Below is a question (paraphrased) which I addressed to two physics lecturers on the subject of slit diffraction. The problem I have is that answer I received from them sounds so ridiculous and impossible that I find it difficult to accept. I asked the question independently to both lecturers, and their answers were in agreement with each other. Furthermore, both are accomplished, respected, and are expert on this subject; and so I have no reason to doubt the truth of what they say. I would be grateful to hear any thoughts that anybody has on this subject. Anyway, here is the question: "Suppose that you pass single photons through a standard double slit apparatus one at a time. Over time, a diffraction pattern builds up on the screen, the screen being made up of an array of closely-packed photon detectors. Additionally, you have an accurate clock which is connected both to the photon emitter and to the detection array. When a photon is emitted, the clock starts; when that photon is detected, the clock stops. The two slits are positioned far apart from each other (see diagram), and hence a photon, in passing through one slit or the other, may take either a short path or a long path to the screen. In other words, the photon may spend either a short or a long period of time "in transit" between emission and detection. The purpose of the clock is to observe when a short transit time occurred, and when a long transit time occurred, and hence to provide unambiguously the (retrospective) knowledge of which slit the photon passed through. Now, isn't it true that in all experiments designed to determine which slit the photon went through, the diffraction pattern is destroyed? That is, no matter how gently and sensitively you try to detect which slit the photon (or electron etc.) goes through, in carrying out this detection you must interfere with the photon slightly, and this interference is what destroys the diffraction pattern somehow? If it is a fact of Nature that one can never both get a diffraction pattern AND tell which slit the photon went through, then what happens in my clock experiment as described above? A clock is passive and does not physically interfere with the photon in any way at all. It merely times the photon's travel time and calculates which slit the photon must have gone through. My question is: By what mechanism does a clock destroy the interference pattern?" The above is heavily paraphrased but my basic question remains the same. Here is the answer given me by both lecturers: If you switch off the clock, you get a diffraction pattern. If you switch on the clock and collect its timing data, you do not get a diffraction pattern. If you switch on the clock but throw away its timing data as soon as each is collected, you do not get a diffraction pattern. If the clock is switched off, but an advanced robot is present in the room (I began to ask quite desperate questions at this point), and the robot, possessing an accurate internal clock, is able to tell how much time passed between photon emission and detection merely by looking at the experiment, then you do not get a diffraction pattern. Presumably, then, the diffraction pattern will appear and disappear if the robot opens and closes its eyes? Or if robots sound too far-fetched, presumably the diffraction pattern will appear and disappear as you switch the clock's power supply on and off? This all sounds too ridiculous to be true! What is going on?