Hi First I should point out that I don't have any scientific knowlege in Quantum Mechanics. I am just enthusiast physicist not professional. I am interested in physics and the only thing that I know for now is Classical Mechanics (Newtonian, Lagrangian and Hamiltonian reformulations) and some Special Relativity. All that I learned form Leonard Susskind's Lectures that are uploaded on youtube. The things that I will ask about QM I all got from popular books so the questions and statements i make can be very inaccurate or even stupid. I am sorry If the questions that I ask have been asken or things like that but I am new to the forum. As far as I know the Wave Functions represents the probability of the particle to be in certain state and it is experimental fact that mesurements cause some change in the WF which make the particle to establish well defined state. The thing that I realy wonder is what kind of action can be described as a mesurement. For example in the double slit experiment with a single particle. When the particle is emited its wave function splits through the slits then interfere with itself and then when it is detected ( a mesurement is performed) it establishes well defined state which we can see as a dot on a screen (if it can detect single particles). If detectors are put to the slits the wave function collapses there and after a long periond of time there is no interference pattern on the screen. But how the slits themselves don't affect the wave functions? They also are made of some kind of particles that have kinetic energy and are interacting with the wave. Also this experiments have also been made with big atoms or molecules. In this case how the molecule itself can hold its wavefunction without collapsing? I mean the particles in the atoms are interacting electromagnethicaly ( they are exchanging photons) and there are quarks and gluons in the protons and neutrons. They all have energy and they all interact. What is the difference between me emmiting some photon to a particle then after the particle emits it back to me and I detect it and a molecule or atom where the particles exchange photons? I am not sure about this but I've heard that quantum effects where particles behave like waves have even been observed in living cells ( for example during photosyntesys) when electrons can move as a waves. Living cells have very high entropy and they are made of many particles interacting which should be counted as a mesurement. So how at some point a mesurement can affect the system and other times it does not affect it? What can be described as mesurement in QM?