Lecture 19 – Modern Physics
- “Classical” post-Newtonian world-view:
 “Absolute” space and time
 Indivisible atoms
 Aether
- Success of 19th century science
- Relativity theory: All measurements are relative to the
movement of the object
- Quantum mechanics: Atoms are both waves and particles
- Predictive success and mathematical accuracy
Wave and Particle Theories
- Wave and particle theories of energy
- 19th century support for a wave theory:
 Light speed and intensity
 Interference patterns
- Wave theories of light assumed the existence of an aether
Troubling Experimental Results
- Earth’s motion through the “aether wind”
- 1887, Albert Michelson: measured speed of Earth
- Sensitive experiment, failed to make measurement
- Reasons for failure:
 Earth does not move
 Aether was dragged along by earth
- William Crookes (1832-1919): rays in cathode tube,1876
 Explanations: particles or vibrations in aether
- 1897 JJ Thomson (1856-1940): cathode rays involved
particles with negative charge:
 Magnet deflects rays, rays impart negative charge
 Mass to charge ratio for particles identical
 Particles thousand times smaller than hydrogen atom
- Max Planck (1858-1947) EM radiation comes in packets or
quanta, not continuous
Relativity Theory
- Different velocity measurements, absolute velocity
- Albert Einstein (1879-1955), special theory of relativity
- All measurements (mass, time, length) depend on the
position and motion of the object to be measured
- Relativity and particle accelerators, electron microscopes &
equipment where particles approach speed of light
- Equivalence of energy and mass, E=MC2
- Gravity and acceleration formally indistinguishable
- Gravity warps the fabric of space due to the mass of a body
- Planets orbit the sun following shortest path in curved space
- Light “bent” by gravity, 1919 solar eclipse experiment
Quantum Mechanics
- Ernest Rutherford (1871-1937) gold foil experiment, strong
positive charge at centre, atom mostly space
- Rutherford and Nils Bohr (1885-1962) “planetary” model for
the atom, with a central nucleus and electrons
- Electron should be unstable & give off continuous radiation
- Quantum theory: orbits of electrons are areas where there is a
probability of an electron being found
- Louis de Broglie (1892-1987): E=mc2, E=hv, all particles
have wavelength
- Erwin Schrodinger (1887-1961) probability waves for
particles
- Werner Heisenberg (1901-1976) “Uncertainty Principle”,
position & velocity of particle not known with accuracy
- Quantum mechanics conflicts with commonsense and
classical physics:
 Matter as both a particle and a wave
 Theory irreducibly probabilistic
- Einstein: QM incomplete, “God does not play dice”
Theory and Practice
- Electronics, materials science, high energy physics and
astronomy
- Direct practical applications not common
- Cultural influence of theories
- Quantum mysticism
- Relativity of knowledge
- Limits of knowledge and predictive success