Ken Hughes said:
This is a genuine question to which I seek a genuine answer. Fundamentally, light is quantised (however we explain the overall wave effect). So, being quantised, a single photon will hit the screen at a point where the probability of this event is at its maximum. The wave must be made up of photons. This is imperative if light itself is made up of photons. The only way we can get a wave (of varying energy levels) is if the number of photons varies in a wavelike manner (sinusoidally) over time, over the path of the wave. To me, that must mean that there is more probability of a photons hitting the screen during the "peak" of the wave where there are many more photons than are at the trough. This seems a more complete explanation than just some "probability wave" existing in some ethereal way. Everything is physical, including light surely?
I'm still puzzled about the level you expect me to answer your question. Although you labeled the thread A, I've not the impression that you have understood enough QT to get satisfied by an answer at A level.
To really understand what a photon is, I'd advice you to follow #14. I'd however suggest another book, because you won't find the answer in a high-energy-particle QFT book but rather in a book about quantum optics. Of course both use the same theory, namely QED, but the physics phenomena treated by the different communities (high-energy physicists vs. quantum opticians) are quite different. I find the book
J. C. Garrison, R. Y. Chiao, Quantum Optics, Oxford University Press
very nice. There are for sure also tons of good manuscripts online.
I already tried to boil it down to a non-mathematical answer, which of course always bears the danger to be misunderstood, because it's impossible to talk about physics, particularly quantum physics, without mathematics. But let me try again:
The meaning of the sentence "light is quantized" is that electromagnetic-field energy is absorbed and emitted by matter consisting of charged particles in "quanta" of energy ##\hbar \omega##, where ##\omega## is the frequency of an em.-field mode you can always decompose the (free) em. field to.
To describe this you do not need to quantize the em. field yet, but semiclassical descriptions are adequate, i.e., you quantize the matter particles (in this case most importantly electrons) and keep the em. field as classical. That said, you see that it's not so simple to give the sentence "light is quantized" a proper meaning. Particularly it doesn't imply that you can say that "light consists of photons" when thinking of a "photon" as a little bullet-like object. This "particle picture" is to a a certain degree possible for massive particles, but not for massless quanta as the photon. Nevertheless the semiclassical approximation is completely adequate to understand why (a) there is an interference pattern (just use classical electrodynamics with adequate boundary conditions for the double-slit) and (b) if you dim down your light source enough there appear only single dots on the screen building up the interference pattern after some time when enough dots have appeared: The dots appear from the absoption of the light quanta by the material of the screen. In this picture, however we have not made use of the full quantum picture of the em. field.
We are forced to use the full theory (a) to understand the phenomenon of spontaneous emission which you necessarily need to get the correct black-body formula by Planck when analyzing it's derivation from the point of view of kinetic theory (as was done by Einstein already in 1917, i.e., before modern QT and quantization of the em. field has been discovered, which happened in 1925 by Born and Jordan, using the groundbreaking idea by Heisenberg) and (b) in our context if instead of dimmed light one uses a true single-photon source (which is not so easy; that's why reliable true single-photon source have become available only in the mid 1980ies and in full glory even later with the use of parametric down conversion exploiting non-linear optics using strong lasers).
In terms of the quantized em. field light emitted from a laser is a quantum state of the electromagnetic field called "coherent state". If the intensity of the coherent state is high, you can neglect the quantum fluctuations at all and you are back at a situation where classical electrodynamics is a very good approximation. If you dim down the laser enough, the coherent state is a superposition, where the vacuum state dominates and it's a good approximation to only take the next term, which is a single-photon state. That means only rarely per unit time (which you can resolve with your detector) you absorb a photon on the plate and thus see "particle-like behavior of light", but it's not really a particle hitting the screen.
Finally, as said already above, nowadays you can also prepare true single-photon states, but for the double-slit setup that again doesn't make any difference compared to the dimmed laser! You still see the single point-like detection events on the screen, because one photon is absorbed each time there's such a detection event. All QED does and can do in this situation is to describe the probability distribution of the location of these detection events, and that's also all we can observe.
Now you write
Ken Hughes said:
In the end, everything must be explicable by physical entities or events. QED is a way of working with the behaviour and characteristics of fields and particles. It does not explain the physical nature of light. I have suggested a physical explanation which I do not believe contradicts QED. I expected a more helpful approach here.
I hope, I'll not disappoint you, but physics is a natural science, and all physics does is to try to systematically observe nature and try to figure out how to systemize the observed patterns of how nature behaves, i.e., in shorter words it uses the observation that there are such regular patterns and that they can be expressed in terms of mathematical models and theories. As it turns out, it's even impossible to adequately talk in "natural language" about realms of our experience which is not directly related to our daily experience, directly with our senses, like the microscopic phenomena related to the atomistic structure of matter (leading to the fundamental building blocks, called elementary particles and radiation, and the description of their interactions in terms of quantum (field) theory) or the very big realm of astronomy and cosmology (where general relativity has to be applied).
In this sense what we have tried to give as an answer already refers to the "physical nature of light". I don't know, what else you expect, i.e., what would be a "more helpful approach here".
