Laser Interferomter Gravitational-Wave Observatory

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Graduate Research at Caltech in
Experimental Gravitational Physics



  • Rana Adhikari
  • Barry C. Barish
  • Ken G. Libbrecht
  • Yanbei Chen
  • Thomas A. Prince
  • Kip S. Thorne
  • Alan J. Weinstein

The LIGO project is a joint effort of physicists at Caltech and MIT with collaborating groups at a number of other universities (the LIGO Scientific Collaboration) and is part of an international effort to develop a new field of science: gravitational-wave astronomy.  LIGO stands for Laser Interferometer Gravitational-Wave Observatory, a facility now in operation that should be capable of discovering gravitational waves, confirming the existence of black holes, testing Einstein's general relativity theory in the strong field limit, and opening a new window onto the astrophysical universe.  Caltech scientists in the LIGO project pursue an active experimental research effort towards developing interferometry techniques with the extraordinary sensitivity required to detect and measure such waves. They also actively develop and employ analysis techniques aimed at identifying gravitational waveforms in noisy data and extracting information about their propertis and astrophysical sources.

The physics of gravitational waves, as predicted by general relativity, has been fairly well understood since the 1960's.  However, theoretical studies of gravitational wave generation in the nonlinear strong-field regime are an active subject of research at Caltech Theoretical Astrophysics and elsewhere. Vigorous efforts are underway to compute the details of the waves from realistic astrophysical sources to provide guidance for the experimental efforts.

Indirect astronomical evidence of gravitational waves comes from high precision measurements of the orbit of a neutron star binary system ("the binary pulsar") for which the Nobel prize was awarded to Joseph Taylor and Russell Hulse in 1993.  Unlike electromagnetic radiation, the interaction of gravitational waves with matter is so weak that generating waves detectable from earth requires stellar masses moving at relativistic velocities.  Among the sources that might be detectable by the LIGO are the supernova collapse of a stellar core to form a neutron star or black hole, the collision and coalescence of two neutron stars or black holes that are orbiting each other, the rotation of neutron stars with deformed crusts, "star-quakes" in neutron-star crusts, and the big-bang origin of the universe.

A gravitational wave is a perturbation in the curvature of space-time that travels at the speed of light.  As it passes, it causes adjacent inertial frames -- and free masses carried along by those frames -- to move relative to one another.  A gravitational wave can be detected by carefully monitoring the resulting fluctuations ΔL in the separation L between two free masses.  The ratio ΔL/L is directly proportional to the wave's amplitude; thus, large separations L result in detectable fluctuations ΔL.  The strongest sources (e.g. black-hole collisions and supernova explosions) are likely to cause fluctuations of magnitude ΔL/L ~ 10-21 between free masses on earth.

Aerial view of Hanford Observatory
LIGO Hanford Observatory

In the LIGO these small relative displacements ΔL are measured using laser interferometry.  Each interferometer has four mirrored test masses arranged in pairs to form two long (L = 4 km), resonant optical cavities situated at right angles to each other, forming a Michelson interferometer. LIGO operates three such interferometer detectors, located at two widely separated sites in the United States (two in Hanford, Washington and one in Livingston Parish, Louisiana), operated in coincidence for the detection of gravitational waves.  By making high-precision measurements over such long baselines, LIGO interferometers will be able to see gravitational radiation originating from distant sources in the universe.  Compared to laboratory-scale apparatus, this should make the difference between detecting a few events per century and detecting several per year.  Because the LIGO detectors will operate over a broad band of frequencies (from 10 Hz to 10 kHz), they should be able not only to detect a gravitational wave but also to reveal the details of the wave's time evolution, i.e. of its "waveform."  From the waveform it will be possible to infer the details of the dynamical evolution of the waves' source that will rarely be obtainable from electromagnetic waves, since most gravitational-wave sources are likely to be surrounded by dense layers of matter (e.g. the core of a supernova or the big bang) or to be made of pure gravity which emits no electromagnetic waves (e.g. colliding black holes).

Aerial view of Livingston Observatory
LIGO Livingston Observatory

The three Initial LIGO detectors have achieved noise-limited strain sensitivity consistent with their design, and are taking data in a series of science runs. The current science run (S5) extends through 2007, and analysis of the data to search for gravitational wave signals is underway. The LIGO Laboratory plans to implement a modest upgrade (Enhanced LIGO) to improve the sensitivity by a factor 2-3 and then take date through 2011. Plans for Advanced LIGO interferometers with an order of magnitude improved sensitivity (resulting in a thousand-fold increase in event rate) are well under way, for installation in 2011-2013. These upgrades hold the promise of discovery and the establishment of a new field of astrophysics.

Advanced R&D is also being pursued at Caltech, JPL, and elsewhere, for a space-based observatory LISA, and other future gravitational wave projects. Caltech Theoretical Astrophysics explores the astrophysics, phenomenology and modeling of gravitational-wave sources and the development of numerical relativity for the simulation of black holes and other extreme spacetimes. Weekly seminars are sponsored by the Caltech LIGO group, Theoretical Astrophysics, and the Caltech-JPL Association for Gravitational-Wave Research CaJAGWR.

The faculty and staff working on the LIGO Project currently divide their effort between development of the existing detectors at the observatories, development of data analysis strategies, and experimental research on Advanced LIGO technologies, prototype interferometers, and associated physics. Many opportunities exist for graduate student research using the Initial LIGO data, and/or pursuing the R&D for Advanced LIGO. Research using prototype advanced interferometers offers unique opportunities for students interested in a wide range of problems in physics. Caltech operates the Thermal Noise Interferometer (TNI), the 40 Meter Prototype, the Seismic Attenuation System (SAS) laboratory, and table-top interferometers.

LIGO 40m  Prototype
LIGO 40m Prototype (Caltech campus)

The 40-meter prototype, located on the Caltech campus, is a fully instrumented gravitational-wave detector. The laboratory is playing a central role in developing and prototyping more complex optical configurations and control systems for Enhanced LIGO and Advanced LIGO. Many opportunities exist for frontier research at this facility, pushing the state of the art in precision measurement science and technology: developing more stable lasers, more sensitive interferometer optical configurations, complex non-linear control systems and electronics, techniques for seismic isolation and reduction of thermal noise in materials at finite temperature, deeper understanding of the limitations on measurement precision due to the quantum nature of matter and light, and methods to extract the gravitational waveforms and their astrophysical information from the detectors' noisy data.

  More information may be found at the Physics, Mathematics, and Astronomy Division, LIGO, and Caltech home pages.