Professor of Physics and Astronomy, Emeritus
UC Los Angeles
1917 – 1997
On 22 August 1997, the community of physical acousticians suffered a great loss with the death in Los Angeles of Isadore Rudnick. His passing followed a brutal, decade-long fight against a progressive form of dementia.
Izzy was born on 8 May 1917 in New York City and received his university education at UCLA, where he earned B.A., M.A. and Ph.D. degrees in physics in 1938, 1940, and 1944, respectively.
After doing postdoctoral work at Duke University and holding a faculty position at Pennsylvania State University, he returned in 1948 to UCLA where he served as a professor of physics for 39 years.
Before his return to UCLA, Izzy significantly advanced our understanding of several areas of classical acoustics – namely, atmospheric sound propagation, attenuation of sound in soil and nonlinear distortion and attenuation of shockwaves in ducts and horns. While at Penn State, he and Clayton Allen developed a special type of siren, which was then the most powerful man-made sound source. The siren’s variety of uses included the study of the biological effects of intense airborne sound and acoustical technique for cleaning clothes.
Izzy’s research in classical acoustics continued at UCLA, with studies of acoustically induced streaming, modes of vibration of elastic bodies and attenuation of sound in seawater.
In 1956 baving recognized the increasing importance of quantum mechanics in graduate education, Izzy went on sabbatical to the Technical University of Denmark to study low-temperature physics. On his return to UCLA, he obtained funds to buy a helium liquefier and began a series of ultrasonic investigations of electron-photon interactions in metals and superconductivity.
In the mid-1960s, Izzy turned his attention to the superfluid hydrodynamics of liquid helium and began a 20-year series of experiments which produced some of the most subtle and precise measurements of the dynamics of this quantum fluid. Superfluid helium supports six distinct sound modes. His research group discovered two of them and exploited the other four.
His helium experiments began with quantitative measurements of the speed and attenuation of 1st sound near the superfluid transition with microdegree resolution at frequencies in the gigahertz range. These measurements pushed forward our understanding of the application of equilibrium thermodynamics to the λ-transition. In terms of the relaxation of the superfluid order parameter, the measurements remain unsurpassed.
Izzy’s group succeeded in making the first experimental observation of 4th sound – a wave that propagates in the superfluid component when the normal fluid is constrained within a porous medium. Exploiting this mode, he determined the decay rate of superfluid persistent currents by measuring the Doppler shift that occurs in the modes of a torus when the persistent currents split the degeneracy of the toroid’s azimuthal modes. By measuring the beat frequency of the split modes, he determined the superfluid flow speed within milliseconds of the creation of the current, thereby extending the measurement interval by over a decade. (This finding was more than just a convenience, since these currents decay by only 10% in the age of the universe!) This 4th sound mode later provided researchers with the first means of demonstrating that a superfluid phase of helium-3 obeys two-fluid hydrodynamics.
One of Izzy’s favorite modes was 3rd sounds, a surface wave that can be excited on atom-thick absorbed films of superfluid helium. He liked the mode partly because it dramatically demonstrated one of his favorite maxims: “Superfluid helium obeys the laws of quantum mechanics on the macroscopic level.” Although he did not discover 3rd sound, he developed a simple thermal technique for its excitation and detection, which made possible exceedingly precise measurements of its velocity and amplitude and allowed its unusual backward-breaking shock waves to be observed.
In 1969, about three years before the Kosterlitz-Thouless theory of two dimensional phase transitions was formulated, Izzy emphasized the universal aspects of the disappearance, as the film thickness is decreased, of 3rd sound at nonzero propagation velocities. These 1969 experiments still comprise the most accurate test of the K-T theory.
An important aspect of the superfluid state is that heat does not diffuse as it does in an ordinary fluid, but propagates, instead, as a wave known as 2nd sound. In 1969, Izzy showed how 2nd sound could be excited and detected mechanically through vibration of a porous foil. The method’s high-quality data and the fact that it did not introduce a constant heating of the fluid facilitated the use of 2nd sound in measuring critical properties of the λ-transition. By simultaneously measuring the speeds of 1st, 2nd, and 4th sound at more than 400 points in the pressure-temperature plane, his students and postdoc measured and entabulated all the thermodynamic properties of superfluid helium. With a precision of 0.2%, these thermodynamic tables are still the best available.
In the last stages of his career, Izzy again returned his attention to non-linear acoustics and focused on shallow water gravity waves. In an updated version of an experiment first performed by Faraday, his group investigated the subharmonic response of a parametrically excited trough of water. As well as observing the extensive befurcation sequence, the team also accidentally discovered a new standing wave soliton.
Izzy’s contributions to low-temperature physics brought him the Fritz London Memorial Award of the International Union of Pure and Applied Physics in 1982 and election to the National Academy of Sciences in 1983.
Izzy was known within the physics community as a low-temperature physicist, but his professional home was always the Acoustical Society of America, which, in 1948, had presented him with its Biennial Award. Izzy went on to serve the Society as vice president and later as president. He received the ASA’s first Silver Metal in Physical Acoustics in 1975 and its Gold Medal in 1982.
Izzy loved to use demonstrations in his teaching. His public lectures at UCLA, given under the title of “An Evening of Demonstration Experiments in Physics,” where always standing room only. He would levitate objects in high-intensity sound fields and make cotton burst into flames as it absorbed the abundant acoustic energy generated by his siren. Though simple in appearance, these demonstrations included fresh discoveries, which quickly became part of the curriculum because, as Izzy put it, “Today’s research is tomorrow’s homework.”
Izzy was the most important and influential physical acoustician of the past 50 years. He was a great role model and mentor to generations of students. Death came to him peacefully, as he sat on a porch facing flowers that he had planted and nurtured long before. Long after, his memory will live in the hearts and minds of his students and colleagues whom he nurtured so well.