HANBURY BROWN, Robert (#196)

He is perhaps best known for his invention of the optical intensity interferometer. This was not only a brilliant conception in itself, but also proved a crucial factor in the advancement of the understanding of fundamental physical optics.

The Intensity Interferometer

Robert Hanbury Brown (always known as Hanbury) was born in India on August 31 1916, and educated at Tonbridge and Brighton Technical College. He went on to London University, where he took a BSc in Engineering in 1935.

As one of the brightest young engineers of the time, he was invited to join the original team working with Sir Robert Watson-Watt on radar development at the Air Ministry research station at Bawdsey Manor (1936-42). Here he helped develop the first airborne radars for night-fighting and the detection of ships and submarines.

Young Robert Handbury Brown

His contributions included his work on the polarisation of radio waves, crucial in determining the optimum configuration of the radar aerials on all the early air-to-surface equipment operated by Coastal Command.

Without his discovery, the range of detection of a surfaced submarine in a rough sea would have been negligible, with catastrophic consequences for the British merchant fleet in the first years of the war.

In 1942 Hanbury Brown was seconded to the US Naval Research Laboratory in Washington as assistant head of the Combined Research Group; here he worked, again in conditions of great secrecy, on America’s expanding airborne radar programme.

Hanbury Brown left government work in 1947 and, after a short period as an engineering consultant, went to Manchester University to join Bernard Lovell who remembered him from his own radar days.

Lovell later wrote that Hanbury Brown came “like a gift from heaven”, and described how he made an immediate positive impression on the senior members of the university administration and the external sources from whom they were demanding funding.

These were the early days in the development of Jodrell Bank. Hanbury Brown himself made astronomical measurements using the 218ft diameter fixed paraboloidal dish, and was thrilled by the detection of radio emission from our own galaxy with a sensitive receiver he himself had built and installed.

With Cyril Hazard he succeeded in detecting similar emissions from the nearest large spiral galaxy to ours, the Andromeda Galaxy, a remarkable achievement by the standards of the day. This work was a pivotal demonstration of the need for a steerable narrow-beam radio dish, and paved the way for what became the famous 250ft Mark I telescope.

While still in his first year at Jodrell, Hanbury Brown began to contemplate the design of a radio interferometer which would solve the intriguing problem of measuring the angular sizes of the two most prominent radio sources in the sky, known as Cygnus A and Cassiopeia A.

For all one knew at the time, these might have been extremely compact, and it did not seem feasible to build a practical interferometer with the necessary baseline, which might have to extend to thousands of kilometres.

The problem was the technical difficulty of combining the radio signals received at two widely separated receivers with the required stability of phase.

Hanbury Brown came up with the idea of measuring the correlation of intensity fluctuations, a process not requiring the phase information necessary for signal amplitude detection. This seemingly simple solution was in fact deeply profound, and he sought the help of Richard Twiss to put it on a firm mathematical basis.

All went well, and in surprisingly little time Hanbury Brown built the interferometer; in 1952 he successfully used it to make the measurements he was seeking. It turned out that the sources proved to be so large that they could be resolved with baselines of only a few kilometres – and he commented that he had used “a sledgehammer to crack a nut”.

Nevertheless, the technique had been shown to work, and its general advantages (a crucial one being that it was tolerant of a turbulent atmosphere) led him and Twiss to apply its use in the even more challenging optical domain to solve the classical problem of measuring the tiny angular sizes of visible stars.

In principle, the theory is the same for all wavelengths; but in practice, working in the radio and optical domains is quite different. Hanbury Brown, with Twiss, proceeded to establish the detailed theory of an optical interferometer, and immediately ran into a barrage of objections, claiming the violation of aspects of established physical knowledge.

But the two men were sure of their ground. As Hanbury Brown observed, trying to explain the paradoxical nature of light is an activity “analogous to preaching the Athanasian Creed”.

They built a simple optical intensity interferometer as a pilot model, using two former Army searchlight mirrors as light collectors, and successfully demonstrated the technique with the measurement of the angular diameter of Sirius.

The way was then clear to plan a sophisticated interferometer which would measure a significant number of stars. Hanbury Brown proceeded to design and build a device with mirrors 23ft in diameter moving on a circular railway 600ft across.

This was a vast instrument compared with the conventional telescopes then available at optical observatories. The need for clearer skies than those of Cheshire, and the fact that Twiss had moved to Australia, led eventually to the shipment of the new interferometer to New South Wales, where it was erected in a suitably remote spot at Narrabri.

Hanbury Brown had been appointed Professor of Radio-Astronomy at Manchester University in 1960. After taking several years leave in Australia to establish the interferometer, in 1964 he accepted the Professorship of Physics (Astronomy) at Sydney University.

Setting up and using the complex instrumentation in the difficult conditions at Narrabri was not easy; it is a tribute to Hanbury Brown’s single-mindedness that he, with his team, overcame high temperatures, floods, and the attentions of insects, animals and birds to produce a series of enduring astronomical results of great importance for the progress of scientific research.

In 1991 Hanbury Brown published his autobiography, Boffin: A Personal Story of the Early Days of Radar, Radio Astronomy and Quantum Optics. He wrote many other books, including Photons, Galaxies and Stars (1985) and Wisdom of Science (1986). In all of them his love of science and his humanity and humour shine through.

Hanbury Brown was held in affectionate respect by the international astronomical community, receiving many scientific awards. He was President of the International Astronomical Union from 1982 to 1985.

Radio astronomers from the 1952 URSI Congress shown at the Hornsby Valley field station. Alex Shain is on the extreme left, seated between Robert Hanbury-Brown and Francis Graham Smith. Opposite them (from left to right) are: Joe Warburton, Marius Laffineur, Charlie Higgins, Chris Christiansen and Jim Hindman. The antennas in the background formed part of the 9.15 MHz array (courtesy: CSIRO RAIA 2842-131)

After retirement he settled in Hampshire, where he continued to write.

He married, in 1952, Heather Chesterman; they had three children. (Obituary courtesy of the Daily Telegraph)