In quantum mechanics, you usually hear the phrase "the act of observing the state modifies it", but they never really define what they mean by "observing".
For example, suppose we have Stern-Gerlach analyzers aligned in the $z$-axis and we shoot atomic beams into the analyzer thus we don't know if the state is spin up or down in the $z$-axis where both have a 50 percent probability. We won't know the state until we "observe" it. By observing, do they mean that we have a measuring device and the act of measuring the state disturbs the system? Does the act of measuring it use photons which interacts with the measurement? Or is it just the act of looking? What exactly is "disturbing the system"? Is the disturbance the actual photons in the measuring device?