Question on "observer" of double-split experiment From what I understand when we detect from which of two splits each electron passes we see two lines on the board. But, the moment we don't electrons become self-interfering waves. A puzzling result. My question is, can you explain me how we detect the trajectory of each electron? We light a beam on it or via magnets? Also, in what way does our measurement act differ from interactions of the electron with the environment? If it doesn't then the only difference would be that our measurement has a direct "purpose" on interacting with the electron? So, if observation is the same as any interaction how does the electron know which is which?