as lunar tides There are some sources of noise which need to be actively damped, such as lunar tides This is achieved by a system of electromagnets attached to each mirror, that provides a force to cancel out an unwanted periodic displacement This is achieved by a system of electromagnets attached to each mirror, that provides a force to cancel out an unwanted periodic displacement

23. 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/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”