The words ‘‘table’’ and ‘‘light,’’ as we ordinarily use them, are what we might call ‘‘classical words.’’ They refer to things that we imagine to have an objective reality between events of observation. We know from our experience with quantum mechanics that this imagined reality is only an approximation, usually a very good one. The word ‘‘photon,’’ however, is a ‘‘quantum word’’ that describes a phenomenon for which no classical work is adequate.
The words ‘‘table’’ and ‘‘light,’’ as we ordinarily use them, are what we might call ‘‘classical words.’’ They refer to things that we imagine to have an objective reality between events of observation. We know from our experience with quantum mechanics that this imagined reality is only an approximation, usually a very good one. The word ‘‘photon,’’ however, is a ‘‘quantum word’’ that describes a phenomenon for which no classical work is adequate.
How can we construct a picture of the photon from the wealth of observation material available to us? The photon appears to have a split personality: it is neither a wave nor a particle but something else which, depending on the experimental situation, exhibits a wave- or a particle-like behavior. In other words, in the photon (as in material particles such as the electron) the particle–wave dualism becomes manifest. Whereas classically the wave and the particle pictures are separate, quantum mechanics accomplishes a formal synthesis through a unified mathematical treatment. Let us look first at the wave aspect familiar from classical electrodynamics, which seems to be the most natural description. It makes all the different interference phenomena understandable, such as the “interference of the photon with itself” on the one hand and the appearance of spatial and temporal intensity correlations in a thermal radiation field on the other (which are obviously brought about by superposition of elementary waves emitted independently from different atoms). It might come as a surprise (at least for those having quantum mechanical preconceptions) that the classical theory is valid down to arbitrarily small intensities: the visibility of the interference pattern does not deteriorate even for very small intensities – the zero point fluctuations of the electromagnetic field advocated by quantum mechanics do not have a disturbing effect – and is valid not only for conventional interference experiments but also for interference between independently generated light beams (in the form of laser light).
An outline is given of the interpretation of quantum mechanical expressions if one assumes that what is called Quantum Mechanics today, refers to an underlying reality as in Chap. 2 , even if many authors claim to have evidence to the contrary, as is usually brought forward. If there exists a sub-microscopic classical world of real things, is that the same classical world as the world of large things such as planets, rocks and people? How does the notion of probability enter into quantum mechanics? 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.
Abstract The use of the word ‘photon’ to describe the quantum of electromagnetic radiation can lead to confusion and misunderstanding. It is often used in the context of interference experiments, for example Young’s slits, in such phrases as ‘which slit does the photon pass through?’ and ‘where do the photons hit the screen when one of the slits is covered up’. The impression is given of a fuzzy globule of light that travels this way or that way through pieces of optical equipment or that light beams consists of streams of the globules, like bullets from a machine gun. Lamb has even argued that there is no such thing as a photon [11 and he has proposed that the word should be used only under licence by properly qualified people! It is, however, difficult to disagree with some of his concerns.
Unlike special relativity, quantum mechanics was not the inspired product of one mind. It developed, in fits and starts, over a period of 25 years (1900–1925), and many cooks stirred the broth. Even now, a century later, quantum mechanics raises profound conceptual questions. Every competent physicist can “do” quantum mechanics, and the predictions it makes are in spectacularly good agreement with experimental results; there can be little doubt that quantum mechanics is “right.” On the other hand, as Richard Feynman once remarked, “nobody understands quantum mechanics.” I'll tell the story in two parts. First the history (how the essential pieces of the puzzle were assembled), and then the implications (what the finished picture has to say about our world). Photons Like relativity, quantum mechanics began with the study of light. In both cases, this historical association is misleading. Relativity, really, has nothing to do with light – to be sure, it involves that magic speed limit, c , and of course the most familiar thing that travels at speed c is light. So light is a convenient vehicle for introducing the subject. But it is perfectly possible to imagine a universe in which there is no such thing as light, and yet relativity might still be valid. Quantum mechanics, too, starts with light – in this case the “quantum” of light (the photon). But, as it turns out, this is just one rather specialized example of much more general principles – in fact, a rather bad example, because photons are by their nature relativistic (they travel at the speed of light), and it would be more natural to begin with the quantum mechanics of slow-moving objects, in analogy with Newtonian mechanics.
I will propose that the reality to which the quantum formalism implicitly refers is a kind of generalized history, the word history having here the same meaning as in the phrase sum-over-histories. This proposal confers a certain independence on the concept of event, and it modifies the rules of inference concerning events in order to resolve a contradiction between the idea of reality as a single history and the principle that events of zero measure cannot happen (the Kochen-Specker paradox being a classic expression of this contradiction). The so-called measurement problem is then solved if macroscopic events satisfy classical rules of inference, and this can in principle be decided by a calculation. The resulting conception of reality involves neither multiple worlds nor external observers. It is therefore suitable for quantum gravity in general and causal sets in particular. Quantum gravity and quantal reality Why, in our attempts to unify our theories of gravity and the quantum, has progress been so slow? One reason, no doubt, is that it's simply a very hard problem. Another is that we lack clear guidance from experiments or astronomical observations. But I believe that a third thing holding us back is that we haven't learned how to think clearly about the quantum world in itself, without reference to “observers” and other external agents. Because of this we don't really know how to think about the Planckian regime where quantum gravity is expected to be most relevant. We don’t know how to think about the vacuum on small scales, or about the inside of a black hole, or about the early universe. Nor do we have a way to pose questions about relativistic causality in such situations.
In quantum mechanics, light cannot be described in terms of waves and particles at the same time. However, the question about light "as it really is" implies a description that is independent from the constraints of measurement. It is rather preferable to incorporate both aspects in a causal ontological theory, especially if it simplifies explanation and improves predictions. Hence, the photon can be described as an extended longitudinal association of discrete indivisible constituents, which are held together by the effect of their own waves on each other. The same waves can then be used to interpret the force of interactions between different pulses of light, or between light and matter. The concept of wavemediated interaction between sub-photonic entities is entirely consistent with the observations of quantum mechanics. The key is to assume that elementary sources of waves never collide, and that they have constant parameters. Novel experimental implications follow from this approach. For example, the speed of propagation of changes within magnetic or electrostatic fields may not be equal to the speed of light. Also, interference fringes should be detectable beyond the volume of overlap of two non-entangled beams, but only in special experimental settings.