A discovery by chance with significant consequences – 125 years ago Conrad Röntgen discovered X-rays

On November 8, 1895, the physicist Wilhelm Conrad Röntgen experimented in his laboratory in Würzburg, Germany, with cathode rays. These consist of electrons which, after being released from a hot cathode, are accelerated by an electric field in a tube. Röntgen noticed that fluorescent objects near the tube began to glow despite having covered the tube with black cardboard. So, he saw light where there should not be any. Röntgen quickly realized that he had stumbled upon something incredibly special here, a new type of radiation. It had a very particular property: it passed through most forms of matter. A few weeks later, with the help of his rays, he succeeded in taking a picture of his wife’s hand, in which her bones and wedding ring were clearly visible. This first “X-ray” in history contributed to the fact that Röntgen’s discovery quickly caused a sensation even outside the circles of physics and triggered a great wave of enthusiasm in wider society. Within just a few weeks, newspapers around the world were reporting on it. His discovery was one of the few in physics that even fascinated farmers, bakers, and foresters. For one profession, however, his rays had truly revolutionary consequences: The physicians were almost beside themselves of excitement when they learned that X-rays enabled them to look inside the living body. Today, every child knows what it means when any part of the body needs to be “x-rayed”.

Since Röntgen did not know the origin and nature of his radiation, he simply called it “X-rays” (the “X” stood for “unknown”). This name has been preserved in the English language until today. After Röntgen had presented his discovery to enthusiastic listeners from all circles of science and society at a meeting of the Würzburg Physical-Medical Society on January 23, 1896, it was suggested that the “X-rays” be renamed “Röntgen-rays” for the German language. For their discovery, Röntgen was awarded the very first Nobel Prize in Physics in 1901 “in recognition of the extraordinary merit he had earned by the discovery of the rays named after him”. The nature of X-rays as electromagnetic waves (with very high frequency and thus high energy of its photons) could only be proven by Max von Laue in 1912, for which he was equally awarded the Nobel Prize for Physics in 1914.

But the discovery of X-rays was not only the beginning of a revolution in medicine. They also turned out to be of great importance for physics. For they were to be an important piece of the puzzle for an image that was still completely unknown to physicists. With it, many more pieces of the puzzle were soon to emerge, which the physicists then assembled in painstaking work. The picture that slowly revealed itself to them, at first in vague contours, but then increasingly clearer, was … the structure of the atom!

Inspired by Röntgen’s results, the French physicist Henri Becquerel began experimenting with a similar setup only shortly after the discovery of X-rays. After Röntgen had shown that his radiation could cause fluorescent phenomena in certain materials, Becquerel expected that fluorescent bodies, after being exposed to normal light, would in turn emit X-rays. To prove his hypothesis, he used various salts of uranium that he knew would fluoresce after exposure to light. In fact, corresponding darkenings appeared on the photographic plates. When he had to interrupt his experiments for a few rainy days because he did not have sufficient sunlight as a light source for fluorescence, he placed the photo plates together with the uranium salts in a dark drawer. When he took them out again a few days later, he experienced a surprise: The photo plates were heavily darkened, although the uranium salts had not been exposed to any light, i.e. had not been able to fluoresce. Some other radiation must have caused the blackening on the photo plates. Without knowing it, Becquerel had discovered the phenomenon of radioactivity. It became clear that radioactive radiation is associated with the transmutation of elements. It must therefore have its origin in the atoms.

The various types of radiation also fascinated the English researcher Joseph John Thomson. Thomson was particularly fascinated by a type of radiation that had been known for a while and which Röntgen had also used in his experiments, the cathode radiation. The physicists had discovered that when an electric field is applied to a metal, radiation is emitted from it which, unlike light, can be deflected by magnetic fields. Thomson concluded that these particles are electrically charged. Thomson called them “corpuscles”. But as early as in 1891, the Irish physicist George Johnstone Stoney had proposed the name “electron” for them. Since these electrons are emitted from metals, Thomson assumed that they too are components of atoms.

With the radioactive rays and the electrons, there were now a number of indications that atoms consist of different components. Starting with Conrad Röntgen’s discovery 125 years ago, physicists set out on the expedition for the holy grail of physics, the development of a consistent model of the atom. With this, things began to take a course in their field that they could not have imagined in their wildest dreams. In the process, they were forced to bid farewell to 250-year-old physical and more than 2500-year-old philosophical certainties. This development was to dramatically change not only physics, but the entire human society.