With the rapid progress of science and technology, physical instruments that can reliably launch individual electrons have been developed. Using this single electron emitter to carry out the double slit experiment can make at most one electron exist between the emitter and the detection screen at any time. Therefore, at most one electron passes through the double slit at a time, rather than a large group of electrons squeezing through the double slit in a short time interval. It is worth noting that the detection screen will display familiar interference patterns after accumulating many electron impact events. It can be inferred from this pattern that a single electron seems to be engraved through two slits at the same time and interfere with itself. This explanation is not consistent with the commonly observed physical behavior of discrete objects. People have never witnessed the tiger passing through two side-by-side fire circles at the same time, which is not easy to agree with intuitively. However, from atoms to more complex molecules, including buckyballs, these microscopic particles will produce similar phenomena.
In the double slit experiment, the probability distribution of particles reaching the position of the detection screen is highly decisive, whether it is electrons, neutrons or any other quantum size particles. Quantum mechanics can accurately predict the probability density of particles arriving at any position of the detection screen, but quantum mechanics cannot predict when and where a particle will arrive at the detection screen. This indisputable result is obtained through repeated experiments. This result gives scientists great confusion, because it is impossible to predict the arrival position of particles, which means that there is no reason for the arrival event of particles. Many physicists are very reluctant to accept this fact. Although quantum mechanics can correctly predict the experimental results, quantum mechanics cannot explain why such phenomena occur and why particles seem to pass through two slits at the same time? Albert Einstein believed that it can be inferred from this that quantum mechanics is not complete, and a complete theory must give a satisfactory explanation to these problems. Niels Bohr retorts that this just shows the advantages of quantum mechanics. Quantum mechanics will not use inappropriate classical concepts to explain this quantum phenomenon. If necessary, quantum mechanics can find and apply new concepts to explain these problems.