In the two-slit experiment, as I understand it, an interference pattern can be generated by sending many electrons (or photons) through the two slits one at a time, and the pattern itself gets built up over time. The explanation is usually this: A single electron travels as a wave and so interferes with itself as it passes through the two slits, and the interference pattern results from multiple electrons interfering with themselves as they pass through the two slits. My question is this: If a single wave were to pass through the two slits and interfere with itself, isn't the middle of the detector screen the only place you get constructive interference? If so, then multiple electrons, shot one at a time, should generate a pattern that exhibits constructive interference only in the middle of the screen. So, what explains the other cases of constructive interference observed across the detector screen?