I just went to an astronomy this past week and one the lectures pertained to the particle zoo. As the professor began the lecture, he quickly covered the basics and eventually went into explaining how electrons and protons "sensed" the presence of other particles. To do so particles emit photons as I understand and particles around say an electron act accordingly i.e. protons are attracted and electrons repelled. One student brought up what I thought was a good question: How do particles differentiate the difference between a photon emitted from an electron or proton so that they can act accordingly? The professor didn;t really have an answer for this.