Apr 242019

On this date in 1914, James Franck and Gustav Hertz presented a paper to the German Physical Society concerning an experiment that was the first electrical measurement to clearly show the quantum nature of atoms, utterly transforming the understanding of reality itself at the deepest level. Debate continues to this day concerning the precise interpretation of quantum phenomena, most of which passes by the average person – unawares. But the implications are stupendous. Franck and Hertz proved experimentally for the first time that energy states do not vary continuously at the sub-atomic level, but move from one state to another with no intermediate transition. This postulation is completely counter-intuitive. At the macro level, energy states seem to vary continuously over a range. You can dim lights gradually, or raise the volume on your music continuously. But at the sub-atomic level, such variations (say, in electrical charge) are not possible. They simply leap from one to another.

Franck and Hertz had designed a vacuum tube for studying energetic electrons that flew through a thin vapor of mercury atoms. They discovered that, when an electron collided with a mercury atom, it could lose only a specific quantity (4.9 electron volts) of its kinetic energy before flying away. This energy loss corresponds to decelerating the electron from a speed of about 1.3 million meters per second to zero. A faster electron does not decelerate completely after a collision, but loses precisely the same amount of its kinetic energy. Slower electrons merely bounce off mercury atoms without losing any significant speed or kinetic energy.

These experimental results proved to be consistent with the Bohr model for atoms that had been proposed the previous year by Niels Bohr. The Bohr model was a precursor of quantum mechanics and of the electron shell model of atoms. Its key feature was that an electron inside an atom occupies one of the atom’s quantum energy levels. Before the collision, an electron inside the mercury atom occupies its lowest available energy level. After the collision, the electron inside occupies a higher energy level with 4.9 electron volts (eV) more energy. This means that the electron is more loosely bound to the mercury atom. There were no intermediate levels or possibilities in Bohr’s quantum model. This feature was revolutionary because it was inconsistent with the expectation that an electron could be bound to an atom’s nucleus by any amount of energy.

In a second paper presented in May 1914, Franck and Hertz reported on the light emission by the mercury atoms that had absorbed energy from collisions. They showed that the wavelength of this ultraviolet light corresponded exactly to the 4.9 eV of energy that the flying electron had lost. The relationship of energy and wavelength had also been predicted by Bohr. After a presentation of these results by Franck a few years later, Albert Einstein is said to have remarked, “It’s so lovely it makes you cry.” On December 10th, 1926, Franck and Hertz were awarded the 1925 Nobel Prize in Physics “for their discovery of the laws governing the impact of an electron upon an atom.” Both physics and philosophy were changed forever.

Since the original experiment involved a vacuum chamber, some uses of vacuums in cooking can be on the agenda. Vacuum cooking sugar is an industry standard in the manufacture of hard candy, and sous vide cooking (cooking items at precise temperatures in vacuum-sealed bags), has had a vogue for a while.   My favorite is the home vacuum cooker which is a cross between a slow cooker and a thermos flask. Food is heated in the inner chamber which is then placed in an outer vacuum box, and sealed. The food continues to cook for hours without any heat source. This video is an advertisement, but you get the idea: