The Birth of the Quantum

Blackbody A body that absorbs all the electromagnetic radiation that falls on it. Example: opening of a cave or oven:

Blackbody Radiation Purpose of study: establish relations between temperature, frequency distribution, and energy of the radiation emitted out of the blackbody Method: Treat radiation like gas molecules and apply thermodynamics, statistical mechanics, and Maxwell’s theory of electromagnetic waves. Best classic theory ran into problem: it predicted infinite energy at high frequency (“ultra-violate disaster”)

Planck Planck: tinkered with existing formula (Wien’s) and somehow got the right formula that corresponded to the experimental results Now: needs to derive the right formula from fundamental physical theories Calculation Trick: treat radiation energy (oscillators on the wall) as if they consisted of finite units: E = pe, hoping that e can eventually be made arbitrarily small But: e turned out to be finite: e=hv, proportionate to the frequency of the radiation h: Planck’s constant.

Planck’s Reactions Glad: that he found the right formula Desperate: That he could not reduce e or h to zero, thus reconciling with classical physics Eventual reconciliation: “My vain attempt to reconcile the elementary quantum with classical theory continued for many years….Now I know for certain that the quantum of action has a much more fundamental significance than I originally suspected.”

Photoelectric Effects Photoelectric effect: electromagnetic radiation, including light, when shone on metal, can eject electrons from it and produce an electronic current

Einstein’s Light Quantum 1905: Light was made of quanta; each light quantum carried the energy E=hv Thus, the frequency of light, not its intensity, determined the speed of the electron being ejected, which corresponded with experiment Equally radical step, if not more than, Planck’s Duality of light and eventually of matter