Old quantum theory
Theory · 1900 to 1925, Germany, Denmark, and Switzerland · Physics, stage 2: Map the theories
The old quantum theory is the set of partial fixes applied to classical physics between 1900 and 1925, before a full quantum mechanics existed. Planck opened it in 1900 by assuming that a hot body emits energy only in discrete amounts, which made the blackbody spectrum come out right. Einstein extended the idea in 1905 to light itself, explaining the photoelectric effect. Bohr applied it to the atom in 1913, allowing electrons only in certain orbits, which predicted the hydrogen spectrum correctly. The results worked but the reasoning did not hold together: quantum rules were bolted onto classical orbits with no principle deciding where. Recognising this as a transitional stage rather than a finished theory is the point of studying it.
What it claims
- Energy is exchanged in discrete packets rather than continuously.
- Light carries energy in quanta proportional to its frequency.
- Electrons in atoms occupy only certain allowed states, and emit or absorb light when they move between them.
- Classical mechanics cannot be repaired piecemeal; the discrete rules do not follow from it.
Key ideas
People
Sources
- Old quantum theory English Wikipedia
- Bohr model English Wikipedia
- Moments of Discovery: The Origins of Quantum Mechanics AIP Center for History of Physics
