Nobel Prizes

in

Astrophysics & Cosmology

(A Twelve Part Series)

Part 3 – Pulsars & Radio Astrophysics

Martin Ryle & Antony Hewish

There was a magic about pulsars... no other things in the sky had such labels on them. Each one had its own distinct pulsing frequency, so it could be identified by anybody, including other creatures, after a long period of time and far, far away.

-       Frank Drake

The Nobel Prize is equated with the pinnacle of human achievement in both popular perception and professional esteem.  Since it was first awarded in 1901, the annual Nobel Prize for Physics has gone to major contributions in Astrophysics and Cosmology related fields only on eleven occasions. The first of these awards (1967) was the subject of the previous article (see here). The next was in 1974, jointly to Martin Ryle and Anthony Hewish for their contributions in Radio Astrophysics.

Radio Astrophysics

Radio astrophysics is the branch of astronomy that studies celestial objects by observing their emission at radio wavelengths. Unlike optical astronomy, which sees thermal radiation from hot objects, radio astronomy reveals non-thermal processes, such as synchrotron radiation from electrons spiraling in magnetic fields. This allows us to ‘see’ phenomena that are invisible in optical light, including cold gas clouds, pulsars, quasars, and the remnant afterglow of the Big Bang.

The field transformed our understanding of the universe, moving it from a static, human-sized place to a dynamic, violent, and evolving cosmos. Two figures were paramount in this revolution: Martin Ryle and Antony Hewish.

The Pivotal Contributions of Ryle and Hewish

Their work, which earned them the 1974 Nobel Prize in Physics, was complementary. Hewish made a specific, stunning discovery, while Ryle developed the tools and methods that made such discoveries possible and turned radio astronomy into a precise, mapping science.

Sir Martin Ryle

Ryle's primary contribution was the development of radio interferometry and aperture synthesis, which dramatically improved the resolution and mapping capabilities of radio telescopes.

· The Problem: Early single-dish radio telescopes had very poor angular resolution, meaning they could tell that a radio source was strong, but not precisely where it was or what its structure was. They produced "blurry" images.

· The Solution - Interferometry: Ryle pioneered the use of multiple radio antennas spread out over a distance and connected together. By combining their signals, these arrays could act as a single, giant telescope with a resolution equivalent to a dish as wide as the distance between them.

Aperture Synthesis*: This was Ryle's masterstroke. By moving the telescopes to different positions over time and combining the data, he could mathematically "synthesize" the resolving power of a telescope kilometers wide. This technique is now the foundation of every major radio observatory in the world, including the Very Large Array (VLA) and the Atacama Large Millimeter Array (ALMA).

Key Scientific Impact:

Using his ever-improving telescopes at Cambridge, Ryle conducted a series of sky surveys (the 1C, 2C, 3C, etc.). His most crucial finding was that faint, distant radio sources (now known to be quasars and radio galaxies) were far more numerous in the past. This provided the first direct observational evidence for the Big Bang theory over its rival, the Steady State theory, by showing that the universe has evolved over time.

[*Aperture Synthesis is a technique in radio astronomy that uses a network of separate telescopes to simulate a single, large telescope, achieving higher angular resolution. Signals from multiple antennas are combined, and their separation and relative orientation are used to reconstruct a high-resolution image of a celestial source. This process effectively synthesizes a large aperture from smaller, widely-spaced components, with the Earth's rotation being used to vary the positions of the antennas and fill out the synthesized aperture.]

Sir Antony Hewish

Hewish's primary contribution was the investigation of scintillation and, through that process, the monumental discovery of pulsars*.

· The Investigation - Scintillation: Hewish was studying the twinkling (scintillation) of compact radio sources caused by the solar wind. To do this accurately, he built a revolutionary radio telescope at Cambridge. It was a giant array of over 2,000 dipoles covering 4.5 acres, designed to be sensitive to rapid variations in radio signals.

· The Discovery - Pulsars: In 1967, his graduate student, Jocelyn Bell Burnell, noticed a curious, repeating signal of sharp, regular pulses arriving every 1.33 seconds. Hewish led the team that confirmed this was not human-made interference but a celestial source. They named it a "pulsar" (pulsating star).

[*Pulsars are rapidly rotating neutron stars that emit beams of electromagnetic radiation, appearing as regular pulses to Earth-based observers, similar to a lighthouse. They form from the collapse of massive stars in a supernova explosion. The beams are created by powerful magnetic fields, and the pulsing effect is caused by the star's rotation, which sweeps the beams across the sky.]

Key Scientific Impact:

The discovery of pulsars was a sensation. They were quickly identified as rapidly rotating neutron stars—the incredibly dense, collapsed cores of massive stars that went supernova. This confirmed a theoretical prediction and provided a new, extreme laboratory for testing physics under conditions of immense gravity and density. Pulsars have since been used to:

· Confirm the existence of gravitational radiation (indirectly, earning a 1993 Nobel Prize).

· Test the theory of general relativity with extreme precision.

· Serve as cosmic lighthouses for navigation.

Legacy

Together, Ryle and Hewish propelled radio astrophysics into maturity.

· Ryle's legacy is the toolkit. His aperture synthesis technique is the bedrock of modern observational astronomy, enabling the creation of high-resolution maps of the radio sky that rival or surpass optical images in detail.

· Hewish's legacy is the discovery. The identification of pulsars opened an entirely new field of study into compact objects and high-energy astrophysics. Their work demonstrated that the universe is filled with energetic, exotic, and dynamic phenomena that can only be fully understood by listening to the radio whispers from space. They truly gave humanity new ears with which to hear the cosmos.

Martin Ryle (1918 - 1984) – A biographical sketch

Martin Ryle was born on September 27, 1918, the second of five children. His father John A Ryle was a doctor who, after the war, was appointed to the first Chair of Social Medicine at Oxford University.

Ryle was educated at Bradfield College and Oxford, where he graduated in 1939. During the war years he worked on the development of radar and other radio systems for the RAF and, though ‘gaining much in engineering experience and in understanding people, rapidly forgot most of the physics he had learned’.

In 1945, J A Ratcliffe, who had been leading the ionospheric work in the Cavendish Laboratory, Cambridge before the war, suggested that Ryle should apply for a fellowship to join his group to start an investigation of the radio emission from the Sun, which had recently been discovered accidentally with radar equipment.

During these early months, and for many years afterwards both Ratcliffe and Sir Lawrence Bragg, then Cavendish Professor, gave enormous support and encouragement to Ryle. Bragg’s own work on X-ray crystallography involved techniques very similar to those they were developing for “aperture synthesis”, and he always showed a delighted interest in the way their work progressed.

In 1948, Ryle was appointed to a Lectureship in Physics and in 1949 elected to a Fellowship at Trinity College. At this time Antony Hewish joined him, and in fact four other members of their team started their research during the period 1948-52.

In 1959, the University recognized their work by appointing Ryle to a new Chair of Radio Astronomy.

During 1964-7, he was president of Commission 40 of the International Astronomical Union, and in 1972 was appointed to the prestigious position of Astronomer Royal.

Martin Ryle explaining aperture synthesis with two model radio telescopes

In 1947, Ryle married Rowena Palmer, and they had two daughters, Alison and Claire, and a son, John. They enjoy sailing small boats, two of which he had designed and built himself.

Some of the awards and honors conferred on Martin Ryle are:

·      1952 Fellow of Royal Society of London

·      1954 Hughes Medal, Royal Society of London

·      1955 Halley Lecturer, University of Oxford

·      1958 Bakerian Lecturer, Royal Society of London

·      1964 Gold Medal, Royal Astronomical Society, London

·      1971 Elected Foreign Member of USSR Academy of Sciences

·      1973 Royal Medal, Royal Society of London

 

Antony Hewish (1924-2021) – A biographical sketch

Antony Hewish was born in Fowey, Cornwall, on 11 May 1924, the youngest of three sons and his father was a banker. He grew up in Newquay, on the Atlantic coast and there developed a love of the sea and boats. He was educated at King’s College, Taunton and went to the University of Cambridge in 1942. From 1943-46, he was engaged in war service at the Royal Aircraft Establishment, Farnborough and also at the Telecommunications Research Establishment, Malvern.  He was involved with airborne radar-counter-measure devices and during this period he also worked with Martin Ryle.

Returning to Cambridge in 1946, Hewish graduated in 1948 and immediately joined Ryle’s research team at the Cavendish Laboratory. He obtained his Ph D in 1952, became a Research Fellow at Gonville and Caius College where he had been an undergraduate, and in 1961 transferred to Churchill College as Director of Studies in Physics. He was University Lecturer during 1961-69, Reader during 1969-71 and Professor of Radio Astronomy from 1971 until his retirement in 1989. Following Ryle’s illness in 1977, he assumed leadership of the Cambridge radio astronomy group and was head of the Mullard Radio Astronomy Observatory from 1982-88.

Hewish’s decision to begin research in radio astronomy was influenced both by his wartime experience with electronics and antennas and by one of his teachers, Jack Ratcliffe, who had given an excellent course on electromagnetic theory during his final undergraduate year and whom he had also encountered at Malvern.

Hewish’s first research was concerned with propagation of radiation through inhomogeneous transparent media and this remained a lifelong interest. The first two radio “stars” had just been discovered and he realized that their scintillation, or ‘twinkling’, could be used to probe conditions in the ionosphere. He developed the theory of diffraction by phase-modulating screens and set up radio interferometers to exploit his ideas. Thus, he was able to make pioneering measurements of the height and physical scale of plasma clouds in the ionosphere and also to estimate wind speeds in this region. Following the Cambridge discovery of interplanetary scintillation in 1964, he developed similar methods to make the first ground-based measurements of the solar wind and these were later adopted in the USA, Japan and India for long term observations. He also showed how interplanetary scintillation could be used to obtain very high angular resolution in radio astronomy, equivalent to an interferometer with a baseline of 1000 km – something which had not then been achieved in this field. It was to exploit this technique on a large sample of radio galaxies that Hewish conceived the idea of a giant phased-array antenna for a major sky survey. This required instrumental capabilities quite different from those of any existing radio telescope, namely very high sensitivity at long wavelengths, and a multi-beam capability for repeated whole-sky surveys on a day-to-day basis.

Hewish obtained funds to construct the antenna in 1965 and it was completed in 1967. The sky survey to detect all scintillating sources down to the sensitivity threshold began in July. By a stroke of good fortune, the observational requirements were precisely those needed to detect pulsars. Jocelyn Bell joined the project as a graduate student in 1965, helping as a member of the construction team and then analyzing the paper charts of the sky survey. She was quick to spot the week-to-week variability of one scintillating source which he thought might be a radio flare star, but their more detailed observations subsequently revealed the pulsed nature of the signal.

One of his interests was the way the daily observations of scintillation over the whole sky could be used to map large-scale disturbances in the solar wind. It was then the only means of seeing the shape of interplanetary weather patterns so their observations made a useful addition to in-situ measurements from spacecraft such as Ulysses (1992) on its way to Jupiter.

Looking back over his forty years in radio astronomy Hewish felt extremely privileged to have been in at the beginning as a member of Martin Ryle’s group at the Cavendish. They were a closely-knit team and besides his own research programs, he was also involved in the design and construction of Ryle’s first antennas employing the novel principle of aperture synthesis.

Teaching physics at the University, and more general lecturing to wider audiences was a major concern for Hewish. He developed an association with the Royal Institution in London when it was directed by Sir Lawrence Bragg, giving one of the well-known Christmas Lectures and subsequently several Friday Evening Discourses. He believed scientists have a duty to share the excitement and pleasure of their work with the general public, and he enjoyed the challenge of presenting difficult ideas in an understandable way.

Hewish got married in 1950. His son became a physicist and obtained his PhD for neutron scattering in liquids, while his daughter became a language teacher.

• Some of the awards and honors conferred on Antony Hewish are:
• Hamilton Prize, Cambridge (1952)
• Eddington Medal, Royal Astronomical Society (1969)
• Dellinger Medal, International Union of Radio Science (1972)
• Michelson Medal, Franklin Institute (1973)
• Hughes Medal, Royal Society (1976)
• Fellow of the Royal Society (1968)
• Foreign Fellow, Indian National Science Academy (1982)
• Honorary Fellow, Indian Institution of Electronics and Telecommunication Engineers (1985)

The Bell Controversy

Dame Susan Jocelyn Bell Burnell (1943- ) is a Northern Irish physicist who, while conducting research for her doctorate, discovered the first radio pulsars in 1967. This discovery later earned the Nobel Prize in Physics in 1974 as described in this article, but she was not among the awardees. She could have been given the award jointly with Hewish and Ryle, but seems to have been ignored, perhaps because she was just a scientific assistant and doctoral student at the time (see here).

Jocelyn Bell, circa 1967. To her right is the radio telescope that she built for her thesis. Picture courtesy Roger W Haworth

Antony Hewish never publicly denied Jocelyn Bell's role in discovering pulsars and acknowledged her crucial part in the research, but he maintained that the Nobel Prize was deserved due to his own contributions. He defended the 1974 award given to him and Sir Martin Ryle by pointing out that he built the telescope, conducted crucial follow-up measurements, and that Bell herself had downplayed the controversy, stating she was not bitter and understood the reasons behind the decision at the time. 

Nevertheless, the denial of the Nobel Prize to Jocelyn Bell is a glaring example of the kind of gender bias people like her had to face even in those post-war reformist days. She spoke about this in an interview much later to El Pais (see here).