Gravitational Wave Observatories By: Matthew Fournier

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 • Einstein’s equation admits wave solutions • Moving masses create a propagating disturbance in the space-time fabric • 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

6. Mysterious, Enigmatic . . . Ripples in Space-Time • Up until now, these waves have escaped direct detection • 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 • This makes them interesting because of the information they could carry • Evidence exists

7. Sources of Gravitational Waves • Come from lots of places: • Binary Stars • Super Massive Black Holes • Supernovae • 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 Bar • Laser Interferometer • There are some other interesting methods

10. Resonant Mass Bar Detectors • 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 • 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 • 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 • LIGO • VIRGO • GEO 600 • TAMA 300 • AIGO • 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. • This is why bar detectors aren’t working so well • I’ll concentrate on interferometers

15. Noise in Interferometers Two Main types of noise: • 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 • I’ll concentrate on displacement noise due to time constraints

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

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

18. Solutions • Isolation stacks: • Reduce seismic, industrial, and tidal noise • 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 • 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

19. Solutions • Mirror Heating: • 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 • 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 • The mirrors in the interferometer can pick up a surface charge by interacting with the laser. • Originates from Compton Effect, Photoelectric Effect, and ionization • This is a problem because it introduces uncontrolled electric fields into the system. • These fields vary uncontrollably due to thermally driven mechanisms • 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

21. Solutions • Laser Noise: • 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 • 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

22. Solutions • 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

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/NumRel/ • Braginsky et al: “Notes about Noise in Gravitational Wave Antennae Created by Cosmic Rays” • R. Weiss “Note on Electrostatics in the LIGO suspensions”