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> ... one might have been tempted to adopt the hypothesis that we are dealing here with a new type of atom, an identifiable entity, uncreatable and indestructible, which acts as the carrier of radiant energy and, after absorption, persists as an essential constituent of the absorbing atom until it is later sent out again bearing a new amount of energy...

> 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.

http://www.nobeliefs.com/photon.htm

http://dx.doi.org/10.1038/118874a0

As far as I know, this original meaning of the word photon is not used anymore, because all the modern variants allow for creation and destruction of photons.

[...] one might have been tempted to adopt the hypothesis that we are dealing here with a new type of atom, an identifiable entity, uncreatable and indestructible, which acts as the carrier of radiant energy and, after absorption, persists as an essential constituent of the absorbing atom until it is later sent out again bearing a new amount of energy [...]
–"The origin of the word "photon""

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.
–"The Conservation of Photons" (1926-12-18)

As far as I know, this original meaning of the word photon is not used anymore, because all the modern variants allow for creation and destruction of photons.

The word photon is one of the most misused word in physics. Much more than other words, it is being used with several meanings and one can only try to find which one is meant based on the source and context of the message. The photon that spectroscopy experimenter uses to explain how spectra are connected to the atoms and molecules is a different concept from the photon quantum optics experimenters talk about when explaining their experiments, and that is different from the photon that the high energy experimenters talk about and that is different from the photon high energy theorists talk about. There are probably more variants in use (not counting personal modifications).

> ... one might have been tempted to adopt the hypothesis that we are dealing here with a new type of atom, an identifiable entity, uncreatable and indestructible, which acts as the carrier of radiant energy and, after absorption, persists as an essential constituent of the absorbing atom until it is later sent out again bearing a new amount of energy...

> 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.

The photon the visible-UV spectroscopy experimenter usually talks about is an object that has definite frequency $\nu$ and energy $h\nu$, its size and position are unknown, perhaps undefined; yet it can be absorbed and emitted by a molecule.

The photon the quantum optics experimenter usually talks about is a purposely mysterious "quantum object" that has no definite frequency, has somewhat defined position and size, but can span whole experimental apparatus and only looks like a localized particle when it gets detected in a light detector.

The photon the high energy experimenter talks about is a small particle that is not possible to see in photos of the particle tracks and their scattering events, but serves well to explain the curvature of tracks of matter particles with common point of origin within the framework of energy and momentum conservation. This photon has usually definite momentum and energy (hence also definite frequency), and fairly definite position, since it participates in the localized scattering events.

Theorists use the word photon with several meanings as well. The common denominator is the mathematics used to describe electromagnetic field and its interaction with matter. Certain special quantum states of EM field - so-called Fock states - behave mathematically in a way that allows one to use the language of "photons as countable things with definite energy" and count how many of them are in a certain state to give one information on the total energy of the system and frequency distribution of this energy.

In the general case, the state of the EM field is not of such a special kind, and the number of photons itself is not definite. This means the primary object of the mathematical theory of EM field is not a set of point particles, but the continuous EM field and photons are merely some kind of emergent phenomenon, when the field is of a special kind.

• It is quite entrenched in the curriculum and textbooks for historical and inertia reasons;

• Experimenters use it to describe their experiments;

• partially because it makes a good impression on people reading popular accounts of physics; it is hard to talk interestingly about $\psi$ function or the Fock space, but it is easy to talk about "particles of light";

• Partially because of how the Feynman diagram method is taught.

Note on the necessity of the concept of photon

Many famous experiments once regarded as evidence for photons were later explained qualitatively or semi-quantitatively based solely based on the theory of waves (classical EM theory of light, sometimes with Schroedinger's equation added). These are for example the photoelectric effect, Compton scattering, black-body radiation and perhaps others.

There always was a minority group of physicists who avoided the concept of photon altogether for this kind of phenomena and preferred the idea that the possibilities of EM theory are not exhausted. Check out these papers for non-photon approaches to physics:

R. Kidd, J. Ardini, A. Anton, Evolution of the modern photon, Am. J. Phys. 57, 27 (1989) http://www.optica.machorro.net/Lecturas/ModernPhoton_AJP000027.pdf

C. V. Raman, A classical derivation of the Compton effect. Indian Journal of Physics, 3, 357-369. (1928) http://dspace.rri.res.in/jspui/bitstream/2289/2125/1/1928%20IJP%20V3%20p357-369.pdf

Trevor W. Marshall, Emilio Santos: The myth of the photon, Arxiv (1997) http://arxiv.org/abs/quant-ph/9711046v1

Timothy H. Boyer, Derivation of the Blackbody Radiation Spectrum without Quantum Assumptions, Phys. Rev. 182, 1374 (1969)

http://dx.doi.org/10.1103/PhysRev.182.1374

The word photon is one of the most confusing and misused words in physics. Probably much more than other words in physics, it is being used with several different meanings and one can only try to find which one is meant based on the source and context of the message. 

The photon that spectroscopy experimenter uses to explain how spectra are connected to the atoms and molecules is a different concept from the photon quantum optics experimenters talk about when explaining their experiments. Those are different from the photon that the high energy experimenters talk about and there are still other photons the high energy theorists talk about. There are probably even more variants (and countless personal modifications) in use.

> ... one might have been tempted to adopt the hypothesis that we are dealing here with a new type of atom, an identifiable entity, uncreatable and indestructible, which acts as the carrier of radiant energy and, after absorption, persists as an essential constituent of the absorbing atom until it is later sent out again bearing a new amount of energy...

> 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.

The photon the experimenter in visible-UV spectroscopy usually talks about is an object that has definite frequency $\nu$ and definite energy $h\nu$; its size and position are unknown, perhaps undefined; yet it can be absorbed and emitted by a molecule.

The photon the experimenter in quantum optics (detection correlation studies) usually talks about is a purposely mysterious "quantum object" that is more complicated: it has no definite frequency, has somewhat defined position and size, but can span whole experimental apparatus and only looks like a localized particle when it gets detected in a light detector.

The photon the high energy experimenter talks about is a small particle that is not possible to see in photos of the particle tracks and their scattering events, but makes it easy to explain the curvature of tracks of matter particles with common point of origin within the framework of energy and momentum conservation (e. g. appearance of pair of oppositely charged particles, or the Compton scattering). This photon has usually definite momentum and energy (hence also definite frequency), and fairly definite position, since it participates in fairly localized scattering events.

Theorists use the word photon with several meanings as well. The common denominator is the mathematics used to describe electromagnetic field and its interaction with matter. Certain special quantum states of EM field - so-called Fock states - behave mathematically in a way that allows one to use the language of "photons as countable things with definite energy". More precisely, there are states of the EM field that can be specified by stating an infinite set of non-negative whole numbers. When one of these numbers change by one, this is described by a figure of speech as "creation of photon" or "destruction of photon". This way of describing state allows one to easily calculate the total energy of the system and its frequency distribution. However, this kind of photon cannot be localized except to the whole system.

In the general case, the state of the EM field is not of such a special kind, and the number of photons itself is not definite. This means the primary object of the mathematical theory of EM field is not a set of point particles with definite number of members, but a continuous EM field. Photons are merely a figure of speech useful when the field is of a special kind.

• it is quite entrenched in the curriculum and textbooks for historical and inertia reasons;

• experimenters use it to describe their experiments;

• partially because it makes a good impression on people reading popular accounts of physics; it is hard to talk interestingly about $\psi$ function or the Fock space, but it is easy to talk about "particles of light";

• partially because of how the Feynman diagram method is taught.

Theorists use the word photon with several meanings as well. The common denominator is the mathematics used to describe electromagnetic field and its interaction with matter. Certain special quantum states of EM field - so-called Fock states - behave mathematically in a way that allows one to use the language of "photons as countable things with definite energy" and count how many of them are in certain state to give one an information on the total energy of the system and frequency distribution of this energy.

In general case, state of the EM field is not of such special kind and the number of photons itself is not definite. This means the primary object of the mathematical theory of EM field is not a set of point particles, but the continuous EM field and photons are merely some kind of emergent phenomenon, when the field is of a special kind.

• it is quite entrenched in the curriculum and textbooks for historical and inertia reasons;

• experimenters use it to describe their experiments;

• partially because it makes good impression on people reading popular accounts of physics; it is hard to talk interestingly about $\psi$ function or the Fock space, but it is easy to talk about "particles of light";

• partially because of how the Feynman diagram method is taught.

(In the Feynman diagram, wavy line in spacetime is often introduced as representing a photon. But these diagrams are a calculational aid for perturbation theory for complicated field equations; the wavy line in the Feynman diagram does not necessarily represent actual point particle moving through spacetime. The diagram, together with the photon it refers to, is just a useful graphical representation of certain complicated integrals.)

Many famous experiments once regarded as evidence for photons were later explained qualitatively or semi-quantitatively based solely based on the theory of waves (classical EM theory of light, sometimes with Schroedinger's equation added). These are for example photoelectric effect, Compton scattering, black body radiation and perhaps others.

There always was a minority group of physicists who avoided the concept of photon altogether for this kind of phenomena and preferred the idea that possibilities of EM theory are not exhausted. Check out these papers for non-photon approaches to physics:

Theorists use the word photon with several meanings as well. The common denominator is the mathematics used to describe electromagnetic field and its interaction with matter. Certain special quantum states of EM field - so-called Fock states - behave mathematically in a way that allows one to use the language of "photons as countable things with definite energy" and count how many of them are in a certain state to give one information on the total energy of the system and frequency distribution of this energy.

In the general case, the state of the EM field is not of such a special kind, and the number of photons itself is not definite. This means the primary object of the mathematical theory of EM field is not a set of point particles, but the continuous EM field and photons are merely some kind of emergent phenomenon, when the field is of a special kind.

• It is quite entrenched in the curriculum and textbooks for historical and inertia reasons;

• Experimenters use it to describe their experiments;

• partially because it makes a good impression on people reading popular accounts of physics; it is hard to talk interestingly about $\psi$ function or the Fock space, but it is easy to talk about "particles of light";

• Partially because of how the Feynman diagram method is taught.

(In the Feynman diagram, a wavy line in spacetime is often introduced as representing a photon. But these diagrams are a calculational aid for perturbation theory for complicated field equations; the wavy line in the Feynman diagram does not necessarily represent actual point particle moving through spacetime. The diagram, together with the photon it refers to, is just a useful graphical representation of certain complicated integrals.)

Many famous experiments once regarded as evidence for photons were later explained qualitatively or semi-quantitatively based solely based on the theory of waves (classical EM theory of light, sometimes with Schroedinger's equation added). These are for example the photoelectric effect, Compton scattering, black-body radiation and perhaps others.

There always was a minority group of physicists who avoided the concept of photon altogether for this kind of phenomena and preferred the idea that the possibilities of EM theory are not exhausted. Check out these papers for non-photon approaches to physics: