I am a biochemist and so my physics knowledge is limited. But for a long time I have been wondering on how exactly the electromagnetic field associated with a single photon propagates in space. I understand (using a cartesian image of the electric and magnetic field change) the oscillating nature of the electric and magnetic fields associated with a single photon as it propagates in a given direction and how the frequency of the oscillation of the value of those fields are associated with (or determine) the energy of that photon. What I have trouble understanding, or visualizing is: what is (the substratum) that is changing as the electromagnetic disturbance propagates in space? Is it an underlying electromagnetic field that permeates the entire universe, and that is disturbed by the amount of energy associated with the moving electromagnetic wave? If not, then how does the disturbance (wave) propagate in space?