Hi I'm a complete novice when it comes to Quantum physics, or any physics for that matter, so I hope this isn't a very silly question. I'm currently reading Brian Greens's excellent book "The fabric of the cosmos" and I'm up to chapter 7 Time and Quantum. On pages 195 and 196 and in Figure 7.5 Green describes an interesting beam splitter experiment in which in photons are fired at a screen via a beam splitter and then two down converters. The down converters produce so called idler photons. The idler photons are allowed to pass through a further series of beam spitters and finally arrive at one of 4 detectors. Basically if the idler photon arrives at sensors 1or 2 we are able to determine the path of the signal photo which will eventually arrive at the screen. However, if the idler photon arrives at detectors 3 or 4 we are unable to determine the path. It turns out if the idler arrives at 1 or 2 no interference pattern is formed, but if it arrives at detector 3 or 4 an interference pattern is formed on the screen for those signal photons. What I would like to know is what would be the result if we preformed the following experiment. The beam is fired at the screen via a beam splitter and two detectors are placed on either path after the splitter to let us know which path the photon took. The detectors are connected to a computer and the results are stored on a hard disk. Obviously if we look at the results and then look at the screen no interference pattern would be formed. But, what would happen if we deleted the results from the hard disk without looking at them and before looking at the screen , so that although the "which path" information was detected it was unknown to us. In this experiment what would we see if we looked at the screen? Would the interference pattern be there or not? Maybe a stupid question and I'm guessing the Interference pattern would not be there. I have to ask though as the experiment described by Greene has got me wondering whether it is the measurement of the quantum event or the knowing of it that causes the interference pattern to not form. That is to say is it the interaction of the measuring device or the interaction of the human consciousness which causes the interference pattern to disappear.