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Quantum mechanics is concerned with the peculiar ways in which matter behaves at scales of 100 nm (ten-thousandths of a millimeter) and less. At these minuscule scales, the particles that form the universe display behaviors that do not make intuitive sense to those used to the simpler laws of classical mechanics (when you push a rock, it moves; but if you push it against a wall, it does not move).
Quantum particles, like photons and neutrinos, can tunnel through matter like passing through a wall. They can become entangled so that a stimulus to one particle causes another to react despite them being separate from one another across spatial distance. They can even exist in a superposition of two mutually exclusive states like Schrodinger’s cat – dead and alive.
Central to the development of our understanding of these and many other quirky discoveries of quantum mechanics has been spectroscopy: the study of the ways in which matter interacts with electromagnetic radiation. This includes, but is not limited to, the ways that various atoms, molecules, and materials react with light.
History of Spectroscopy
Spectroscopy began with Sir Isaac Newton in the eighteenth century. Newton discovered that visible light consists of a blend of different colors (or frequencies of radiation) which can be split when the light is passed through a prism. This discovery showed that radiated energy sources like light interact with matter in a physical manner.
Newton’s discovery – and many observations, inferences, and theories of physicists since him – led to the discovery in the late nineteenth century that different molecules absorb different frequencies of light and the development of spectroscopes which could record these absorption levels. Because the molecular structure (covalent bonds) of each molecule absorbs radiation at a defined frequency, spectroscopes can be “tuned” by changing the frequency of emitted light, so that only one type of molecule in the subject matter is identified.
Careful observation of how light interacts with different materials – early spectroscopy – enabled scientists in the early twentieth century to infer certain non-intuitive structures and behaviors of the universe’s smallest particles. These inferences developed alongside the development of spectroscopic methods of observing matter’s interactions with radiation to enable our current understanding of quantum mechanics.
Spectroscopy and Quantum Mechanics
Johann Jakob Balmer in 1885 demonstrated how a mathematical formula could represent the spectral lines of atoms of hydrogen, and Johannes Rydberg soon expanded the idea to represent other elements mathematically using the information provided by spectroscopic observation.
Then in 1913, Niels Bohr proposed a model of the subatomic structure of the hydrogen. He built on the spectroscopic observations of Balmer, Rydberg, and others to develop a quantum theory of the element’s composition.
The model that Bohr described provided the theoretical substantiation for the empirical results of spectroscopy. It showed that spectral line frequency could be understood as a difference between energy levels, an inference that Erwin Schrödinger and Werner Heisenberg would use to explain atomic and molecular structure in detail. This was the basis of quantum theory in the early twentieth century.
Wolfgang Pauli’s 1924 proposal that a magnetic moment is present in atomic nuclei was made to account for the empirical discovery of atomic hyperfine structure – in which energy levels in atoms shift as a result of interactions between the nucleus and electrons – with spectroscopy twenty five years earlier. Since then, spectroscopy has enabled more study of hyperfine structure, enabling quantum theorists to measure the nuclear moments of many more particles.
Applications of quantum mechanics – such as the atomic clock first built in 1949, the redefinition of a meter in 1983 and the assignation of a fixed value to the speed of light in the same year – have since been enabled by modern spectroscopy. The discovery of ultrafast spectroscopic methods, where the frequency can provide much more accurate measurements, has led to our precise understanding of the mechanics at the universe’s quantum scales.
Sources and Further Reading