The Unitary Dynamical Group in Schrödinger's Picture

Up to now we have been concerned with the description of a physical system at a fixed instant of time. The time evolution of quantum systems constitutes a many-faced, wide issue which, however, is somewhat tangential to the main aim of this volume. We shall touch upon it only briefly, restricting ourselves to the more familiar aspects of the problem.

We assume, to begin with, that the physical system has no superselection rules, so that states and physical quantities correspond one-to-one to density operators and self-adjoint operators, respectively, on the Hilbert space. To describe the dynamical evolution of the system one has to specify the way the representatives of states and physical quantities evolve in, which is fixed in time, since the physical system is supposed to preserve its identity.

There is an old tenet according to which there are two kinds of time evolution: the discontinuous, nondeterministic evolution undergone by the quantum system when a measurement is made on it, and the continuous, deterministic evolution caused by the interaction with external forces or with other quantum systems. The first kind of evolution will be dealt with in Chapter 8, where we shall find it less alarming than is often claimed.

The development of the three pioneer radio astronomy groups at Sydney, Jodrell Bank and Cambridge offers historians and sociologists of science an unusual opportunity to make a comparative study of concurrent ‘research schools’. What emerges is an intriguing story of the interrelationship of technical strategy and commitments, social structure, and scientific style.

The main advantage in studying a research school (as opposed to, say, a more loosely defined ‘specialty network’ or ‘invisible college’) is that any school is an unproblematic group – it consists of specifiable people, working together. The three radio astronomy groups also shared a number of other common factors, which simplifies any comparative analysis. To begin with, they all embarked, essentially from scratch, into a virgin area of research in which they shared the same initial clues – namely, the original observations of Jansky, Reber and Hey – solar and galactic radio emissions; the existence of discrete sources, or ‘radio stars’; and radar reflections from meteors. What is more, since all the researchers had been engaged in wartime radar development, they shared the same technical resources and insights, and cannibalised or adapted the same equipment. Many of them had been active colleagues. They understood the same patois of electromagnetic theory — especially as it related to aerial design — and they all had access to, and familiarity with, at least some elements of the crucial ‘Fourier transform’ theory. The two British groups were both sections of prestigious university Physics Departments, and both Martin Ryle (at Cambridge) and Bernard Lovell (at Jodrell Bank) were given effectively carte blanche support by eminent superiors — J.A. Ratcliffe at Cambridge, Patrick Blackett at Manchester.

Fifty years ago Karl Jansky accidentally discovered that the Milky Way is a copious source of radio waves. This eventually led to detailed study of extraterrestrial radio waves, which for three reasons has unquestionably been one of the key developments in the astronomy of our century. First, there are of course the startling results themselves, revealing a panoply of unexpected phenomena. After radio galaxies, quasars, pulsars, the cosmic background radiation, and complex interstellar molecules, the Universe would never again be the same. Second, the style of research of the radio researchers eventually also changed the way that traditional astronomy was done; here I refer to the use of electronics and the attitude that primary training in astronomy was not necessary for success. Third, the achievements of radio astronomy provided much of the basis for the desire to scan the skies at every electromagnetic frequency possible, a program which has dominated much of subsequent observational astronomy. When an historian of the distant future characterizes the astronomy of our own era, major emphasis will surely be placed on this continual opening of the electromagnetic window and our resultant expanded view of the Universe, a process which began with radio astronomy.

It is thus both important and fitting that we pause after fifty years and collect the reflections of the pioneers of radio astronomy. To this end a score of major participants have contributed to the present volume their recollections and analyses of how the field developed. While recollection by itself is not history, these articles yield invaluable glimpses, not otherwise obtainable, of the spirit of the times, and also serve as starting points for delving further into the history.

As in other countries, the first discoveries of Jansky and Reber in radio astronomy made little impression in Australia. Rather, interest in the subject was first aroused by news of the wartime discoveries of radio emission from the Sun by J. S. Hey in England and G. C. Southworth in the United States. As World War II wound down in 1945, the CSIR (later CSIRO) Radiophysics Laboratory in Sydney was ending its work on the development of radar equipment for the South West Pacific theatre of war. A wise decision was made to keep this highly experienced and imaginative group of people together and to look for new scientific and technical challenges. With the strong administrative encouragement of E, G. (Taffy) Bowen and the scientific foresight of J. L. Pawsey, the group turned its curiosity to radio astronomy, and in this way Australia got an early start in the opening up of this new branch of science (see the contribution by Bowen elsewhere in this volume). Those were exciting days and new discoveries came rapidly as existing equipment could be turned over to the exploration of the almost unknown radio sky.

“COSMIC NOISE”

Some experiments had been made in 1944 by Pawsey and Ruby Payne-Scott from the roof of the Radiophysics Laboratory building in the grounds of Sydney University, but the first significant observations were made by pointing antennas of Air Force and Army radar stations near Sydney at the Sun. The first years of solar work in Australia are described by W. N. Christiansen elsewhere in this volume.

In the decade following World War II radio astronomy evolved from a minor curiosity to a strong scientific discipline, from small groups of equipment-oriented radio physicists and electrical engineers to major laboratories whose tenor was as much astronomy as radio techniques. This evolution took place in a similar fashion in many countries, but there were two nations where radio astronomy especially flourished. These were England and Australia. While it is not surprising to find England at the forefront of a scientific field in the middle of the twentieth century, Australia's presence requires more explanation.

The main elements in the Australian success story emerge from the articles in this section, by five of the key participants. First, the Radiophysics Laboratory in Sydney had in fact very close ties to the mother country and her strong tradition in radio science — many of the staff members were originally British or trained in Britain. Second, the Laboratory had been at the forefront of radar development during World War II and, when the war ended, was not dissolved. Rather, the strong team was kept intact under CSIRO aegis while new recruits and directions for peacetime radio research were sought. Third, dynamic and wise leadership was provided by “Taffy” Bowen and Joe Pawsey — two men whose contrasting personalities and styles of science led to just the right mix for exploring and exploiting the most profitable avenues into the radio sky.

The final section consists of a group of articles addressing the early development of radio astronomy in various broader contexts. David Edge, a radio astronomer turned sociologist of science, has frequently used his former field as a case study for achieving new insights into the social forces which make science what it is. In his present contribution he summarizes a portion of this work as it applies to the contrasting styles of research found at Cambridge, Jodrell Bank, and Sydney.

William McCrea never himself became embroiled in the 1950s controversy over radio source counts and their cosmological implications, but he was there as it all happened. In his article he traces the development of cosmology in the twentieth century, in which he himself played a major role, and then discusses how the new radio data affected the cosmologist's perception of the Universe.

In the third article Hendrik Van de Hulst does not so much write about his own substantial contributions to radio astronomy, but rather examines how ideas in astrophysics evolve over a period of decades, that is, with frequencies in the nanohertz range. Although this frequency range is somewhat lower than radio astronomers normally consider, he argues that in fact such long-term changes, often overlooked, are vital to the proper understanding of the development of a science.

Owen Gingerich, trained as an astrophysicist and now an historian of science, closes the volume with a look at radio astronomy's overall effect on twentieth-century astronomy.

Radio noise from space was detected by Karl Jansky in 1931, working at the Bell Telephone Laboratories (Jansky 1933), This revolutionary discovery broke the barrier confining astronomical knowledge to the information contained, and the relevant physics, within the narrow band of wavelengths accessible (an octave and a half), and to positions and motions under purely gravitational forces. Jansky's wavelength was ten million times longer than that of light. His signals were radiated from the galactic center, 10,000 parsecs distant. The long wavelengths he used resulted in low angular resolution. There was no radial velocity information, no sharp spectral features (the first line was found twenty years later). For such reasons, and perhaps because he was an electrical engineer, no astronomer beat a pathway to his door; in fact I have never met any astronomer who personally knew him. Public recognition came only as an article in the New York Times (May 5, 1933) and a radio interview. His relevant bibliography includes only seven entries over the years 1932 to 1939, and he died young (see the article by Sullivan in this volume for further information on Jansky). As a summer resident of New Jersey seashore resorts in the early 1930s, I wore golf knickers, possibly even a hip flask, and drove an open car with a rumble seat (oh nostalgia!) past the giant antennas of the transatlantic radio transmitters for which Jansky's studies of noise background were to find the best operating wavelengths. Although I felt no premonitory twinges, I met my wife there, soon became interested in Jansky's results, and my life became linked with that place and time.

My interest in radio astronomy began after reading the original articles by Karl Jansky (1932, 1933). For some years previous I had been an ardent radio amateur and considerable of a DX [Distance Communication] addict, holding the call sign W9GFZ. After contacting over sixty countries and making WAC [“worked all continents”], there did not appear to be any more worlds to conquer.

It is interesting to see how the mystifying peculiarities of short-wave communications of 1930 gradually have been resolved into an orderly whole. The solar activity minimum of the early thirties must have brought with it abnormally low critical frequencies. Many a winter night was spent fishing for DX at 7 Mc/s when nothing could be heard between midnight and dawn. It is now clear that the MUF [maximum usable frequency] over all of North America was well below 7 Mc/s for several hours. An hour after sunset when the west coast stations disappeared 14 Mc/s went dead. These years would have been a very fine time for low-frequency radio astronomy. The now appreciated long quiesence of the sun during the latter half of the seventeenth century (Schove 1955) would have been even better!

One further recollection is that on these quiet nights it was always possible to make the receiver quieter by taking off the antenna. This receiver uses a regenerative detector and one RF [radio frequency] stage. The detector and RF stage are tuned separately so that the latter may be gradually tuned across the former. When this was done with the antenna off, no appreciable change could be heard in the sound of rushing water.

The decade of the 1970s saw the four-hundredth anniversary of Kepler, the quinquecentennial of Copernicus, the tercentenary of the Greenwich Observatory, and the Einstein centennial. Now, at the beginning of the 1980s, we are celebrating the fiftieth anniversary of the beginnings of Karl Jansky's work in what was to become known as radio astronomy. The difference between those earlier anniversaries and the present one is that most of us have been, if not active participants, at least interested bystanders as the discoveries at radio wavelengths have made their impact upon astronomy. What we lack in historical perspective is perhaps compensated by the immediacy of our own experiences.

RECOLLECTIONS OF EARLY RADIO ASTRONOMY

J.S. Hey, in his 1973 book The Evolution of Radio Astronomy, has singled out the years 1950–51, as particularly crucial. In 1949 the Australian group of J.G. Bolton, G.J. Stanley, and O.B. Slee had given the first three optical identifications for discrete radio sources: Taurus A, the Crab Nebula; Virgo A, the elliptical galaxy M87; and Centaurus A, the peculiarly distorted galaxy NGC 5128. In 1950 H. Alfven and N. Herlofson introduced the idea that the observed radio emission from the discrete radio sources was synchrontron radiation, K.O. Kiepenheuer interpreted the radio emission of the Milky Way as synchrotron radiation from cosmic ray electrons spiraling in the interstellar magnetic field, and M. Ryle, F.G. Smith, and B. Elsmore published their preliminary list of fifty so-called radio stars in the northern hemisphere.

INTRODUCTION

In this volume we celebrate the fiftieth anniversary of the beginning of radio astronomy, when Karl Jansky observed cosmic radio emission for the first time. In the Soviet Union, radio astronomy began in 1946 with theoretical papers by V. L. Ginzburg and I. S. Shklovsky. Then in the following year Soviet scientists carried out their first observations of an extraterrestrial radio source, the Sun.

I must say some words about favourable conditions for the development of radio astronomy in the 1940s in our country. There existed a high level of theoretical physics, in particular of electrodynamics, combined with a deep interest in the problems of radio wave propagation, ionosphere and plasma physics, statistical physics, and radio engineering. In particular, investigations made by the scientific school of L. I. Mandel'shtam and N. D. Papaleksi paved the way.

Mandel'shtam and Papaleksi addressed the idea of radar measurements of the distance to the Moon at least twice – in 1925 and in 1943. On the first occasion they had to admit the impossibility of such an experiment with existing radio equipment. Their estimates made in 1943, however, were more reassuring. (By the way, they also analyzed the possibilities of trying an optical reflection experiment; the advantage of monochromatic pulses was especially noted (Papaleksi 1946).) The first successful lunar radar experiments, however, were carried out in 1946 in the U.S.A. and in Hungary; the first Soviet radar astronomy was also in 1946, but used for the study of meteors (Levin 1946).

Two powerful influences acted on the early development of radio astronomy at Cambridge. The first was the existing radio research under J.A. Ratcliffe, directed primarily at the ionosphere. J.W. Findlay, who is known amongst radio astronomers as the inspiration for the 300 ft transit telescope at Green Bank and, later, as one of the designers of the Very Large Array, was a member of this research group before and after the 1939–45 war. The second influence was wartime experience in radar, when Martin Ryle in particular developed his genius for experimental methods which were at once bold, original and economical. His most important wartime work was in airborne counter-measures, involving the analysis of enemy radar and the desperate scramble to provide aircraft with warnings of radar-directed fighter attack. Ratcliffe would maintain that his own contribution was to attract Martin Ryle to the Cavendish, and to encourage him to develop his own techniques in investigating radio waves from the sun. Nevertheless Ratcliffe's influence in our understanding of radio, and even more of Fourier analysis, were other vital ingredients.

Radio research re-started in the Cavendish in 1945. Ryle, who was an Imperial Chemical Industries Research Fellow, was joined by Derek Vonberg; both were registered for Ph.D.s, although neither ever wrote a thesis. Their first approach to the measurement of solar radio waves was to build a radio version of the Michelson interferometer which would distinguish the sun from other extraneous sources of noise. They built a switched receiver known as the Cosmic Radio Pyrometer (Ryle & Vonberg 1948) in which a controllable noise diode was switched against the input noise signal.

Aim

This paper is an attempt at what with a sophistication understandable to radio astronomers might be termed nanohevtz astronomy: the art of registering the coming and going of astronomical convictions in periods of the order of 109 seconds = 30 years. This approach is complementary to the common one, where the history of science is described by focusing on the sudden discoveries and the rapid breakthroughs of insight. This complement is as necessary as are the added measurements at very short spacings in the Fourier synthesis of extended sources. Otherwise a broad underlying valley or elevation might be misjudged and the basic structure misinterpreted. And – to continue this metaphor – the historical development of science is indeed such an extended source with a highly complex structure.

How can we ‘measure’ these low-frequency components? The history of science that can be reconstructed from published papers is far from complete. A substantial part of the development of scientific knowledge is not documented this way. The relative importance of this ‘hidden’ part varies with the field: it is probably smallest in the gathering of new data, moderate in the development of new techniques and largest in the growth of theoretical insight and interpretation. In this ‘hidden’ part the ideas originate, grow (rightly or wrongly) into an accepted theory, or (again rightly or wrongly) are discarded. The scene is: sleepless nights, the private work room, the class room, visits, phone calls, bull sessions, workshops, team meetings, some correspondence, the hall where a symposium is being held, or the coffee shop across the road.

THE IMMEDIATE POST-WAR ACTIVITIES OF THE DIVISION OF RADIOPHYSICS

Radio astronomy began in Australia at the Division of Radiophysics of CSIR (Council for Scientific and Industrial Research, later renamed CSIRO, Commonwealth Scientific and Industrial Research Organization) as a direct outcome of its involvement in radar research and development during World War II. The Division was established in 1939 with responsibility for developing radar for the Australian Army, Navy and Air Force – and later for the American Forces in the Southwest Pacific. What were the ingredients which led in 1946 to the development of radio astronomy?

The first and by far the most important of these was the decision by the Chairman of CSIR, Sir David Rivett, that at the conclusion of World War II, CSIR would be devoted only to peace-time research, and that defence research would be carried out by other agencies. This meant that a highly developed laboratory with a superlative staff became available for a wide range of researches and practical developments in a peace-time environment.

It would be easy to underestimate the importance of this decision. In later years we were to be reminded of its wisdom by the fate of some overseas laboratories which were not so fortunate. For many years they carried joint responsibility for civilian and military research. When a new research proposal came up, it often led to a bureaucratic argument as to whether it was predominantly civilian or military – and what priority it should be given. This argument would run right down through the staff structure, to the detriment of the job itself.