I have some specific, related questions, for which I haven't been able to find answers: 1. At any instant in time, is the physical size or extent (e.g., length, width, diameter) of the electromagnetic field of a single photon (of a given wavelength) traveling through a vacuum determinate and invariable? 2. Is there an equation which relates the physical size of the field to the photon's wavelength? 3. What shape is the field at an instant in time, e.g. is it spherical? Is the length of the field (longitudinal in the line of travel) the same as in the dimensions transverse to the line of travel (colloquially, the width and height)? 4. At any instant in time, does the field physically encompass one, less than one, or more than one full wave phase cycle? 5. Does the maximum amplitude of the field vary depending on location within its boundaries, for example is the maximum amplitude greater near the physical "center" of the field than at the extreme outer edge? I wouldn't think so. I understand that one cannot pin down the specific "point" location of a photon at any instant in time without observing the photon (thereby causing the wave to collapse). But that is a separate issue from the question of the physical size of the wave's field at any instant in time. In the double-slit experiment each single photon interferes only with itself, not with other photons. Since the experiment implies that a single photon's electromagnetic field passes through both slots simultaneously, it seems to me that the physical width of the field (at least in the dimensions transverse to the photon's line of travel) must be at least as large as the distance between the two slots. As I understand it, in order for interference to occur there is a maximum limit to the distance between the slots, which is related to the photon's wavelength. I would appreciate if anyone knows the equation for the maximum slot distance. And whether it equals the physical width of the photon's field. I'm primarily interested in understanding the instantaneous physical length of the field along the photon's line of travel. It seems to me that the double-slit experiment doesn't shed light directly on that point (so to speak). I'm not asking about the concept of polarization, unless it directly relates to the physical size of the field in one or more dimensions.