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).