I understand how interference can arise when we start with a coherent light source where all the photons emitted are in phase. As they traverse the double slit, the distances to the screen from each of the double slits is different. The relative phases of the incident waves then sums to give the intensity.
And I have read various threads here
and it seems in these answers, they are explaining the out of phase photons as being out of phase because the emanate from different locations on the source.
One commenter said "The first slit is an attempt to make the source look like a point. If the slit has a finite width the double slits produce displaced fringe patterns depending on where the light emanating from the single slit has come from".
Another commenter said if "you have an incoherent light-source, i.e. a non-point or extended source, and you place in its path a small enough opening, you're isolating light that was emitted, relatively speaking, from a single point on that non-point source, and hence that is already relatively spatially coherent."
But even from a given point on a filament, there is thermionic emission occurring concurrently at all depths into the filament(y coordinate) and heights(z coordinate) on the filament. This y and z coordinate spatial variability cannot be eliminated if your slits only diffract waves based on their x coordinate location.
There is also the fact that atoms are much smaller than the light wavelength they are emitting. So atoms next to each other or even a few atoms apart can emit much larger light waves slightly out of phase in time but they will not be resolvably different in their direction or their diffraction.
If we consider light emanating in a very specific direction and could measure the phase of light at some distance away from the source, then graph the phase of that light vs time we would see a band(the sum of all the phases of the unsynchronized photons arriving), not a sine wave as we we would with laser light.
So, at any given instant, the photons traversing the slits are not in the same phase even though they have an identical source direction. There is a population of photons (or waves). This population would only be 'coherent in direction' but not phase. As the waves diffract through the slits, then at every specific horizontal location on the screen we should be observing the interference pattern of two populations. Shouldn't this give a smear?
Yet we clearly see an interference pattern. I read the above referenced threads and their comments several times, but remain confused. One commenter even said 'The interference is washed out by random phase shifts! This is why the experiment is done with a laser,' but Thomas Young did this experiment in May 1801, centuries before lasers were around. His actual apparatus had a single slit far behind the double slit.
I suspect such a single slit would 'filter out' all spatial phase variation of light arriving at the double slit. In other words, the light impinging on the double slit would be 'the same wave'. Yet temporal phase variation is not affected by this. Neither is phase variation that is spatial and smaller than the wavelength.
As I am relatively simple, I would appreciate a simple answer.