At radio wavelength "photons" are not really needed. The description of this part of the electromagentic spectrum as waves is completeley satisfying. In turn, in the range of cosmic radiation, the description of photons as "particles" is sufficient. However, this may not be intrinsic basic physics but caused by the choice of detectors. Radio detectors are solid metal rods - antennae. The energy density and the wavelength of a radio photon are much smaller than that of the antenna. In the range of cosmic radiation, the detectors are essentially single atoms. The energy density of such a photon is much higher, the wavelength much smaller. The primary process of photon detection at optical wavelengths detectors usually occcurs at a single atom, a limited group of atoms or a band in a solid state detector. There, the energy density is comparable to the energy density of the detected photon. Depending on the detailed conditions, a wave or a beam of particles is perceived.
At radio wavelength "photons" are not really needed. The description of this part of the electromagentic spectrum as waves is completeley satisfying. In turn, in the range of cosmic radiation, the description of photons as "particles" is sufficient. However, this may not be intrinsic basic physics but caused by the choice of detectors. Radio detectors are solid metal rods - antennae. The energy density and the wavelength of a radio photon are much smaller than that of the antenna. In the range of cosmic radiation, the detectors are essentially single atoms. The energy density of such a photon is much higher, the wavelength much smaller. The primary process of photon detection at optical wavelengths detectors usually occcurs at a single atom, a limited group of atoms or a band in a solid state detector. There, the energy density is comparable to the energy density of the detected photon. Depending on the detailed conditions, a wave or a beam of particles is perceived.
It has already been noted (section 2.8) that high frequency electromagnetic radiation can exhibit particle-like behaviour. For example, in the sensitive type of detector known as a photomultiplier tube (described in Wehr and Richards, Physics of the Atom , 2nd edn, Addison-Wesley, section 10.5), a beam of low-intensity X-rays is detected as a sequence of isolated sharp pulses, such as would be produced by a stream of particles arriving at the detector. As was pointed out, it is not surprising that it is usually high-frequency radiation which travels in particle form; as shown in figure 2.39, a wave group of high-frequency radiation can be compressed into a very short time span. However, when we speak of a particle, we mean something more than an entity which arrives in a short time span. A particle has a definite size, or more exactly it has a well-defined energy and momentum. It was discovered around about 1900 that the same is true of an X-ray wave packet: all packets of a given frequency radiation carry the same energy and momentum. This revolutionary discovery means that we must treat the wave packets as particles on exactly the same footing as more familiar particles such as electrons. The electromagnetic particle is called a photon . There is a wealth of experimental evidence which may be used to define the properties of a photon. We shall be highly selective, however, and simply present two particularly clear-cut experiments, the photoelectric effect and Compton scattering, which enable one to define the energy and momentum of the photon in terms of the frequency and wave number of the corresponding wave. These experiments will be described in detail in section 4.1.
Light absorption Whereas receiving radio waves is a macroscopic process and hence belongs to the area of classical electrodynamics – in a macroscopic antenna an electric voltage is induced whereby a large number of electrons follow the electric field strength of the incident wave, in a kind of collective motion – the detection of light, so far as the elementary process is concerned, takes place in microscopic type objects such as atoms and molecules. As a consequence, the response of an optical detector is determined by the microstructure of matter. In particular, it is impossible – due to the enormously high frequency of light (in the region of 10 15 Hz) – to measure the electric field strength. What is in fact detectable is the energy transfer from the radiation field to the atomic receiver, and this allows us to draw conclusions about the (instantaneous) intensity of light. We might ask what we can say about the above-mentioned absorption process from an experimentalist's point of view. Among the basic experiences that provide an insight into the structure of the micro-cosmos is the resonance character of the interaction between light and an atomic system. The atomic system, when hit by light, behaves like a resonator with certain resonance frequencies; i.e. it becomes excited (takes up energy) only when the light frequency coincides with a value that is characteristic for the particular atom. Hence, an incident light wave with an initial broadband frequency spectrum that has passed through a gas exhibits in its spectrum dark zones, the so-called absorption lines.
This paper explores the nature of the photon through the double-slit thought experiment, demonstrating that it is not a wave-particle duality. The essence of the photon is that of a pure particle. The double-slit thought experiment concerning photons is pivotal in proving that photons, along with other particles such as electrons, atoms, and macromolecules, are purely particulate in nature. Between the double-slit partition and the screen, photons do not produce coherence or the superposition of probability wave functions. Instead, they generate alternating bright and dark fringes through repeated oscillations between the slits and the detection screen. Within space, photons can interfere with each other's paths based on their momentum, simultaneously altering their trajectories and even their velocities. Thus, a beam of photons possessing greater momentum may carry along a portion of lower-momentum photons, propagating them over considerable distances. By examining the wavelength of electromagnetic waves, the intrinsic relationship between electromagnetic waves and photons is revealed. This establishes that electromagnetic waves are fundamentally pure particle-wave phenomena of photons. The wavelength of an electromagnetic wave essentially represents the spatial distance between consecutive photons as they advance in formation through space. The longer the wavelength of an electromagnetic wave, the greater the distance between successive photons, causing them to diverge more widely in space, as seen with radio waves. Conversely, shorter wavelengths result in closer spacing between photons, leading to more concentrated emission and a pronounced directionality, exemplified by gamma rays. Within the visible spectrum of electromagnetic waves, humans can occasionally transmit information bearing specific content via photons. This information, conveyed through photons, can traverse distances ranging from metres to thousands of kilometres, where it is received and discerned by another individual.
This paper explores the nature of the photon through the double-slit thought experiment, demonstrating that it is not a wave-particle duality. The essence of the photon is that of a pure particle. The double-slit thought experiment concerning photons is pivotal in proving that photons, along with other particles such as electrons, atoms, and macromolecules, are purely particulate in nature. Between the double-slit partition and the screen, photons do not produce coherence or the superposition of probability wave functions. Instead, they generate alternating bright and dark fringes through repeated oscillations between the slits and the detection screen. Within space, photons can interfere with each other's paths based on their momentum, simultaneously altering their trajectories and even their velocities. Thus, a beam of photons possessing greater momentum may carry along a portion of lower-momentum photons, propagating them over considerable distances. By examining the wavelength of electromagnetic waves, the intrinsic relationship between electromagnetic waves and photons is revealed. This establishes that electromagnetic waves are fundamentally pure particle-wave phenomena of photons. The wavelength of an electromagnetic wave essentially represents the spatial distance between consecutive photons as they advance in formation through space. The longer the wavelength of an electromagnetic wave, the greater the distance between successive photons, causing them to diverge more widely in space, as seen with radio waves. Conversely, shorter wavelengths result in closer spacing between photons, leading to more concentrated emission and a pronounced directionality, exemplified by gamma rays. Within the visible spectrum of electromagnetic waves, humans can occasionally transmit information bearing specific content via photons. This information, conveyed through photons, can traverse distances ranging from metres to thousands of kilometres, where it is received and discerned by another individual.
In a consistent description of the quantum measurement process, whether the wave or particlelike aspect of a system is revealed depends on the details of the measurement chain, and cannot be interpreted as an objective fact about the system independent of the measurement. We show precisely how this comes to be in the measurement of gravitational radiation. Whether a wave or particlelike aspect is revealed is a property of the detector employed at the end of the quantum measurement chain, rather than of the meter, such as a gravitational-wave (GW) antenna or resonant bar, used to couple the radiation to the detector. A linear detector yields no signal for radiation in a Fock state and a signal proportional to the amplitude in a coherent state—supporting a wavelike interpretation. In contrast, the signal from a detector coupled to the meter's energy is nonzero only when the incident radiation contains at least a single graviton. Thus, conceptually simple modifications of contemporary GW antennae can reveal wave-particle duality in the measurement of gravitational radiation.
Arguments about particles or waves gave way to a recognized need for both particles and waves in the description of radiation. The confusion over particle versus wave properties was resolved by Louis de Broglie. He postulated that since light has a dual wave–particle nature, matter also has this nature. The idea was to perform a diffraction experiment with electrons. An interference pattern was demonstrated not only with electrons, but also with many kinds of material particles like protons and neutrons. However, any attempt to observe an interference pattern with neutral atoms was found difficult by the fact that atoms carried no charge as electrons do and could not penetrate through condensate matter like neutrons. The de Broglie wavelength of an electron in the smallest orbit turns out to be exactly equal to the circumference of the orbit predicted by Bohr. The interpretation of the interference phenomenon becomes more subtle when the light beams are replaced by individual photons or particles.