Apologies if this has been asked before (I did check and I believe it wasn't). I have a question about the particle/wave duality of photons (or other particles). Depending on what and how we measure the photon turns out to be either a wave or a particle. Recently I saw some web page (and I can't remember where) that maybe neither is true. Both the wave perception and the particle perception are just that: perceptions, strengthened by our perhaps limited ability to observe. What if the reality of the photon is something else, something "on top" of our two perceptions?? Could someone direct me to a site that could tell me more about it (or maybe debunk the whole idea?)?
This is the idea: when you see the high school equations that describe a parabolic motion, you consequently visualize a flying stone in your mind. Physics does not care about what you imagine, but rather it deals with the equations and their ability to tell you the distance the stone will reach. Because the equations were invented and tuned for that purpose, the numeric results of that mathematical tricks are equal to what happens in nature.
Around the 1930s, some men eventually managed to build a consistent sets of mathematical tricks that correctly give the same results as nature, when dealing with a wide range of problems for which the known theories had failed. It is known as Quantum Mechanics. But, unlike the high school parabolic shot, the equations of QM are there but nobody can imagine a graphical representation.
Whatever it may be happening down there, there is no possible way of depicting it in your human brain. It is just so. You may spend your life thinking about a wave as a transparent diffuse entity that suddenly feels observed and transforms into a tiny hard ball... But the result will be nothing but a headache. With Quantum Mechanics you see the equations and check the result, but there is never a parallel mental image of a flying thing or whatever.
There is however good news, because the most basic of the theory is not difficult to grasp. But bear in mind that it consists on math (by the way, Physics may be well defined as the science that invents mathematical models that resemble the behavior of nature). Google for a small collections of video lectures called "Quantum Entanglements 1" by Susskind. After that, the third volume of the Feynman Lectures is a good choice (but that requires more work).
Of course, there is room for much more than pure math alone, but that room is only accessible in a mentally healthy way after you know the math. With no knowledge of the math, words about Quantum Mechanics are meaningless, and intuitive ideas are unavoidable wrong. Whatever meaning you give to the words "wave" and "particle", there is no way to combine them in a satisfactory explanation without at least some elementary knowledge of Quantum Mechanics.
The statement "Depending on what and how we measure the photon turns out to be either a wave or a particle." is quite old-fashioned, and unhelpful, in my opinion.
Let's consider an electromagnetic wave of frequence $\nu$. Classically, this wave can carry an arbitrarily small amount of energy, i.e., the electromagnetic field can have arbitrarily small wiggles in it, with frequency $\nu$. But this classical picture is wrong: quantum mechanics tells us that a wave of frequency $\nu$ can only have energies $n h \nu$, where $n$ is a non-negative integer and $h$ is Planck's constant. We say that "$n$ photons are present".
Suppose we pass the electromagnetic wave through a narrow slit. Classically, we expect diffraction, and quantum mechanically... exactly the same! All that changed with quantum mechanics is the possible values of the energy contained in the wave. So our $n$ photons 'behave like a wave'.
Now imagine we have an apparatus (perhaps a photographic film) set up to detect the electromagnetic wave. Classically, any amount of energy could be deposited anywhere on the film, but quantum mechanically, we know that absorption occurs via interactions with single atoms/molecules/whatever. Since the electromagnetic field can only carry discrete amounts of energy, and energy is conserved, it must be absorbed in 'units' of $h\nu$. In this sense, it looks like individual particles impinging on the apparatus, each of energy $h\nu$.
Assuming you believe quantum field theory, specifically quantum electrodynamics, a photon is neither a particle nor a wave, so your web page is correct in this respect though I hesitate to give it too much credit without seeing its reasoning.
A photon is an excitation in the photon quantum field. It can interact in ways that resemble a particle and it can interact in ways that resemble a wave, but is incorrect to say it is a particle or a wave or some mixture of the two. It is an excitation in a quantum field.
Photons, electrons, quarks,... are always particles [*] and never are waves. As the CERN site correctly notes everything in the universe is made of particles. To be concrete the photon is a boson particle.
The source of the wave-particle duality misconception is pointed out in Ballentine recent textbook on quantum mechanics:
A more detailed discussion is given in the Klein site.
[*] Particle here means quantum particle not Newtonian particle. Quantum particles do not behave as a tiny ping-pong balls, but follow the laws of quantum mechanics.
Duality Thought Experiment to explain Duality through Quantum Causality
This is in regard to shooting single photons or electrons through a double-slit and the result that show the duality of particle and waveform properties coexisting.
Take Schrodinger's Equation and wherever a term is used, break it down into Mathematics Series functions of units representing space (length) and time (seconds).
Cross-reference Physics "Constants" to these Mathematics Series.
The Constants represent Relativistic Singularities and they are proposed to be formed from differences in non-relativistic quantum causality systems. Relativistic Systems floating upon moderated systems of quantum causality.
A Non-Relativistic Singularity is a constant, but it is not referenced directly to anything observable, only indirectly through the connections with the Relativity Singularities (relativistic physics constants).
Quantum causality exists in 3 fundamental forms:
A quantum null or QC0 A non-evolving causal artifact; Simple Causality or QC1 An evolving causal artifact, Evolving Causality or QC-1
Non-Relativistic Quantum Singularities are made from Static connected systems of QC-1, QC0, and QC1. The Singularities do not change but their effect is to moderate causal formations.
Time is most closely related to the evolving systems of quantum step events. Space is most closely related to the non-evolving systems of quantum step events.
Space/Time is then relative systems of non-evolving quantum causality varying as moderated by Singularities in relation to relative systems of evolving quantum causality systems.
This provides a tool for separable treatment of space/time.
Hence, this provides a basic mathematics connectedness from quantum causality to quantum mechanics. While Schrodinger's Equation provides connectedness to particle physics.
From this, intuitively non-observable systems of causality connect single electrons in both relativistic and non-relativistic mathematics space considerations. The non-relativistic connections as modeled by Shrodinger's Equation moderate certain causal distributions of quantum step events (standing waves as cyclic causality systems for instance).
While the common relativistic connections of evolving causality systems, the same causality systems as related to our eyes, are visually references as particles.
An intuitive proof: Try to think of ANYTHING you can observe that fundamentally is NOT in an evolving state.