Back in 1965, physicist Richard Feynman proposed a "thought-experiment" to try to illustrate the properties of the sub-atomic particle called the "electron."
Physicists and some other scientists have to resort to thought-experiments in order to test hypotheses that couldn't be tested under normal conditions. Or in some cases, under any conditions that the science of the time can produce.
Feynman did this with the electron, suggesting what's called the "double-slit" experiment to determine if an electron behaves like a wave or like a particle. In his experiment, a single electron was fired through a slit onto a surface that recorded its impact. The surface with the slit had two identical slits in it which could be covered or uncovered individually and as a pair. If the recording surface showed one pattern after a series of electrons had passed through it, it would mean that electrons were particles. If it showed a different pattern, then it would mean electrons were not particles at all, but waves.
In 1961, German physicists had conducted the experiment with a continuous stream of electrons rather than a series of single electrons, and their results showed that electrons were waves and particles at the same time. This is supposed to be impossible, but Feynman took it one more step and said the German team got those results because every individual electron is a wave as well as a particle and it doesn't "make up its mind," so to speak, until an experiment makes it do so. That was definitely impossible, but his thought-experiment showed how this quality of wave-particle duality was not only possible but was in fact the way things were. Technology in 1965 didn't permit scientists to conduct an actual double-slit experiment, but Feynman's logic was consistent and it matched the observations scientists could make.
By 1974, a device called a "biprism" was used by Italian physicists to demonstrate that Feynman's hypothesis was true. Technology still couldn't make a true double-slit experiment, but the biprism had the same effect and experiments using it showed exactly what Feynman had predicted. A similar experiment in 1989 also verified the results of the thought-experiment, which was by then 25 years old.
The same Italian team got even closer to Feynman's conditions in 2008, and in 2012 physicists at the University of Nebraska replicated them almost exactly as Feynman described. Their results also were as Feynman predicted -- almost 50 years after he predicted them. Technology can do great things -- but keeping abreast of the human imagination is one at which it labors just to try to catch up.