Ok, bearing in mind that I only have a brief understanding of quantum mechanics (no formal education, only from reading about concepts in books), so I could be way off here, I have a question regarding probability wave functions and light waves.
From my understanding, every particle of matter has a probability wave which fills the entire universe, that's to say the only reason a particle in my fingernail is in my fingernail is because that's where the highest probability of it being is (with the chance that it's anywhere else in the universe being infinitesimally small), so my question is what is a light wave? Also from my understanding, a light wave is simply a probability wave for the location of a photon, and only upon observation does that wave collapse and does the photon 'take on' a definite position in space, so if I compare a particle of matter to a particle of energy (a photon in this case), how come the particle of matter has a probability wave that fills the entire universe, and the photon have only a small defined wave which virtually makes up a straight line in space? Is the light wave just an area (straight line?) of space where the probability is magnitudes higher than everywhere else and the photon's probability wave is still permeating the rest of the universe, or is the photon's location definitely somewhere in the area of space defined by the light wave?
For example, could a photon that's part of a laser beam ever interact with a photon from a laser beam travelling parallel to the first beam? (Disregarding any other physical theories that could cause the two beams to converge through other means, I'm wanting to stay on the topic of Schroedinger probability waves)
I might well be completely misunderstanding/mixing concepts here, so that might be the reason for my confusion, if so, please enlighten me!