230 likes | 345 Views
Gravitational Wave Observatories By: Matthew Fournier. 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. Outline. Problems with detection Noise sources
E N D
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
Outline • Problems with detection • Noise sources • Solutions
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
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
Sources of Gravitational Waves • Come from lots of places: • Binary Stars • Super Massive Black Holes • Supernovae • Inflation
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
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)
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
Interferometers • Several projects throughout the world, including • LIGO • VIRGO • GEO 600 • TAMA 300 • AIGO • Space missions are in the works, LISA
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
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
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
Solutions? Or is it Hopeless? • I think it’s hopeless. • Put it in Space! • More solutions . . .
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
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
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
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
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
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”