Do Photons Exist?
The answer is ambiguous.
First, from Wikipedia:
Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. An individual photon can be described as having right or left circular polarization, or a superposition of the two. Equivalently, a photon can be described as having horizontal or vertical linear polarization, or a superposition of the two.
Spin is the QM term for polarization.
A 'photon' is the QM description for one wavelength.
That wavelength is there whether we call it a photon or leave it as a wavelength having properties like polarization and intensity. The light wavelength can be polarized regardless of whether it is called spin.
This 'massless particle' is a QM concept that simply makes descriptions of some light behaviors easier.
For example the photoelectric effect, driven by a specific threshold wavelength for the target material, can be described easier by just using 'photon' for that threshold wavelength.
An absorption line is a similar behavior with a specific threshold wavelength for the target material or element. Because the wavelength is absorbed by an atom's electron changing its state, 'photon' is rarely used for an event with a lost wavelength.
With the photo electric effect the electron absorbing the threshold wavelength leaves the atom rather than just changing its state. The excess energy beyond the state change is passed to the electron in its kinetic energy so there is a minimum energy for the light higher than this wavelength to get an electron emission.
This observed threshold for energy to get an electron emission helped result in: 'a concept first put forward by Max Planck that light waves consist of tiny bundles or packets of energy known as photons or quanta.'
The energy in light is related to its frequency. Wikipedia's entry for photoelectric effect has a 'Diagram of the maximum kinetic energy as a function of the frequency of light on zinc'
A photon emitter device generates individual wave lengths, no particles.
A photon detector device detects a particular wave length, not a particle.
The term 'photon' is commonly accepted as one wavelength of 'the classical polarized sinusoidal plane electromagnetic wave.'
The term 'photon' is widely accepted (exists) but no actual photon 'particle' exists.
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