The historical development of all of these ideas is a long and complicated story, so I won't attempt to trace it but there are some nice experiments we can look at to illustrate the concepts. The double-slit experiment is a classic example of particle-wave duality. The setup is simple: a barrier with two gaps in it, a source, and a detector. Famously, this experiment was carried out on light by Thomas Young in the early 1800s, an important demonstration that light propagates as a wave.Ĭlassically, there are two versions of the experiment we can set up. If the source emits particles - macroscopic ones, like bullets for example - then after many trials, we'll find a bimodal distribution, with most of the impacts tightly distributed around the two straight-line paths from the source to the detector through the gaps. On the other hand, if our source emits a wave (using water as a medium, let's say, or light as Young showed), then the wave encounters both slits, diffracts from them, and we expect to find an interference pattern at the detector, with peaks if the difference in length traveled from the two slits is equal to \( n \lambda \), and troughs if the difference is \( (n 1/2) \lambda \). So naturally, the question arises: are fundamental particles like waves, or are they like bullets? Of course, you know that the answer is that they are both particles and waves, depending on circumstances! If we run the double-slit experiment with an extremely low source intensity, we will see individual point-like particle detections on our screen, but the intensity of the arriving particles will show the characteristic interference pattern of a wave source. Richard Feynman, who played a large role in promoting the double-slit experiment using electrons and its variations, called it "the heart of quantum mechanics". (By the way, if you've never seen a video of this experiment, you should watch it. So electrons are particles - we clearly see their individual impacts on the screen - and they are waves, as evidenced by the interference pattern that builds up. There are lots of interesting variants on this experiment involving measurements and quantum mechanics, but we won't go into that just yet.
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