I know it sounds like a foolish question but I have a reason for asking and I'm hoping someone here, can give a convincing response.
Here is why I pose the question...it seems to me that all this discussion of photons, and other sub-atomic particles possessing mass or negative mass is really a discussion about the mass of shadows.
Although we can't really weigh shadow we can see it, we can measure its area, observe its outline. a photon- which is massless - may seem heavier than shadow. It is really driving me crazy when I think about it.