The Nobel Prize in Physics
autobiography and work
Discovery of the first binary pulsar and subsequent tests of GR
Pulsars were discovered, albeit accidentally, by Jocelyn Bell at Cambridge in 1967. The apparently sporadic bursts of radio emission appeared during the course of a survey to investigate the effects of interplanetary scintillation of radio sources. Working as a graduate student in a team lead by Anthony Hewish, Bell soon realized that the emission always occurred at the same position in the celestial sphere indicating that the source was not of terrestrial origin. Subsequent observations with greater time resolution showed the emission to be a train of pulses with a precise repetition period of 1337 ms. The Cambridge team published their discovery the following and, soon afterwards, announced the discovery of 3 more pulsars found from subsequent inspection of the remaining survey data.
Prior to the discovery, neutron stars were a purely theoretical concept - first proposed by Walter Baade and Fritz Zwicky to be the collapsed remains of a massive star after it has exploded as a supernova. The existence of neutron stars, and the prediction made by Baade-Zwicky that neutron stars would be associated with supernova remnants was dramatically confirmed with the measurement at the Arecibo Observatory of the short rotational period (33 ms) of the Crab pulsar in 1968. The pulsar lies at the center of the Crab nebula, shown on the right, this is the remains of a nearby supernova explosion witnessed by Chinese astronomers in 1054 AD. Using the rotating neutron star model, Thomas Gold of Cornell University, USA was able to show that the Crab pulsar is the dominant energy supply to its surrounding nebula. The connection between pulsars and rotating neutron stars is now universally accepted.
In over 30 years since the discovery, pulsars have proved to be exciting objects to study and, presently, over 1000 are known. Most of these are ``normal'' in the sense that their pulse periods are of order one second and, with few exceptions, are observed to increase secularly at the rate of about one complete period in 1,000,000,000,000,000! This is naturally explained as the gradual spin-down of the neutron star as it radiates energy at the expense of its rotational kinetic energy. A small fraction of the observed sample are the so-called ``millisecond pulsars'' which have much shorter periods (< 20 ms) and rates of slowdown of typically only one period in 10,000,000,000,000,000,000, proving to be extremely accurate clocks. In addition, some pulsars are known to be members of binary systems in which the companion is another neutron star, a white dwarf and even a main sequence star.
Pulsar Research at Arecibo
From the early days researchers at Arecibo Observatory joined the effort to search for new pulsars and to understand the physical processes responsible for this remarkable phenomenon. Their measurement of the periodicity of the Crab pulsar proved that neutron stars exist in the Universe and that they are the end point of stellar evolution for massive stars. A few other landmark contributions by Arecibo pulsar astronomers are summarized here.
During a systematic search of the galactic plane with the Arecibo telescope in July 1974, Russell A. Hulse and Joseph H. Taylor discovered an extraordinary of 59 ms pulsar PSR B1913+16. It soon became clear that the timing properties of this source could only be understood if the pulsar were in a highly eccentric 7.75 hour orbit around another neutron star. The intense gravitational field generated by the neutron stars results in several special and general relativistic effects unobtainable in a terrestrial physics laboratory, or even in Solar System experiments. Perhaps the most important effect is the emission of gravitational radiation from the system at the expense of its gravitational binding energy. According to Einstein's general theory of relativity, the separation of the stars should decrease by 3 mm each orbit due to this process.
From regular observations, the orbital decay was measured by Taylor and collaborators within 6 years of the discovery and is in agreement with general relativity to better than a percent after nearly 20 years of observations. The discovery of this fascinating binary system and the subsequent measurements provide the only experimental evidence to date for the existence of gravitational radiation. In recognition of this achievement, Hulse and Taylor were awarded the 1993 Nobel Prize in Physics. For more about the importance of this discovery, see the Press release from the Swedish Academy and Cornell's Astronomy Course web page. Click here for more information on PSR 1913+16.
The discovery of the first `millisecond pulsar' PSR 1937+21 was made at Arecibo by Don Backer, Shri Kulkarni collaborators in 1982. This remarkable neutron star with a spin period of just 1.5578 ms rotates about its axis almost 642 times per second. This value introduced fundamental constraints to equations of state for cold matter at supra-nuclear densities. The discovery revitalized pulsar astronomy since it demonstrated that a potentially large population of similar objects exists which had been missed by pulsar searches conducted during the 1970s which had sampling rates > 20 ms. These objects have extraordinary rotational stability, which opened up new avenues of research: high precision timing and solar system studies, high precision tests of GR and the nature of space-time itself, stellar evolution, tests of nuclear physics and search for long-period gravitational waves.
Although subsequent pulsar surveys with better sensitivity to short period objects have found a large number of similar objects, for 24 years PSR B1937+21 stayed as the most rapidly rotating neutron star known. The next shortest period object, PSR B1957+20 was discovered at Arecibo in 1988 by Andy Fruchter and collaborators. This 1.6074 ms pulsar turns out to be in a near circular 9 hr orbit around a low-mass companion star. The pulsar is eclipsed by its companion for about 50 minutes per orbit. For a few minutes before an eclipse becomes complete, and for more than 20 minutes after the signal reappears, the pulses are delayed by as much as several hundred microseconds- presumably as a result of propagation through plasma surrounding the companion. Detailed studies of this system suggest that the companion is being gradually ablated by the relativistic `wind' of the pulsar. This system may be a progenitor for single millisecond pulsar like PSR 1937+21.
The Arecibo discoveries continued in 1992 with the announcement of the first extra-solar planetary system known. This orbits around the millisecond pulsar PSR B1257+12, and was discovered by Alex Wolszczan and Dale Frail. Timing observations of this system have so far revealed the presence of at least three Earth-mass bodies in orbit around the neutron star.
Medicine Hat is a small town in Southwestern Alberta founded just over 100 years ago in a valley where the Canadian Pacific Railway crossed the South Saskatchewan River. I was born there on November 2, 1929 and raised in comfortable if somewhat Spartan circumstances. My father was the son of a Northern Irish carpenter and his Scottish wife who homesteaded on the Canadian prairies; my mother was an American, the daughter of Norwegian immigrants to the northern United States who moved to a farm in Alberta shortly after the first World War. During my early years our family of three was part of a large family clan headed by my Scottish grandmother. I attended schools named after English Generals and Royalty - Kitchener, Connaught, Alexandra.
Although I read quite a bit and found mathematics easy, I was not an outstanding student. In high school I did reasonably well in mathematics and science thanks to some talented and dedicated teachers.
I was nearly ten years old when World War II began. That conflict had a great effect on our town, and on me. In rapid succession the town found itself host to an R.A.F. flight training school, a prisoner of war camp and a military research establishment. The wartime glamor of the military, the sudden infusion of groups of sophisticated and highly-educated people, and new cultural opportunities (the first live symphonic music I ever heard was played by German prisoners of war) all transformed our town and widened the horizons of the young people there. I developed an interest in explosives and blew three fingers off my left hand just before hostilities ended in Europe. The atomic bomb that ended the war later that summer made me intensely aware of physicists and physics.
Higher education was highly prized in the society of a small prairie town and I was expected to continue on to university. After some difficulties over low grades in some high school subjects, I was admitted to the University of Alberta in Edmonton. I registered in a special program emphasizing mathematics and physics and gradually became interested in experimental physics, continuing my studies towards a Masters degree at the same institution. My thesis research was a rather primitive effort to measure double b-decay in an aging Wilson cloud chamber. Between sessions at the University, I spent two summers as a research assistant at the Defense Research Board installation near Medicine Hat working with Dr. E.J. Wiggins, who encouraged me to continue my studies either in eastern Canada or in the United States.
Those were interesting years, and during this time I met, courted and married Rita Bonneau - a partnership which has enriched my life in every way. Together we decided to try California, and I was accepted into the graduate program at Stanford, while she found work teaching in a military school in order to support us both. The first two years at Stanford were exciting beyond description - the Physics Department at Stanford included Felix Bloch, Leonard Schiff, Willis Lamb, Robert Hofstadter, and W.K.H. (Pief) Panofsky who had just arrived from Berkeley. I found that I had to work hard to keep up with my fellow students, but learning physics was great fun in those surroundings. At the end of the second year I joined the High Energy Physics Laboratory where the new linear accelerator was just beginning to do experiments. My thesis work was accomplished there under Prof. Robert F. Mozley, on a rather diffcult experiment producing polarized g-rays from the accelerator beam and then using those g-rays to study p-meson production.
In 1958 I was invited to join a group of physicists at the ?cole Normale Sup?rieure in Paris who were planning experiments at an accelerator (similar to the linac at Stanford) which was under construction in Orsay. I stayed in France for about three years working on the experimental facilities for the accelerator, and then participated in some electron scattering experiments. My wife began a new career there as a librarian at the Orsay laboratory, a career which was interrupted for a while when our son, Ted, was born in 1960. We returned to the United States in 1961 but a continuing connection to French physics and physicists has been a significant element in my life since that time - including a Doctorate (Honoris Causa) very kindly conferred upon me in 1980 by the Universit? de Paris-Sud.
Upon our return to the United States, I joined the staff of the Lawrence Berkeley Laboratory at the University of California. After less than a year in Berkeley, I moved back to Stanford where work on the construction of Stanford Linear Accelerator Center (SLAC) was just beginning. At SLAC, I started working on the design of the experimental areas for the new accelerator. By 1963 I had joined the group considering the requirements for electron scattering apparatus in the larger of two experimental areas. I worked closely with Pief Panofsky, and with collaborators from the California Institute of Technology and the Massachusetts Institute of Technology. I spent the next decade helping to build equipment and taking part in various electron scattering experiments, a number of which are the subject of the 1990 Nobel lectures. This was a period of intense activity, but also one of intense enjoyment for me. I was surrounded by people I liked and admired, and deeply involved in experiments which generated interest in laboratories and universities all over the world. I count myself extremely fortunate to have been at SLAC at that time.
I became a member of the SLAC faculty in 1968. In 1971, I was awarded a Guggenheim fellowship and spent an interesting sabbatical year at CERN, where I was impressed by the great progress that European science had made in the decade since I had worked in France.
Well before my trip to CERN, colleagues in the group at SLAC had become interested in testing some of the invariance properties of the electromagnetic interaction, a field which would absorb our efforts for most of the 1970s. When Charles Prescott joined the group in 1970, he began a serious study of ways to test parity conservation in the interaction between an electron and a nucleon. The electroweak theories of Weinberg and Salam predicted levels of nonconservation that looked extremely hard to measure. We attempted an experiment with the existing Yale polarized source, but the measurements did not reach the desired level of sensitivity. I was not very encouraging to my colleagues who wished to pursue the experiment to higher levels of accuracy. After the theoretical work of Veltman and van't Hooft and the discovery of neutral currents at CERN (during the year I was there) and at NAL (now Fermilab), the interest in experiments on parity conservation greatly intensified. In 1975 a new method for producing polarized electrons was discovered by a group in Colorado which included E.L. Garwin of SLAC. In 1978, after building a source for the linac based on the new method, we were able to demonstrate a violation of parity in close agreement with the electroweak predictions.
After the parity experiments, our group presented two proposals for large experimental facilities at PEP, the e+e- collider then being built at SLAC. Both those proposals were rejected. The group was finally successful in proposing a relatively small PEP detector, but I did not take part in that experiment.
sponsors link тюнинг авто
This is my BrainyGoose:
United States, IL, Chicago, English, Italian, Genry, Male, 21-25, bodybulding, swiming.