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Glen Rebka

16 January 2015

Glen Anderson Rebka Jr., 83, died Tuesday, January 13, 2015 at Ivinson Memorial Hospital in Laramie, Wyoming. He was born September 19, 1931, in Cincinnati, Ohio, the son of Glen A. and Ruth (née Wasmer) Rebka. Glen leaves behind his wife of over 50 years, Ann Fischer.

He graduated from Harvard University summa cum laude in 1953 and was a member of Phi Beta Kappa. He received his master’s degree in 1954 and his doctorate in nuclear physics in 1961 also from Harvard. In his doctoral dissertation, Glen worked with his advisor, Robert Pound, to verify Albert Einstein’s prediction that gravity can change the frequency of light. This experiment is widely considered to have started an era of precision tests of General Relativity.

Among his outstanding publications are most notably ‘Gravitational Red-Shift in Nuclear Resonance’, ‘Apparent Weight of Photons’, and ‘Variation with Temperature of the Energy of Recoil-Free Gamma-Rays from Solids’. For this work, Glen was awarded the Eddington Medal for 1965 from the Royal Astronomical Society (London). After completing his studies at Harvard, Rebka joined the faculty at Yale University. A few years later he joined the Physics Department at the University of Wyoming. During his tenure at UW, he helped to shape the department’s vision for the future and to rejuvenate and strengthen the curriculum. Rebka served as department chairman from 1983-1991. When he retired in 1997, he was recognized as a Distinguished Emeritus Professor. That year he was elected a Fellow of the American Physical Society.

The so called Pound-Rebka experiment aims at detecting the gravitational red-shift in an earth-bound experiment. A tower in the Jefferson Physical Lab at Harvard provided the necessary change of gravitational potential over its height of approximately 23 meters. The idea of the experiment was to measure the resulting relative frequency shift. The major experimental challenge was to achieve the required frequency resolution. With Recoil-Free Moessbauer spectroscopy, which was discovered just one year prior, this resolution could be achieved. The 14 keV γ-transition of 57Fe has an intrinsically narrow line width of Γ ~ 10-8 eV and a fractional FWHM of 1 ˙ 10-12. With an emitter source at one end of the tower and an absorber at the other and a further division of the line width by studying resonant absorption, Rebka and his supervisor Robert Pound, mounted the source on a speaker and modulated it at between ten and fifty Hertz, hence at a velocity greater than the Doppler velocity, corresponding to Δν = 2.5 ˙ 10-15. A scintillation counter was synchronized with the velocity of the source. The observed difference in recorded counts is a measure of the frequency shift between emission and absorption. Pound and Rebka found 2 ∆ν = (5.13 ± 0.51) ∙ 10-15 accurate within 10% versus the predicted red-shift. Later improvements reduced the uncertainty to 1% (Snyder et al. PRL 13, 539, 1964).

The main publications of this work in Physical Review Letters show an, for the time, unusually high citation record and continue to be referred to in many monographs on Relativity and Space to this day. In other aspects of his work, Glen spent twenty summers at the Los Alamos National Laboratory participating in experiments. He continued to publish in the field of K+ mesons and magnetic resonance methods.

After his retirement, Glen maintained an office on campus, continued his research, served as interim head of the department, and taught courses in the physics curriculum for more than a decade. His particular joy was the Modern Physics course. In 2008 the Department of Physics and Astronomy at the University of Wyoming held a 2-Day Rebka Scientific Conference to honor his scientific and academic career and his contributions to the University of Wyoming. This event was attended by many former and current faculty, students, and staff. The event has been recorded on the webpage http://wyoskies.uwyo.edu/rebka/site/Rebka_Scientific_Conference.html.

Glen was always a most accommodating and engaging colleague. The most frequently voiced comment the writer of this obituary did come across during his research was: “Glen was such a presence, he had such a stature and people always wanted to know what his opinion on a subject was.” Glen thoroughly enjoyed teaching students of all ages. A long-time member of First United Methodist Church, he and his wife Ann taught a class of second and third graders for nearly thirty years. Glen will be remembered for his quiet, unassuming manner, his kindness, and his deep and abiding interest in all his students.

R. Michalak, Laramie, Wyoming


Glen Rebka
Wyoming Skies

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