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This year marks the 50th anniversary since the first scientific measurements were produced with the Narrabri Stellar Intensity Interferometer, which was constructed in the early 1960’s by Robert Hanbury Brown and Richard Twiss. A collaboration between the Universities of Sydney and Manchester, the interferometer was the culmination of a series of experiments which pioneered the technique of intensity interferometry. The immediate controversy surrounding the quantum implications of the technique enveloped some of the most eminent physicists of the day, sparking a debate about nonlocal effects and optical coherence. A full explanation of the workings of the intensity interferometer in a quantum context was finally put forward by Roy Glauber, ultimately earning him the 2005 Nobel Prize in Physics. The intensity interferometer rekindled the field of high resolution stellar imaging, which had been extinguished for a half century (following the failure of Pease’s 50-foot beam on Mt Wilson), while delivering the first ever measurements of the sizes of normal stars – establishing an effective temperature scaling relationship which has underpinned stellar astronomy for 50 years. This directly paved the way for the next generation of Michelson Stellar Interferometers. Intensity interferometry itself has found application in several fields (notably particle physics), and plans are in active development for modern reprises within stellar interferometry. However undoubtedly the greatest legacy lies in the Hanbury Brown Twiss (HBT) effect being the foundational experiment for what is now known as Quantum Optics – a field which underpins a huge sector of the technology which enables our modern world. This invited review discuses the development of the interferometer, including the controversy that its underlying principles generated within the contemporary physics community. The core scientific output generated by the instrument is presented, together with the impact of the device upon the subsequent course of stellar astrophysics and its role in resurrecting stellar optical interferometery.
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