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The Waviness of Light

A Lecture for John Little's Retirement
Richard M Sillitto

Delivered in the University of Strathclyde, in 1986
at an event chaired by Richard Thornley

to Dick

     First, thank you for allowing me to participate in your tribute to John Little.  I've known John since 1946, when he was an undergraduate in Edinburgh, and I returned from war service as a research student.  Since then we've kept in touch, and I very much hope that in these 40 years John may have learnt a little from me - I'm acutely conscious of having learnt a lot from him, and I'd like to feel that at least a small fraction of my debt to John has been cancelled out.

I'd like to tell you a little story about John which has more to do with my purpose here today than may appear at first sight.

One Saturday morning, many many years ago, my wife and I were walking down the North Bridge in Edinburgh.  In those days the Bridges was one of Edinburgh's principal shopping streets, and Patrick Thompson's ("P.T's"), along whose frontage we were walking, encapsulated the standards and aspirations of comfortable, middle-class Edinburgh.

Ahead of us we were faintly aware of an unusual pattern of movement on the crowded pavement.  As we drew closer, we saw, above the crowd, a familiar head with a shock of black hair, apparently moving to and fro across the crowd in simple harmonic motion.  Soon we could see that this apparent SHM was in fact the projection, perpendicular to our line of sight, of a uniform circular motion - how tolerant those other pedestrians were!  When we were within hailing distance I was just about to speak to John when he, noticing me, pointed - as he gyrated - to a notice in the shop window which read:
Walk round.
Looking back on that event, it seems to me that this quite typifies John's approach to discourse - on science and all those other topics which catch his attention.

Discussing any topic with John, you're liable to find that after an amiable start, an element of cross-purposes seems to appear.  John has started to walk round the subject, and is seeing it now from a standpoint different from the one you initially shared.  You have to choose whether to stay put and allow the cross-purposes to get crosser, or to follow him round, and share his experience of the diversity of the subject.  It's reminiscent of the work of the French Cubist painters early in this century who discovered the trick of displaying in a single picture aspects of the subject which can be seen only from a number of different viewpoints.  Once you accept the apparent incongruity of such compositions, you realise that the painter is showing us more about the subject than the static observer can hope to perceive.
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There are difficulties about this combination of apparently incompatible aspects.  The holographer who tries to reconstruct the real image from a Fresnel hologram with too wide a beam, overlays on the screen different views of the subject, and generates diffuseness; whereas the painter can separate them spatially, and combine them into a harmonious composition.  Scott Fitzgerald, the young American writer of the 1920s, wrote that it was a mark of genius "to be able to hold in the mind simultaneously two contrary views, ... and continue to function."

But quantum physics requires us to do just this, and - like the Cubists? - has devised formal procedures for doing it - they comprise what's called "the transformation theory".

When I was asked to review the developments in some branch of physics during John's working life, it seemed to me that I could link John's interest in the superficially paradoxical view of nature which quantum theory challenges us with to my abiding interest in optics which has been, in this period, one of the most exciting and most spectacularly developing branches of physics.

Of course, this has been an exciting period of progress in many fields. In society we have seen
the decline of the safety pin and the rise of the Velcro fastener
the decline of the picture palace and the rise of the VTR
the decline of spelling and the rise of the word processor
the decline of the Scottish educational system ... ? and the rise of 'trivial pursuits'.
In optics we have seen - I hope and trust - the decline of the photon and a deeper understanding of the wave-picture: there may be room for photons in high-energy physics, but they've been a confounded nuisance, and at times a source of bad temper and destructive cross-purposes, in optics.

We've seen the development of coherence theory, and out of it, part way along its progress, an elegant, and powerful, and profoundly satisfying discipline of quantum optics.  We've seen the development of the laser, of non-linear optics, of the optical image processor, of phase-conjugate optical devices, any minute now the optical computer;   on the High Street, the laser light shows at Christmas, and shops overflowing with binoculars, cameras, zoom lenses and what-not at prices which, in real terms, are trivial compared with those of 1946.

But what I want to share my thoughts with you about is not optical hardware, but how the way we think about light has developed over the past 40 years.  Much of what I say - in particular the last part, will relate to work by an old friend E. Wolf, whom John I know, remembers as one of his teachers in Edinburgh.