Chapter Ten. What Is Light, Really?

Brian Culshaw

The question ‘is light a ray, a wave or a particle?’ has long fascinated philosophers and has intensified since the concept of the photon first emerged at the beginning of the twentieth century. The pragmatic answer to the question is ‘it depends what you are looking for’ and is really about how best to conceptualise light in particular circumstances. We have already mentioned such distinctions briefly in Chapter 1. This chapter will extend these basic concepts to introduce the material which follows in later chapters.

David A. Grandy

Marta García-Matos, Lluís Torner

“The sun shone, having no alternative, on the nothing new.” Samuel Beckett's acclaimed opening words for Murphy appear to be somehow pessimistic. Yet actually, quite often, a distinctive control over light beams puts forward a path to the new. This book takes a glance over some of those paths. Exploring the natural phenomena around us and across the Universe; pushing the limits of our understanding of Nature; and using the knowledge acquired to diagnose and cure disease, and to create devices and machines that make life safer, healthier, and more fulfilling is a program that many, throughout history, have found as captivating as compelling. Advancing such programs requires tools, which must be more and more sophisticated as the limits of knowledge move farther from our own scale and intuition. Really small or really large things, as well as really fast or really slow events, are particularly challenging, as neither our senses nor our ordinary gadgets are equipped to tackle them. We need powerful tools and technologies to enter such territories. The farther we aim to reach, the higher the performances that our toolkit must deliver. Light is one of these wonderful tools. It is ubiquitous and universal, and can be outstandingly accurate and precise. During the past decades we have learnt not only how to generate and control light in exquisite ways – especially since the invention of the laser half a century ago – but also how to transmit it and display it in ways that used to be the realm of science fiction and novels. As a result, light-based technologies are, literally, everywhere. And what is available today is just the beginning. The book consists of sixteen chapters of the same length and structure, each addressing a particular scientific and technological challenge in which some of the multifaceted existing light–matter interactions take a leading role. Readers can go through the chapters following any order they want. Each chapter opens with a short story that aims at motivating a context in which the overall challenge to be addressed has an impact.

David W. Snoke

After having shown in previous chapters that wave-particle duality is not a fundamental problem for quantum mechanics, this chapter introduces the really strange effect of quantum mechanics, namely “nonlocal correlations” that appear to act over long distances faster than the speed of light. The “Copenhagen” interpretation of quantum mechanics is introduced, which puts human knowledge in a special role, and some of the philosophical objections to it.

Rudolf Kingslake