Okay, I had a rather 'interesting' 'discussion' with a professor at my university yesterday. Nothing that he said made any sense to me at all, and I feel I understand the principles quite well, so I came away from the 'discussion' feeling that maybe I understand it better than HE does. I don't want to be arrogant, and it's hard to believe that someone in his position could misunderstand something basic, so I then think maybe I really do not understand. So I'm going to ask here what others believe. I was performing an experiment. It was the double-slit experiment using one photon at a time. I said that there is no way to be absolutely sure that there is only one photon in the tube at a time, and I wanted to know if there was some kind of statistical analysis that could be applied to determine the likelyhood of two photons being in the tube at the same time. In the lab manual it said to set it up so that, ON AVERAGE, there is only one photon in the tube at a time. I figure that without quantifying what this actually means, the results of the experiment are meaningless. Anyway, he grilled me for ages about why there can only be one photon in there at a time, and I just couldn't see the logic. I could turn down the bulb so low that I only record two photons after a year, and I still don't know that they didn't both come out at the same time. Anyway, the really weird part came next. He told me that the value of counts that I had calculated as a maximum value was too low. I had about 137 counts per second as my maximum value. I explained that I was trying to, as the manual suggested, have on average one photon IN THE TUBE at a time, not just at the end where the detector is. He told me that I should have about 10^9 counts as my maximum value . So I was like wait, we are using a slit in front of the detector. I move the slit using a micrometer 50 micron at a time. If the slit records one photon there could have been many more that 'interfered with each other' before they hit the end where the detector is. He said they don't count because if they don't enter the detector slit they die and they might as well have never existed. I was getting a bit confused because this didn't make sense to me. I said that I am trying to determine that one photon can be aware of both slits. Let's assume on the contrary that it is the case that the interference pattern is caused by many photons going through the slits and then 'bouncing' off each other. In this case, they will cause an interference pattern, but only one might reach the detector slit. In which case this professor would say that only one photon had been in there. So I don't find that convincing. Then what he said to me was, "They DON'T bounce off each other." I said, "Yeah, I know." And then tried to explain that I was using it as a way to explain that I think it DOES matter if there are photons in there that aren't detected, and then again explained my idea. Again, he told me that I don't understand the experiment and they don't just bounce off each other. I said that I never said they do, and he told me I just did. So again I tried to explain what I was getting at, using a different approach. Maybe he is right and for some reason the photons that don't enter the detector slit don't matter, but he certainly never gave me a convincing argument. All he did was patronize me. I mean, I learnt in high school that one photon can have wave properties. I don't think I would be able to get to third year without understanding that. Maybe there are other things that I don't understand about the experiment. If so I am all ears. Hopefully I've explained this all well enough. It's hard to explain without waving my hands, hehe.