Although interferenceis intrinsically a classical wave phenomenon, the superposition principle whichunderlies all interference is also at the heart of quantum mechanics. Feynmanhas referred to interference as really “the only mystery” of quantum mechanics.Furthermore, in some interference experiments we encounter the idea of quantumentanglement, which has also been described as really the only quantum mystery.Clearly interference confronts us with some quite basic questions ofinterpretation. Despite its long history, going back to Thomas Young at thebeginning of the 19th century, optical interference still challenges ourunderstanding, and the last word on the subject probably has not yet beenwritten. With the development of experimental techniques for fast and sensitivemeasurements of light, it has become possible to carry out many of the Gedankenexperiments whose interpretation was widely debated in the 1920s and 1930s inthe course of the development of quantum mechanics. Although this articlefocuses entirely on experiments with light, interference has also been observedwith many kinds of material particles like electrons, neutrons, and atoms. For a given metal, there exists a certain minimum frequency of incidentradiation below which no photoelectrons are emitted. This frequency is calledthe threshold frequency. Increasing the frequency of the incident beam andkeeping the number of incident photons fixed increases the maximum kineticenergy of the photoelectrons emitted. The number of electrons emitted alsochanges because the probabilitythat each impacting photon results in an emitted electron is a function of thephoton energy. However, if just the intensity of the incident radiation isincreased, there is no effect on the kinetic energies of the photoelectrons.Ifthe photon energy is too low, the electron is unable to escape the material.Increasing the intensity of the light increases the number of photons in thebeam of light and thus increases the number of electrons excited but does notincrease the energy that each electron possesses. The energy of the emittedelectrons does not depend on the intensity of the incoming light (the number ofphotons), only on the energy or frequency of the individual photons. It isstrictly an interaction between the incident photon and the outermost electron.
Although interferenceis intrinsically a classical wave phenomenon, the superposition principle whichunderlies all interference is also at the heart of quantum mechanics. Feynmanhas referred to interference as really “the only mystery” of quantum mechanics.Furthermore, in some interference experiments we encounter the idea of quantumentanglement, which has also been described as really the only quantum mystery.Clearly interference confronts us with some quite basic questions ofinterpretation. Despite its long history, going back to Thomas Young at thebeginning of the 19th century, optical interference still challenges ourunderstanding, and the last word on the subject probably has not yet beenwritten. With the development of experimental techniques for fast and sensitivemeasurements of light, it has become possible to carry out many of the Gedankenexperiments whose interpretation was widely debated in the 1920s and 1930s inthe course of the development of quantum mechanics. Although this articlefocuses entirely on experiments with light, interference has also been observedwith many kinds of material particles like electrons, neutrons, and atoms. For a given metal, there exists a certain minimum frequency of incidentradiation below which no photoelectrons are emitted. This frequency is calledthe threshold frequency. Increasing the frequency of the incident beam andkeeping the number of incident photons fixed increases the maximum kineticenergy of the photoelectrons emitted. The number of electrons emitted alsochanges because the probabilitythat each impacting photon results in an emitted electron is a function of thephoton energy. However, if just the intensity of the incident radiation isincreased, there is no effect on the kinetic energies of the photoelectrons.Ifthe photon energy is too low, the electron is unable to escape the material.Increasing the intensity of the light increases the number of photons in thebeam of light and thus increases the number of electrons excited but does notincrease the energy that each electron possesses. The energy of the emittedelectrons does not depend on the intensity of the incoming light (the number ofphotons), only on the energy or frequency of the individual photons. It isstrictly an interaction between the incident photon and the outermost electron.
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).
The idea of the photon has stirred the imaginations of physicists ever since 1905 when Einstein originally proposed the use of light quanta to explain the photoelectric effect. This concept is formalized in the quantum theory of radiation, which has had unfailing success in explaining the interaction of electromagnetic radiation with matter, seemingly limited only by the ability of physicists to perform the indicated calculations. Nevertheless, it has its conceptual problems—various infinities and frequent misinterpretations. Consequently an increasing number of workers are asking, “to what extent is the quantized field really necessary and useful?” In fact the experimental results of the photoelectric effect were explained by G. Wentzel in 1927 without the quantum theory of radiation. Similarly most electro-optic phenomena such as stimulated emission, reaction of the emitted field on the emitting atom, resonance fluorescence, and so on, do not require the quantization of the field for their explanation. As we will see, these processes can all be quantitatively explained and physically understood in terms of the semiclassical theory of the matter–field interaction in which the electric field is treated classically while the atoms obey the laws of quantum mechanics. The quantized field is fundamentally required for accurate descriptions of certain processes involving fluctuations in the electromagnetic field: for example, spontaneous emission, the Lamb shift, the anomalous magnetic moment of the electron, and certain aspects of blackbody radiation. (The Compton effect also fits here, but see later under references 8b and c.) Here we will outline how the photon concept originated and developed, where it is not required and is often misused, and finally where it plays an essential role in the understanding of physical phenomena. In our discussion we will attempt to give a logically consistent definition of the word “photon”—a statement far more necessary than one might think, for so many contradictory uses exist of this elusive beast. In particular consider the original coining of the word by G. N. Lewis: “[because it appears to spend] only a minute fraction of its existence as a carrier of radiant energy, while the rest of the time it remains an important structural element within the atom…, I therefore take the liberty of proposing for this hypothetical new atom which is not light but plays an essential part in every process of radiation, the name photon!” (our exclamation point). Clearly the present usage of the word is very different.
The puzzles of quantum mechanics are: 1. what is the medium for the photon and for the electron wave? i.e. what is waving? 2. particle interference pattern 3. observer effect. 4. entanglement. In Quantum Erasure and Double-Slit experiments, how does the emitter know to direct the photon two both slits or only to one slit? And how does the detector know where to detect the photons to form an interference pattern and not a bell-shaped (Gaussian) pattern or vice versa? This paper proposes a few, closely related, simple and subtle laws of nature governing quantum phenomena. Some of the findings in this paper are: 1. A photon is emitted in two steps: the creation of photon path and the emission of the photon 2. Although (almost) all of the energy of the photon is localized in space, the photon extends to infinity. An extremely small energy of the photon, which we call the 'precursor wave' in this paper, extends to infinity. The instant the photon is emitted, its precursor wave creates a path to infinity; it appears instantaneously in all its path to infinity, with infinitely small energy density. The precursor wave serves as the 'highway' for the propagation of the main energy packet. However, to conserve the principle of causality, we assume that the tip of the photon propagates at a speed billions times the speed of light. Thus, there are two speeds associated with the photon: the speed we know as c and the speed at which the photon length increases. 2. A photon will be emitted only towards an absorber. A photon is emitted by the interaction of the emitter and a detector. It is obvious that a photon will not be emitted if there is no emitter. Nor will a photon be emitted if there is no detector . Once a photon is emitted, however, it exists objectively, autonomously. The photon is emitted only after its path is determined by the interaction of the emitter and the absorber, at the instant of emission. 3. The photon energy will be concentrated where there is higher wave amplitude, i.e higher intensity of oscillation of electric and magnetic fields. This results in 'dragging' of the electromagnetic energy by the wave, hence eliminating the need for any medium. It is also proposed that single photon interference is a quantum phenomenon whereas two photon interference is a classical phenomenon. Similar laws hold for the electron. The 'medium' for the electron wave is the electron (the electron mass density field) itself. Where there is higher intensity of electron wave/oscillation, there will be higher electron mass density. This means that the electron wave 'drags' the electron with itself. Therefore, no exotic 'medium' is required for the photon and for the electron wave.
Photons and electrons One of the great technical achievements of the second half of the 20th century was the invention of the laser. With its help extremely intense light could be produced. Furthermore, it even became posssible to excite a single resonator mode so that both the frequency and the propagation direction of the emitted radiation field are extraordinarily sharp. The mere existence of such high-intensity single-mode fields – the real problem that was ingeniously solved was to generate them – is no matter of surprise. We know from classical electromagnetic theory that the amplitude of a monochromatic electromagnetic field can take on arbitrarily large values. This is a direct consequence of the linearity of Maxwell's equations. What does this mean for quantized radiation fields? Certainly, the mentioned property of classical fields must also be retained in the quantum mechanical description. Since the electromagnetic energy is quantized in ‘packets’ of magnitude h ν (photons), the number of photons in a given mode must be unlimited, in principle. All this is very well known. However, it becomes noteworthy when we compare the behaviour of photons with that of electrons. Amazingly, it turns out that an electronic quantum state can be occupied by one electron, at maximum. (Strictly speaking, in the description of the electronic state the spin variable must be included.) This drastic difference in the collective behaviour of photons and electrons could, in fact, not be expected from what we know from single photons and electrons. Both of them have particle-like as well as wave-like properties. However, when many of them are present, they exhibit quite different a demeanour.
The double slit interference experiment has been famously described by Richard Feynman as containing the "only mystery of quantum mechanics". The history of quantum mechanics is intimately linked with the discovery of the dual nature of matter and radiation. While the double slit experiment for light is easily undertsood in terms of its wave nature, the very same experiment for particles like the electron is somewhat more difficult to comprehend. By the 1920s it was firmly established that electrons have a wave nature. However, for a very long time, most discussions pertaining to interference experiments for particles were merely gedanken experiments. It took almost six decades after the establishment of its wave nature to carry out a 'double slit interference' experiment for electrons. This set the stage for interference experiments with larger particles. In the last decade there has been spectacular progress in matter-wave interefernce experiments. Today, molecules with over a hundred atoms can be made to interfere. In the following we discuss some of these exciting developments which probe new regimes of Nature, bringing us closer to the heart of quantum mechanics and its hidden mysteries.
In 1905, Albert Einstein showed that the photoelectric effect could be explained by the hypothesis that the energy of a light beam was distributed in discrete bundles, later known as photons. Photon-counting experiments provide a fairly direct measurement of the photon probability distribution for all kinds of light embraced by the quantum theory. The field of quantum optics occupies a central position involving the interaction of atoms with the electromagnetic field. Young's experiment is one example of a phenomenon that can be explained by both classical and quantum theory. In Young's experiment, each photon must be capable of interfering with itself in such a way that its probability of striking the second screen at a particular point is proportional to the calculated intensity at that point. Multiphoton transitions are found whenever electrons bound in atoms or molecules interact with sufficiently intense electromagnetic radiation.