Science 240 Fall 1998

The Nature Of Scientific Thought

Outline For Lectures #9 & #10

The Origins of Quantum Mechanics: The Classical Perspective and the Need for a New Theory

Determinism vs. Indeterminism.

Kosso A&R pp. 116-126; 110 - 116.

1. Why Quantum Mechanics is of Special Interest
   1. Provides the current framework of physics.
   2. Has interesting and even bizarre implications for the nature of reality
   3. Concrete case of Realism (Einstein) vs. Instrumentalism (Bohr)
2. The Classical Perspective
   1. The Wave/ Particle Dichotomy
      1. 2 mutually exclusive and exhaustive categories
      2. Properties of Particles
      3. Properties of Waves
   2. The View that Light is the waving motion of electro-magnetic fields had been "proven"
      • Young's 2 slit experiment of 1803.
   3. Protons, electrons, tables, chairs are all particles.
   4. Energy, momentum, and spatial location are continuous variables.
3. Experimental Challenges to the Classical Perspective
   1. Overview.
   2. 1900: Planck's data on black body radiation.
      1. Classical Theory:
      2. Actual Data:
      3. Planck's proposal

         Energy is absorbed by a black body and emitted as light in terms of discrete units (particles). Energy of a light particle = h x frequency.h is Planck's constant: 6.63 x 10(-27) erg-sec. A bridge between the wave concept and the particle concept.

   3. 1905: Lenard's data on the photoelectric effect and Einstein's particle explanation.

      (You fill in the details)

      1. Classical predictions
      2. Actual data
      3. Einstein's proposal
   4. 1913: Bohr Invents a new quantized model of the atom.
   5. 1924: De Broglie's Hypothesis: Maybe all matter has wave-like properties.

      Suggests the following: particle momentum x c = particle energy = wave energy = h x frequency. Since c/wavelength = frequency, we get the wavelength of matter = h/ mv.

   6. 1927: The Davidsson-Germer 2 slit experiment.

      Original version with individual electrons. Later thought- version with individual photons.

      Conclusion: photons and electrons each show particlelike and wavelike behavior, but not at the same time or under the same conditions. Thus neither is a pure wave or a pure particle.

      1. Set-up:
      2. Actual Data:
         1. With only 1 slit open at a time.
         2. With 2 slits open at the same time.
         3. Alternate randomly and rapidly between two open slits or just 1 open slit.
      3. Puzzles
4. Determinism vs. Indeterminism
   1. Three interpretations of probability
      1. The frequency interpretation - applies to populations
      2. The dispositional interpretation - applies to individual objects
      3. The subjective interpretation
   2. Understanding Determinism
      1. Definition
         1. Relative determinism
         2. Absolute determinism
      2. Relation to prediction
      3. Examples of deterministic systems
         • The solar system
   3. Understanding Indeterminism
      1. Definition
         1. Relative indeterminism
         2. Absolute indeterminism
      2. Relation to prediction
      3. Some examples
         • coin tossing
         • your genetic heritage
         • radioactive decay