Plancks Assumption Opens the Quantum Door

1
Plancks Assumption Opens the Quantum Door

• 1900 Planck models spectrum of blackbody
  radiation using nonclassical assumption energy
  can only come in discrete parcels
• 1905 Einstein explains photoelectric effect by
  incorporating Planck hypothesis lends credence
  to a particle theory of light

• 1911 Rutherford posits nuclear model for atom
  akin to solar system, orbiting electrons can in
  principle be at any radius.
• Serious problems Classically unstable cant
  predict observed line spectra
• 1913 Bohr proposes allowed orbits embedding
  idea of discreteness into model of nuclear
  structure

2
Successes of Bohr Model

• Solves puzzle of blackbody radiation
• Provides foundation for chemistry structure of
  valence (outermost) electrons in atom determines
  chemical properties

• Correctly predicts observed discrete atomic
  spectra

? Continuous spectra
? Discrete spectra

• Bohrs explanation Spectra are created when an
  excited electron jumps from one allowed
  orbit to another
• These allowed orbits are stable (nonclassical)
• Yet, suggests classical solar system
• Model only applies to one electron atoms
• Still no underlying explanation for why this
  particular structure is present

3
Reaction to Bohr Model

• Clearly it is onto something
• Explains periodic table and chemical interactions
  in terms of electron shell configuration (why a
  particular shellwho knows?)
• But, many still do not believe in the quantum
• Nuclear and atomic processes (radioactivity,
  emission) seem to be unpredictable (uncaused?)
• General belief is that Bohr model is purely
  descriptive, giving you superficial predictive
  ability, but no explanatory foundation chance
  is a reflection of epistemological inadequacies
  and that a deeper, objective, causal theory will
  be found soon

4
A Contrast in Ideas
Quantum
Classical
5
Development of the Quantum Theory

• Quantum theory developed, between 1900 and 1930
  in a willy-nilly method, and couldnt make a
  clean break with classical ideas - too much
  Newtonian baggage
• 1913 Solar system model of atom (Bohr)
• 1926 Wave Equation (Schrodingers wave
  formulation of mechanics - popular, as it
  provided a classical anchor for physicists to
  latch on to)
• The wave function contains all information
  about the present and possible future states
  of a system.
• The amplitude of the wave function is
  interpreted as the probability of realizing a
  certain outcome