What was the old quantum theory?

The old quantum theory is a collection of heuristic corrections and semi-classical approximations applied to classical physics between 1900 and 1925. It emerged to explain individual phenomena like black-body radiation and atomic spectra that classical physics could not resolve. While never fully complete or self-consistent, it successfully paved the way for modern quantum mechanics through tools like the Bohr-Sommerfeld quantization condition.

What we know

• The old quantum theory comprises a collection of results and heuristic corrections to classical mechanics developed between 1900 and 1925. [1, 2]
• The main tool of the old quantum theory was the Bohr-Sommerfeld quantization condition, which selected allowed states for a classical system. [2]
• The final accomplishments of the old quantum theory included Edmund Stoner's determination of the periodic table and the Pauli exclusion principle, based on Arnold Sommerfeld's enhancements. [2]
• Early quantum ideas arose to explain phenomena such as blackbody radiation, the photoelectric effect, and solar emission spectra. [3, 5]

Still open

• The theory was never completely self-consistent or complete, operating instead as a collection of heuristic fixes.

Where to go next

• what caused itHow did Max Planck solve the black-body problem?
• what followedWhat came after the old quantum theory in 1925?
• how it workedHow does the Bohr-Sommerfeld quantization condition work?
• how we knowWhich observations forced the abandonment of classical physics?
• the bigger pictureHow does quantum field theory build on these foundations?
• an unexpected connectionHow does money resemble quantum mechanics?National currency is effectively quantized because money can only take specific discrete values, much like energy states in quantum systems.

Sources

• [1]old quantum theory (Wikidata Q2566926) · CC0 1.0
• [2]Old quantum theory · CC BY-SA 4.0
• [3]History of quantum mechanics · CC BY-SA 4.0
• [4]Quantum field theory · CC BY-SA 4.0
• [5]Quantum mechanics · CC BY-SA 4.0