In another thread, which I do not want to derail, I claimed that a free, non-interacting particle is not physical and that it cannot be detected or observed. It was pointed out to me that apparently I'm the only one on here who doubts about the possibility of detecting a non-interacting particle's energy, mass or spin. Now, I stand on my ground and insist on the non detectability of free particles. What I'd like someone to explain me is how to detect such a prticle. Please, before answering consider that you cannot see see such a particle (it does not interact with light), cannot bend its trajectory (no interaction with the EM field in general), you cannot have a bound state, you cannot pile it up and make a ball out of such matter, and so on and so on. Am I really the only one who knows that you cannot detect a non interacting particle?