Published on November 20, 2007
Ph237 - Gravitational WavesWeek 1: Overview: Ph237 - Gravitational Waves Week 1: Overview Kip S. Thorne, Caltech, 7 & 9 January 2001 Via video feed from Cambridge England Physical Nature of Gravitational Waves - 1: Physical Nature of Gravitational Waves - 1 Waves push freely floating objects apart and together Local inertial frames do not mesh Like non-meshing of Cartesian coordinates on Earth’s surface Earth’s curvature causes non-meshing Spacetime curvature causes inertial-frame non-meshing Gravitational waves are ripples of spacetime curvature Physical Nature of Gravitational Waves - 2 : Physical Nature of Gravitational Waves - 2 Great richness to a wave’s spacetime curvature: Heuristically: Stretch and squeeze of space Slowing and speeding of rate of flow of time … Measure stretch and squeeze with light beams Does light wavelength get stretched and squeezed the same as mirror separation, so no effect is seen? NO! Spacetime curvature influences light differently from mirror separations. Mathematically: Curvature described by rank-4 Riemann tensor, Rabgd Physical Nature of Gravitational Waves - 3: Physical Nature of Gravitational Waves - 3 Stretch and squeeze are: transverse to direction of propagation Equal and opposite along orthogonal axes (trace-free) Force pattern invariant under 180o rotation Contrast with EM waves: invariant under 360o rot’n (Spin of quantum) = (360 degrees) / (invariance angle) = 1 for photon, 2 for graviton Irreducible representation of Little Subgroup of Lorentz grp Two polarizations: axes rotated 90o EM 45o GW plus cross E E Physical Nature of Gravitational Waves - 4: Physical Nature of Gravitational Waves - 4 Each polarization has its own gravitational-wave field These fields’ evolutions h+(t) & hx(t) are the waveforms Waveforms carry detailed Information about source DL / L = hx Double time integral of certain components of Riemann tensor Propagation of Gravitational Waves: Propagation of Gravitational Waves High-frequency waves (wavelength l << radius of curvature R of background spacetime; geometric optics): propagate at light speed => graviton has rest mass zero (like photon) Redshifted and grav’ly lensed, like light If l ~ R, scattered by spacetime curvature Absorption by matter in our universe: Negligible … even back to big bang Dispersion due to interaction with matter: Negligible Example: Universe filled with neutron stars or black holes: In propagating around the universe once: Dispersion delays the GW by about one wavelength l The Gravitational Wave Spectrum: The Gravitational Wave Spectrum Spectrum of known and expected sources extends over 22 decades of frequency Promising sensitivities are being achieved in four frequency bands Some Sources in our Four Bands:: Some Sources in our Four Bands: HF LIGO LF Doppler LISA VLF Pulsar Timing ELF CMB Anisotropy The Big Bang Singularity in which the Universe was born, Inflation of Universe Exotic Physics in Very Early Universe: Phase transitions, cosmic strings, domain walls, mesoscopic excitations, … ? Massive BH’s (300 to 30 million suns), Binary stars Soliton stars? Naked singularities? Small BH’s (2 to 1000 suns), Neutron stars Supernovae Boson stars? Naked singularities? Caltech Faculty Involved in GW Research: Caltech Faculty Involved in GW Research LIGO (high frequencies, ~10 Hz to ~1000 Hz): Barish, Drever, Libbrecht, Weinstein, Kip LISA (low frequencies, ~ 10-4 Hz to ~ 0.1 Hz): Prince, Phinney, Kip. + heavy JPL involvement Doppler tracking (very low frequencies) Kulkarni Cosmic microwave polarization anisotropy Kamionkowski, Lange, Readhead CaJAGWR: Caltech/JPL Association for Gravitational Wave Research Seminars ~ every other Friday [alternate with LIGO seminars] http://www.cco.caltech.edu/~cajagwr/ Links to LIGO, LISA, and other GW sites Multipolar Decomposition of Waves: Multipolar Decomposition of Waves Expand h in multipole moments of source’s mass and mass-current (momentum) distributions: M0, M1, M2, …; S1, S2, … h is dimensionless; must fall off as 1/r => h ~ (G/c2)(M0/r) & (G/c3)(M1/r) & (G/c4)(M2/r) & … & (G/c4)(S1/r) & (G/c5)(S2/r) & … Theorem in canonical field theory: ( Waves’ multipole order ) (spin of quantum) = 2 for graviton . Mass can’t oscillate Momentum can’t oscillate Mass quadrupole Moment dominates Angular Momentum can’t oscillate Current quadrupole r Strengths of Waves: Strengths of Waves Source: mass M, size L, oscillatory period P, quadrupole moment M2 ~ M L2 Quadrupole moment approximation: h ~ (G/c4)(M2/r) ~ (G/c4)(M L2/P2) /r ~ (G/c4)(internal kinetic energy) / r ~ (1/c2) (Newton potential of [mass-equivalent] kinetic energy) ~ (1/c2) (Newton potential of [mass-equivalent] potential energy) Higher multipoles: down by (v/c) to some power Magnitude: Colliding BH’s or NS’s @ r ~ 100 Mpc ~ 3 x 108 ltyr ~ 3 x1027 cm [Mass-equivalent] Kinetic energy ~ Msun h ~ few x 10-22 International Network of Bar DetectorsNow in Operation [~1000 Hz]: International Network of Bar Detectors Now in Operation [~1000 Hz] U. Rome - Nautilus How a LIGO Interferometer Works: How a LIGO Interferometer Works Schematic description of detector: Fabry-Perot Cavity Fabry-Perot Cavity Beam Splitter Phase of excitation Cavity eigenfrequency - Laser eigenfrequency LIGO : LIGO Collaboration of ~350 scientists at ~30 institutions Hanford Washington LIGO: LIGO Livingston, Louisiana First searches for GW’s: 2002 to 2006 -- sensitivity where plausible to see waves Upgrade to advanced interferometers: ~2007; 3000 higher event rate new search: 2008 ... -- sensitivity where should see rich waves from wide variety of sources LIGO Organization: LIGO Organization LIGO Laboratory Responsible for Facilities; and for Design, Construction, & Operation of Interferometers Caltech & MIT; Director: Barry Barish [Caltech] LIGO Scientific Community (LSC) Formulates science goals Carries out Interferometer R&D ~350 scientists and engineers in ~25 institutions Caltech, California State University, Carleton, Cornell, FermiLab, U. Florida, Harvard, Iowa State, JILA (U. Colorado), LSU, Louisiana Tech, MIT, U. Michigan, U. Oregon, Penn State, Southern U., Stanford, Syracuse, U. Texas-Brownsville, U. Wisconsin-Milwaukee, ACIGA (Australia), GEO600 (Britain & France), IUCAA (India), NAOJ-TAMA (Japan), Moscow State U. & IAP-Nizhny Novgorod (Russia) Spokesman: Rai Weiss [MIT] International Network of Interferometric Detectors: International Network of Interferometric Detectors Network Required for: Detection Confidence Waveform Extraction Direction by Triangulation LIGO Hanford, WA LIGO Livingston, LA GEO600 Hanover Germany TAMA300 Tokyo VIRGO Pisa, Italy LIGO’s International Partners: LIGO’s International Partners LIGO: Initial Interferometers: Have been installed (Hanford 4km, 2km; Livingston 4 km) Are being debugged; first search underway (at poor sensitivity) LIGO: Initial Interferometers Square root of Spectral density of h(t) [“theory of random processes”] Seismic Isolation: Seismic Isolation Test-Mass Mirror and its Suspension: Test-Mass Mirror and its Suspension Mirror Installation and Alignment: Mirror Installation and Alignment Protection from Elements: Protection from Elements LIGO: From Initial Interferometers to AdvancedR&D underway; install in ~2007: LIGO: From Initial Interferometers to Advanced R&D underway; install in ~2007 Initial Interferometers Advanced Interferometers Reshape Noise Advanced IFOs: The Technical Challenge: Advanced IFOs: The Technical Challenge In advanced interferometers: Monitor motions of 40 kg saphire mirrors to: ~10-17 cm ~ 1/10,000 diameter of atomic nucleus ~10-13 of the wavelength of light ~ the half width of the mirror’s quantum wave function Quantum Nondemolition (QND) Technology Branch of quantum information science LISA: Laser Interferometer Space Antenna: LISA: Laser Interferometer Space Antenna Joint American/European US: Managed at GSFC (Md) Payload & Science: JPL/Caltech Tom Prince: Mission Scientist Launch: 2011 Three “drag-free” spacecraft 5 million km separations 1 Watt laser, 30cm diameter telescopes Relative motions of spacecraft: ~ 1 million wavelengths / sec Light beams beat against each other (heterodyne detection); beat signal fourier analyzed LISA: The Technical Challenge: LISA: The Technical Challenge Monitor the relative motion of the satellites’ “proof masses”, 5 million kilometers apart, to a precision ~ 10-9 cm [in frequency band f ~ 0.1 - 10-4 Hz ] ~ 10-5 of the wavelength of light accelerations ~ 10-16 g Guarantee that the only forces acting on the proof masses at this level are gravitational, from outside the spacecraft LISA Noise Curve: LISA Noise Curve Frequency, Hz Shot noise Random forces on proof masses White-dwarf binary Stochastic background Gravitational-Wave Data Analysis: Gravitational-Wave Data Analysis Matched filtering: If waveforms slip by ~ 1 radian, it is obvious in cross correlation LIGO: up to ~20,000 cycles (~100,000 radians) LISA: up to ~200,000 cycles (~1 million radians) Theoretical challenge: compute waveforms to this accuracy If waveforms poorly known: Must use other analysis methods: significant loss of signal strength! e.g. Flanagan’s excess power method: filter h(t) then square & integrate. Theoretical waveform Waveform in Noisy data Scientific Goals of LIGO and LISA: Scientific Goals of LIGO and LISA Astronomy: Open up a Radically New Window Onto the Universe Physics: Convert the study of highly curved spacetime From a purely theoretical enterprise (exploring general relativity theory) To a joint observational/theoretical enterprise Examples: Sources organized by science we expect to extract, not by when they might be detected -- The Inspiral of a White Dwarf (WD), Neutron Star (NS), or Small Black Hole (BH) into a Supermassive BH: The Inspiral of a White Dwarf (WD), Neutron Star (NS), or Small Black Hole (BH) into a Supermassive BH Astrophysical phenomenology: Occurs in nuclei of galaxies Provides a probe of the environments of supermassive holes Rates: a few per year; perhaps far more Frequency band and detectors: Low frequencies; LISA Information carried by the waves: High-precision map of the spacetime curvature of the supermassive BH Science to be done: Map black holes, test “no hair theorem”, test theory of evolution of black-hole horizons when gravitationally perturbed, observe extraction of spin energy from black holes. Method of computing waveforms: Black-hole pertubation theory; radiation-reaction theory LISA Inspiral Example: Circular, Equatorial orbit; 10 Msun / 106 Msun; fast spin -- @1 Gpc [optimistic](pessimistic: signal 10 times weaker): LISA Inspiral Example: Circular, Equatorial orbit; 10 Msun / 106 Msun; fast spin -- @1 Gpc [optimistic] (pessimistic: signal 10 times weaker) 1 yr before plunge: r=6.8 rHorizon 185,000 cycles left, S/N ~ 100 1 mo before plunge: r=3.1 rHorizon 41,000 cycles left, S/N ~ 20 1 day before plunge: r=1.3 rHorizon 2,300 cycles left, S/N ~ 7 Might lose factor 10 in S/N, even more, due to nonoptimal signal processing Frequency, Hz h LISA Science Requirement Inspiral Waves:Why might signal processing be non-optimal?: Inspiral Waves: Why might signal processing be non-optimal? Corresponding Waveform [schematic]: Typical Orbit in last year: Extreme sensitivity of orbit to initial conditions => ?? Coherent matched filtering no longer than a few days ?? Less? Many distant inspirals may give troublesome stochastic background; hard to separate strongest inspirals To explore & quantify this: need waveforms. Will take ~2 years of concerted effort to produce them & quantify loss of S/N Binary Black Hole Mergers : Binary Black Hole Mergers Binary Black Hole Mergers [cont.] : Binary Black Hole Mergers [cont.] Astrophysical phenomenology: Stellar-mass holes: in bodies of galaxies (``field’’), in globular & other clusters. Supermassive holes: as result of merger of galaxies Frequency band and detectors: Stellar-mass: High frequencies; LIGO & partners Supermassive: Low frequencies; LISA Rates, Signal to noise ratios: LIGO, initial interferometers: seen to 100Mpc, ~1/200yr to ~1/yr; S/N ~ 10 or less LIGO, advanced interferometers: seen to z~0.4, ~2/mo to ~15/day; S/N ~ 10 to 100 LISA: seen to z~10s (earliest objects in universe), ~few/yr; S/N ~ 100 to 100,000 Binary Black Hole Mergers [cont.] : Binary Black Hole Mergers [cont.] Information carried by the waves: Inspiral: Masses, spins, surface areas, and orbits of initial holes Merger: The highly nonlinear dynamics of curved spacetime Ringdown: Mass, spin, surface area, … of final hole Science to be done: Test Penrose’s cosmic censorship conjecture Test Hawking’s second law of black hole mechanics (horizon area increase) Watch a newborn black hole pulsate, radiating away its excess “hair” Probe the nonlinear dynamics of spacetime curvature under the most extreme of circumstances that occurs in the modern universe Probe demography of black hole binaries Methods of computing waveforms: Inspiral: post-Newtonian expansion; merger: numerical relativity; ringdown: black-hole perturbation theory Neutron-Star / Black-Hole Mergers: Neutron-Star / Black-Hole Mergers Astrophysical phenomenology: Stellar-mass objects: in field, in globular & other clusters. Frequency band and detectors: High frequencies: LIGO and partners Rates: Initial IFOs: 43Mpc, 1/2500yrs to 1/2yrs Advanced IFOs: 650Mpc, 1/yr to 4/day Information carried by waves: Inspiral: masses, spins, orbit Tidal disruption of NS: neutron-star structure (e.g. radius) Science to be done: Probe neutron-star structure, equation of state of matter Methods of analysis: Inspiral: post-Newtonian; disruption of NS: numerical relativity Neutron-Star / Neutron-Star Inspiral: Neutron-Star / Neutron-Star Inspiral Astrophysical phenomenology: Main-sequence progenitors in field, capture binaries in globular clusters Frequency band and detectors: High frequencies: LIGO and partners Rates: Initial IFOs: 20Mpc, 1/3000yrs to 1/3yrs Advanced IFOs: 300Mpc, 1/yr to 3/day Information carried by waves: Inspiral: masses, spins, orbit Merger: probably lost in LIGO’s high-frequency noise Science to be done: Test relativistic effects in inspiral [also for NS/BH and BH/BH] Methods of analysis: Post-Newtonian expansions Spinning Neutron Stars: Pulsars: Spinning Neutron Stars: Pulsars Astrophysical phenomenology: Pulsars in our galaxy Frequency band and detectors: High frequencies: LIGO and partners Detectability: Governed by ellipticity, spin Ellipticities thought to be e <10-6; possibly 10-5 Information carried by waves: NS structure Behavior in quakes Methods of analysis: Slow-motion, strong-gravity ~ Spinning Neutron Stars:Low-Mass X-Ray Binaries in Our Galaxy [LIGO]: Spinning Neutron Stars: Low-Mass X-Ray Binaries in Our Galaxy [LIGO] If so, and steady state: X-ray luminosity ~ GW strength Combined GW & EM obs’s => information about: crust strength & structure, temperature dependence of viscosity, ... Rotation rates ~250 to 700 revolutions / sec Why not faster? Bildsten: Spin-up torque balanced by GW emission torque Neutron-Star Births:R-Mode Sloshing in First ~1yr of Life [LIGO] : Neutron-Star Births: R-Mode Sloshing in First ~1yr of Life [LIGO] NS formed in supernova or accretion-induced collapse of a white dwarf. If NS born with Pspin < 10 msec: R-Mode instability: Gravitational radiation reaction drives sloshing Physics complexities: What stops the growth of sloshing & at what amplitude? Crust formation in presence of sloshing? Coupling of R-modes to other modes? Wave breaking & shock formation? Magnetic-field torques? …. Depending on this,GW’s may be detectable out to Virgo (supernova rate several per year). BUT recent research pessimistic GW’s carry information about these COMPACT BINARIES IN OUR GALAXY: LISA: COMPACT BINARIES IN OUR GALAXY: LISA Census of short-period compact binaries in our Galaxy; rich astro WD/WD @ Galaxy Ctr 3000 WD/WD binaries will stick up above the WD/WD noise AM C Vn The First One Second of Universe’s Life: The First One Second of Universe’s Life Waves from Planck Era, Amplified by Inflation: Waves from Planck Era, Amplified by Inflation Cosmological phenomenology: Vacuum fluctuations (at least) created in Planck era Amplified by interaction with background spacetime curvature of universe during inflation Frequency band and detectors: All bands, all detectors Strength predictions: “Standard Inflation”: detectable only in ELF band (CMB) “Pre-big-bang”, etc: more optimistic Information carried: Physics of big bang, inflation; equation of state of very early universe Methods of analysis: Cosmological perturbation theory; quantum gravity Exploring the Universe’s First Second: Exploring the Universe’s First Second Waves from standard inflation: too weak for LISA or LIGO/VIRGO/GEO or pulsar timing, in next 15 years BUT: Crude string models of big bang suggest stronger waves AND: There may be a rich spectrum of waves from phase transitions and spacetime defects in the very early universe. Phase Transitions in Very Early Universe: Phase Transitions in Very Early Universe Cosmological Phenomenology: As universe expanded, fundamental forces decoupled from each other; phase transition at each decoupling produced gravitational waves; GW’s redshifted with expansion Frequency bands and detectors: LISA probes Electroweak Phase Transition (~100 GeV) at universe age ~10-15 sec LIGO probes any phase transition that might have occurred at ~109 GeV and age ~10-25 sec Science: Probe high-energy physics, e.g. strength of electroweak phase transition; probe topological defects & evolution of inhomogeneities produced by phase transition Mesoscopic Oscillations in Very Early Universe: Mesoscopic Oscillations in Very Early Universe Recent speculations about our observed universe as a 3- dimensional defect (brane) in a higher dimensional universe: All fundamental forces except gravity are confined to the brane. Gravity is confined to some distance b< 1 mm from the brane, in the higher dimensions, and feels the shape of the brane.