The one all-pervasive thing in our world of modern technology is the chip: a tiny semiconductor with transistors, resistors, etc. This calls for a clear and quantitative understanding of crystals. We also see the application of x-rays in a variety of contexts: from dental work and checking the chest to extra-terrestrial astronomy.
In a strange sort of way, crystals and x-rays had a common connection in the work of Max von Laue (born: 9 October 1879). Soon after the discovery of x-rays in 1894, the question arose as to their ultimate nature. Are they tiny particles like electrons or were they waves? And if they are waves, are they longitudinal waves like sound waves or transverse waves like light waves? And again, if they are waves, how can one measure their wavelength?
The question of whether an entity is wave are particle can be determined by doing an experiment to check if it diffracts (i.e. bends around an obstacle). For example sound is clearly a wave, because you can hear someone talking on the other side of the wall: sound waves bend around the obstacle. Light waves do this too, but the dimension of the obstacle has to be of the order of magnitude of the wavelength. Thus light can bend around a hair, or the lines on a diffraction grating, but not a door. But there was reason to believe that if they were waves, the wavelength of x-rays would be much, much smaller than the smallest obstacle we can construct. We simply can’t etch a diffraction grating in which the line-spacing are of the order of magnitude of x-ray wavelengths.
In the first decade of the twentieth century, one knew neither the ultimate nature of x-rays nor the periodic atomic arrangements in crystals. It occurred to von Laue that if crystals were patterned atomic structures and x-rays were waves, then the former could serve as naturally occurring three dimensional diffraction gratings to study and measure x-rays. With a couple of assistants he put this idea to experimental tests. Thus it was that at the University of Munich in 1912, he and his assistants obtained the first photographic impressions of x-rays. This work was of such significance that in just a couple of years it was recognized with a Nobel Prize for Von Laue.
These results on x-ray diffraction established beyond a reasonable doubt that x-rays are in fact electromagnetic waves of extremely small wavelengths. Moreover, Von Laue’s experiments also led to a more quantitative study of crystal structures.
It is refreshing to recall that Von Laue was one of the enlightened scientists of the Nazi era in Germany who protested the dismissal of Einstein, resisted the admission of the Nazi Johannes Stark to the Prussian Academy, and who boldly condemned the characterization of relativity as Jewish science. It is said that he used to leave his house always carrying two things – a briefcase and a book, a file or a bag – one for each hand, so that he would not have to raise a hand to give the Heil Hitler salute. Because of his bold stance he lost respectability among the guardians of Aryan purity.
More importantly, Von Laue already saw in the early 1930s German science slipping fast into very bad times. In that deranged phase in Germany’s history, Von Laue’s was among the few voices of reason that spoke out boldly and openly against the insanity that was fast gripping the land.
Von Laue lived through the nightmare, as so many other decent Germans did, with much pain and sorrow. But when it was all over, he was interrogated by the Allied forces. And when there was a rebirth of civilized Germany, Von Laue played an active role in the organization of scientific centers.
October 9, 2013