I don't know how many people here are familiar with quantum physics, so I'll focus on the basic double slit experiment. The experiment in question shows that certain particles behave both as a particle, and as a wave.
For example, an electron will provide a wave pattern (the same pattern as a wave) when fired through the double slits, while we know that they are particles (and if an instrument interferes to observe, they exhibit the same behavior as a particle).
It is also known that light does the same: it acts as a wave, but we also know that light is composed of particles called photons. My question is why: electrons, for example, possess a mass. It is very small, but they do possess it: the fact electrons may act as a particle is then not surprising.
How can a photon exist, then, if it does not have mass (which it must not to travel at the speed of light)? All known particles, however small, have some mass.
I first came upon this question while studying the concept of 'Solar Sails', which are propelled by the light from the Sun. This initially puzzled me, as how could a massless particle impart momentum, but it was quickly answered once the idea of a particle moving at light-speed and the relative formulae were used. (Momentum appears in another involved formula, for those interested, and can be calculated separately to find it would indeed have a value for a relativistic particle).
However, this doesn't seem to explain a non-mass exhibiting the behavior of a particle. What, for example, would compose a photon if it is not made of any substance involving mass?
I realize this question sounds bizarre, as fundamental particles like quarks and electrons and photons are not thought of to be made up of anything, but the fact remains that, as a particle, it occupies some position in space, and does so as an actual, tangible thing, and even if it can sometimes behave as a wave (as electrons, entities with mass, do) it still exists as some form of particle. If it can have a volume, however miniscule, which it must if it is a particle, how could it not also possess mass?
This is not to question the results of the experiments: simply to query. Why does a non-mass still behave enough like a particle to exhibit quantum behavior?