I have a very basic question about the double slit experiment. Some websites that I have seen state that the simple act of measuring which slit the particle goes through, is enough to collapse the wave function, and it isn't necessary for anyone to actually observe the results of that measurement. Other websites seem to imply that the wave function doesn't collapse until the results of the measurement are actually observed. I was wondering which of these is true. Is the act of measuring alone, sufficient to collapse the wave function, or is observation necessary? Are there any websites that give a good basic outline of the two slit experiment, and any subsequent experiments that attempt to better understand what is actually happening?