The revolution of physics that Bill Shockley watched at Caltech not only undermined Newton’s orderly, rational and predictable universe, but stemmed from one of the most intriguing and beautiful arguments in physics: What is light? Is it a wave of something or a stream of particles? These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
The revolution of physics that Bill Shockley watched at Caltech not only undermined Newton’s orderly, rational and predictable universe, but stemmed from one of the most intriguing and beautiful arguments in physics: What is light? Is it a wave of something or a stream of particles? These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Scientific inquiry took millennia to ascertain the true nature of light, and it is not possible here to attempt to recapitulate all of the philosophizing, hypotheses, arguments, and experimentation involved in that great debate. The main point of controversy was whether light should be considered as a particle or as a wave. Empedocles of Acragas (492–432 b.c.e .) was of the opinion that light is a “streaming substance” emitted by luminous bodies, and that light travels at a finite speed ( Sambursky, 1958 ). In the seventeenth century, Isaac Newton postulated that light is composed of particles and that color is connected with the size of those particles; however, his contemporary Christiaan Huygens argued that light is a wave that transfers energy but not substance ( Einstein & Infeld, 1966 , p. 105ff). It turns out that light is indeed a wave propagating via electric and magnetic fields, but in addition to exhibiting properties of waves (such as interference) it also exhibits properties of particles (such as momentum and localization). When treating light as particles, the individual quanta are called photons .
Two years ago I indicated in this journal* [1] that the conclusions drawn from the formalism of quantum mechanics, and in particular from the uncertainty relations, [2] do not seem to necessarily indicate a failure of the causality principle, because the conceptual structures on which these conclusions are based stem from Newton’s mechanics and, like all of classical mechanics, are of empirical origin. In my opinion this reasoning only refutes applying those [classical] concepts to atomic processes. There is one argument in the discussion of this subject, however, that I had not commented on at the time, which initially sounds quite convincing. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
There is something profoundly amiss between the nature of reality and the ability of the human mind to comprehend it. The incongruity does not arise from mere ignorance, but from paradoxical observations. For instance, single electrons fired one at a time toward a pair of narrow slits can go through both slits simultaneously, as evidenced by the ensuing interference pattern. Yet, this pattern is absent if only a single slit is present or if the passage of the electron through the slits is recorded (Feynman, 1985)! This familiar peculiarity of the wave particle, and the fatal intrusiveness of observing it, is now further compounded by the demonstration that quantum mechanical effects can violate physical “locality” (Aspect et al., 1982; Bell, 1986); that is, the influence of observation can be conveyed at a speed exceeding the velocity of light. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Have you ever tried to define light? If so, your definition is probably related to vision, the ability to see things. The Merriam-Webster English dictionary defines light as “something that makes vision possible”. However, the definition of the “nature of light” is a complex term already discussed by philosophers of Ancient Greece. In the 17th century, the debate on the matter focused on whether light was a particle or a wave, based on the different properties of light that had been gradually discovered over time. Isaac Newton (1642−1727) was a defender of the corpuscular theory of light, which considered light to be formed by particles, i.e., small pieces of matter, like dust or grains of sand. He was the author of Opticks (1706), the front cover of which appears in Fig. 1.1. Christian Huygens (1629–1695), on the other hand, supported the theory that light is a wave. These are two completely different definitions of the same physical concept. A range of experiments were developed to attempt to clarify this dilemma, but it was only until 1900 when Max Planck (1858-1947) introduced the basis of a theory that would go on to revolutionize scientific thinking, marking the beginning of modern physics: quantum theory. Developed during the 20th century thanks to advances made by scientists such as Niels Bohr, Born, Heisenberg, Schrödinger, Pauli, Dirac, Einstein or de Broglie, quantum theory clarified that light is neither a particle nor a wave, but that it has a dual nature: it behaves like a wave as it spreads but like a particle in its interactions with matter. In this book, we will specify where necessary whether light behaves as a particle or as a wave, although in most cases we refer to rays of light, which always represent the direction of propagation of light.
Although many of the solitons described in previous chapters were discovered and studied in the 20th century, they could have been discovered and understood by physicists of the previous century. What I have in mind, of course, is the distinction between classical (19th century) and quantum physics (20th century). It happened that the quantum revolution in physics started at the frontier of the two centuries. At the end of 1900 the German physicist Max Planck (1858–1947) boldly introduced the notion of quanta of energy in the processes of emission of light by atoms. He also introduced a new fundamental constant in physics, the Planck constant h . The significance of this revolutionary step was not recognized until much later. Planck himself did not realize that he had started one of the greatest revolutions in physics. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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.