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Keith Riles U. Michigan Physics Dept.

Looking for Ripples in the Fabric of Space-Time: LIGO- the L aser I nterferometer G ravitational-Wave O bservatory. Keith Riles U. Michigan Physics Dept. Presentation supported by the Michigan Space Grant Consortium and Jackson Community College. Jackson Science Café

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Keith Riles U. Michigan Physics Dept.

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  1. Looking for Ripples in the Fabric of Space-Time: LIGO- the Laser Interferometer Gravitational-Wave Observatory Keith Riles U. Michigan Physics Dept. Presentation supported by the Michigan Space Grant Consortium and Jackson Community College Jackson Science Café Hudson Grill – December 8, 2011

  2. Courtesy: Prof. Andrew Davidhazy - Rochester Institute of Technology What are Gravitational Waves? Recall that a wave is a disturbance that propagates: What exactly is disturbed by a gravitational wave? Space itself! Before we discuss wave propagation, we need to understand how space can be disturbed by anything!

  3. Space deformation But the essence of these equations is summed up nicely by general relativity pioneer John Wheeler: “Matter tells spacetime how to curve, and curved space tells matter how to move" Einstein’s Field Equation: Gμν = 8 π Tμν Mathematically, this equation represents ten different coupled, partial differential equations relating the curvature of space (embodied in Gμν) to energy and momentum (embodied inTμν) Except for handful of highly symmetric problems (e.g., spheres!), these equations must be solved numerically, not algebraically And numerical solutions are tough too!

  4. Courtesy: LIGO Laboratory Space deformation “Rubber-sheet model” of space: Heavy mass warps the “surface”, creating a depression Another mass rolls toward the depression And deepens it!  Highly “non-linear” equations

  5. Making waves Now imagine two very compact stars (neutron stars or black holes) in a tight binary orbiting system: Courtesy Jet Propulsion Laboratory Space is “swirled” by the orbiting stars, creating a ripple that propagates to distant regions of the universe (to us we hope!)

  6. Observed 17-Hz pulsar PSR 1913+16 · Unseen companion (neutron star) · Making waves Are there really such star systems out there? YES! In 1974 Joseph Taylor and Russell Hulse discovered a binary system with two neutron stars, one of which is a pulsar Orbital Period (“year”) is 7.75 hours

  7. Making waves The Taylor-Hulse binary system’s orbit shrinks about 3 mm every revolution Present orbital extent Orbit slowly shrinking  Coalescence in about 300 million years So why is the orbit decaying? Weisberg et al, 1981 Gravitational wave emission  Energy loss!

  8. Making waves Why are we so sure we understand the decay of this binary system? By measuring the precise timing of pulsar signals, one can infer the present orbital parameters Einstein’s Theory of General Relativity then predictsthe rate of decay from gravitational radiation energy loss Did Einstein get it right?

  9. Making waves YES! Graph at right shows the change in orbital period (seconds) over 25 years of observations Smooth curve is absolute prediction from General Relativity (no free parameters!) Dots are measured data

  10. Making waves Can we detect this orbital system’s waves here on Earth? NO On Earth we can hope to see only waves with frequencies greater than ~10 Hz The characteristic frequency for this system is ~1/(4 hours) ~ 70 μHz

  11. Making waves Well, what if we waited around for 300 million years? What might we “see”? A Chirp! Graphs show waveform for 4 different 1-second intervals near the end of the inspiral, a.k.a., “death spiral” (in arbitrary but consistent units)

  12. Making waves Last nine seconds of inspiral

  13. ΔL L Nature of Gravitational Waves How is the strength of a gravitational wave described? By fractional change in distance, i.e., strain Denote time-dependent dimensionless strain displacement (tiny!) by h(t): ΔL(t) ~ h(t) x L

  14. Courtesy: Dr. Peter Shawhan - Caltech Gravitational Waves from “Out There” Imagine two neutron stars: • Each with mass equal to 1.4 solar masses • In circular orbit of radius 20 km (imminent coalescence) • Resulting orbital frequency is 400 Hz (!) • Resulting GW frequency is 800 Hz Einstein predicts:

  15. 1 or million x million x million x thousand or diameter of an atom distance from Earth to Sun How small is 10-21 ? 0.000000000000000000001  Experimental challenge!  Well suited to high-precision interferometry

  16. Courtesy: Prof. Andrew Davidhazy - Rochester Institute of Technology Interference Waves are characterized by their interference – with other waves or with themselves

  17. Interference Constructive Partial Destructive

  18. Audio Interference Demonstration With two audio speakers driven at same frequency (coherently), an interference pattern is set up in the room Speaker 1 Speaker 2 Try plugging one ear and moving the other ear to sample the minima and maxima of the pattern Courtesy: Wiley & Sons: Halliday/Resnick/Walker text

  19. Interference at detector depends on difference in lengths of two arms Gravitational waves cause length differences! ΔL(t) ~ h(t) x L Suspended Interferometers What is an interferometer? Michelson Interferometer  Demonstration Courtesy: Wikipedia

  20. LIGO Observatories Hanford Observation of nearly simultaneous signals 3000 km apart rules out terrestrial artifacts Livingston

  21. LIGO Detector Facilities • Stainless-steel tubes • (1.24 m diameter, ~10-8 torr) • Protected by concrete enclosure Vacuum System

  22. LIGO Detector Facilities LASER • Infrared (1064 nm, 10-W) Nd-YAG laser from Lightwave (now commercial product!) • Elaborate intensity & frequency stabilization system, including feedback from main interferometer Optics • High-quality fused silica (25-cm diameter) • Suspended by single steel wire • Actuation of alignment / position via magnets & coils

  23. LIGO Detector Facilities Seismic Isolation • Multi-stage (mass & springs) optical table support gives 106 suppression • Pendulum suspension gives additional 1 / f 2 suppression above ~1 Hz

  24. The road to design sensitivity at Hanford…

  25. Retrofit with active feed-forward isolation system (using technology developed for Advanced LIGO) • Fixed Solution: Harder road at Livingston… Livingston Observatory located in pine forest popular with pulp wood cutters Spiky noise (e.g. falling trees) in 1-3 Hz band creates dynamic range problem for arm cavity control  40% livetime

  26. achieved What Limits the Sensitivityof the 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 • Best design sensitivity: • ~ 3 x 10-23 Hz-1/2 @ 150 Hz < 2 x 10-23

  27. Status of the Search • No discoveries yet, but • Still examining data we have taken • Major upgrade of LIGO under way now  Advanced LIGO Will increase volume of “visible universe” by factor of 1000  Guaranteed discovery – but not until ~2015

  28. What will the sky look like?

  29. What will the sky look like?

  30. What will the sky look like?

  31. What will the sky look like?

  32. What will the sky look like?

  33. What will the sky look like? Stay Tuned!

  34. Courtesy: Prof. Andrew Davidhazy - Rochester Institute of Technology Waves Before looking at gravitational waves, let’s first review wave motion itself Loosely speaking, waves are oscillating disturbances that propagate:

  35. Examples of Waves • Easily visible: • Surface water waves • Plucked guitar string • Jerked rope • Harder to see, but intuitively plausible: • Seismic waves (ground vibration) • Sound waves (in air, liquid and solids) Courtesy: Wiley & Sons – Halliday/Resnick/Walker text • More abstract (but measurable!): • Electromagnetic waves = Light(oscillating electric & magnetic fields) • Gravitational waves(oscillation of space itself!)

  36. Wavelength, Frequency, etc. • Sinusoidal waves are characterized by well defined: • Wavelength – distance between adjacent “crests” in “snapshot”: • Period – Time between successive crests at fixed location • Frequency – Number of crests per second passing fixed location • Speed = Wavelength / Period ( = Wavelength × Frequency )

  37. Wavelength, Frequency, etc. Audible sound waves have frequencies in the range 20 Hz – 20,000 Hz - depending on the listener’s age - and cumulative exposure to heavy metal, rap, etc At room temperature, sound speed in air is about 340 m/s (~760 mph) Sound demonstrations: 440 Hz (middle A -- λ = 2.5 feet) 1,000 Hz ( λ = 13 inches ) 4,000 Hz ( λ = 3.3 inches ) 20,000 Hz ( λ = 0.7 inch )

  38. Electromagnetic Spectrum Courtesy: Wiley & Sons – Halliday/Resnick/Walker text Human Eye Sensitivity

  39. Piano middle A note (“440 Hz”) Pipe organ middle A note (“440 Hz”) Sample piano note spectrum Sample pipe organ note spectrum 440 Hz 440 Hz Wavelength, Frequency, etc. Most waves are NOT pure sinusoids! Human speech, automobile engines, bird chirps, even instrumental notes contain rich spectra of frequencies

  40. Sample 2 (middle-aged man) (6-year-old girls) Courtesy: Lizzie & Katie Riles Wavelength, Frequency, etc. Samples of human speech with spectra Sample 1

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