The Search for Gravitational Waves

Discovering gravity waves experimentally will open a new era of astronomy. An extremely sensitive Caltech detector is preparing to join the hunt for these very faint signals from space

On the measurement of a weak classical force coupled to a quantum-mechanical oscillator. I. Issues of principle

The monitoring of a quantum-mechanical harmonic oscillator on which a classical force acts is important in a variety of high-precision experiments, such as the attempt to detect gravitational radiation. This paper reviews the standard techniques for monitoring the oscillator, and introduces a new technique which, in principle, can determine the details of the force with arbitrary accuracy, despite the quantum properties of the oscillator. The standard method for monitoring the oscillator is the "amplitude-and-phase" method (position or momentum transducer with output fed through a narrow-band amplifier). The accuracy obtainable by this method is limited by the uncertainty principle ("standard quantum limit"). To do better requires a measurement of the type which Braginsky has called "quantum nondemolition." A well known quantum nondemolition technique is "quantum counting," which can detect an arbitrarily weak classical force, but which cannot provide good accuracy in determining its precise time dependence. This paper considers extensively a new type of quantum nondemolition measurement—a "back-action-evading" measurement of the real part X_1 (or the imaginary part X_2) of the oscillator's complex amplitude. In principle X_1 can be measured "arbitrarily quickly and arbitrarily accurately," and a sequence of such measurements can lead to an arbitrarily accurate monitoring of the classical force. The authors describe explicit Gedanken experiments which demonstrate that X_1 can be measured arbitrarily quickly and arbitrarily accurately. In these experiments the measuring apparatus must be coupled to both the position (position transducer) and the momentum (momentum transducer) of the oscillator, and both couplings must be modulated sinusoidally. For a given measurement time the strength of the coupling determines the accuracy of the measurement; for arbitrarily strong coupling the measurement can be arbitrarily accurate. The "momentum transducer" is constructed by combining a "velocity transducer" with a "negative capacitor" or "negative spring." The modulated couplings are provided by an external, classical generator, which can be realized as a harmonic oscillator excited in an arbitrarily energetic, coherent state. One can avoid the use of two transducers by making "stroboscopic measurements" of X_1, in which one measures position (or momentum) at half-cycle intervals. Alternatively, one can make "continuous single-transducer" measurements of X_1 by modulating appropriately the output of a single transducer (position or momentum), and then filtering the output to pick out the information about X_1 and reject information about X_2. Continuous single-transducer measurements are useful in the case of weak coupling. In this case long measurement times are required to achieve good accuracy, and continuous single-transducer measurements are almost as good as perfectly coupled two-transducer measurements. Finally, the authors develop a theory of quantum nondemolition measurement for arbitrary systems. This paper (Paper I) concentrates on issues of principle; a sequel (Paper II) will consider issues of practice

Interview with Ronald W. P. Drever

An interview in five sessions, January through June, 1997, with Ronald W. P. Drever, professor of physics (now emeritus) in the Division of Physics, Mathematics, and Astronomy. Dr. Drever graduated with first honors from the University of Glasgow in 1953 and received his PhD there in 1958. He moved from Glasgow to Caltech in 1977 to help establish the gravitational-wave project later known as LIGO (Laser Interferometry Gravitational-Wave Observatory)—first as a visiting associate, then a half-time professor (1979-1984), becoming full-time in 1984. He discusses his postdoctoral work at Glasgow on the anisotropy of inertia; a fellowship at Harvard with R. V. Pound measuring gravitational redshift; and collaboration with John Jelley of Harwell looking for radio and light pulses from supernovae and the Crab pulsar. Recalls his interest in Joseph Weber’s experiments to detect gravitational waves and his own bar-detector work at Glasgow; his switch to interferometers; his “friendly rivalry” with the gravitational-wave group at the Max Planck Institute in Munich; his adaptation of Fabry-Perot cavities vs. the delay-line technique of MIT’s Rainer Weiss. Recalls his collaboration with John Hall, of JILA, in Boulder, CO. Discusses his recruitment to Caltech by Kip S. Thorne; designing Caltech’s 40-meter prototype interferometer; his various innovations; his disagreements with Weiss, Thorne, and particularly Robbie [Rochus E.] Vogt, LIGO director 1987-1994; his July 1992 dismissal from LIGO; his grievance hearing before Caltech’s Academic Freedom & Tenure Committee, and its eventual outcome. The interview concludes with comments on his current research and on the prospects for LIGO and allied gravity-wave projects

Interferometry

The following recommended programs are reviewed: (1) infrared and optical interferometry (a ground-based and space programs); (2) compensation for the atmosphere with adaptive optics (a program for development and implementation of adaptive optics); and (3) gravitational waves (high frequency gravitational wave sources (LIGO), low frequency gravitational wave sources (LAGOS), a gravitational wave observatory program, laser gravitational wave observatory in space, and technology development during the 1990's). Prospects for international collaboration and related issues are also discussed

Quantum Nondemolition Measurements of Harmonic Oscillators

The complex amplitude X1+iX2≡(x+ip / mω)e^(iωt) of a harmonic oscillator is constant in the absence of driving forces. Although the uncertainty principle forbids precise measurements of X1 and X2 simultaneously (ΔX1ΔX2>~ℏ / 2mω), X1 alone can be measured precisely and continuously ("quantum nondemolition measurement"). Examples are given of measuring systems that do this job. Such systems might play a crucial role in gravitational-wave detection and elsewhere

The Physics of LIGO

In the spring term of 1994, I organized a course at Caltech on the The Physics of LIGO (i.e., the physics of the Laser Interferometer Gravitational Wave Observatory). The course consisted of eighteen 1.5-hour-long tutorial lectures, delivered by members of the LIGO team and others, and it was aimed at advanced undergraduates and graduate students in physics, applied physics and in engineering and applied sciences and also at interested postdoctoral fellows, research staff, and faculty

Recycled light improves gravity-wave detection

The detection of gravitational waves from coalescing binary neutron stars and other astrophysical sources presents an outstanding challenge to experimental physics. Several major experimental facilities are being built to observe the tiny ripples in space caused by the waves. A team of researchers from the Max Planck Institute for Quantum Optics at Garching in Germany, Glasgow University in the UK and Hannover University, also in Germany, has recently demonstrated some ingenious optical techniques on a prototype detector (G Heinzel et al. 1998 Phys. Rev. Lett. 81 5493). The work is a significant step towards achieving the nearly incredible measurement sensitivity aimed for in gravity-wave research-equivalent to detecting a change less than the radius of an atom in a distance as large as that from the Earth to the Sun