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Natural Science - Year II

Unit 56: Einstein and Relativity

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History Weblecture for Unit 56

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

History Lecture for Unit 56: The Revolution called Relativity

For Class


The Breakdown of the Mechanical Model

In the mid-nineteenth century, as we have seen, the classical mechanics model of the universe begans to suffer some challenges. Perhaps the last expression of this view of the universe was Maxwell's set of elegant equations describing the behavior of electrical and magnetic fields and their interactions. But Maxwell himself contributes to the discussion of thermodynamics and blackbody radiation, and so straddles the gap between the old physics and the new.

Relativity's Roots

While Einstein is given most of the credit for formulating the theory of relativity and recognizing its implications, the trouble with classical mechanics starts long before Einstein.

Read Stephen Hawking's summary of the History of Relativity.

  • Why were classical mechanics and electromagnetic field theory (light) in conflict?
  • How did Michelson's experiment attempt to measure the speed of light?
  • How would the experiment show the existence of an ether?
  • What is wrong with the ether hypothesis as science?
  • Why did the experiment "fail"?
  • What is the Lorentz-Fitzgerald contraction?
  • Why would you expect light to move faster if it were beamed from a moving platform?
  • If the speed of light is absolute, what are the implications for light observed from a moving and stationary source?
  • What are the two primary postulates of relativity? Who originally proposed them?
  • What theories does relativity come in conflict with? Have the discrepancies been resolved?

Einstein's Contributions

If you did not do so earlier, read Einstein's Biography at the Nobel Prize site.

  • What discoveries or theories drew Einstein to the problem of observing events "in time"?
  • What proofs could Einstein offer for his theory?

An outline of the situation: Points to Consider

There is a lot of material in the web readings for both the history and science sections this time, so I'm not adding more reading, but I have assembled an outline to help you synthesize materials from the different sources.

  1. The late nineteenth century concepts rested on classical mechanics as envisioned by Newton and Leibniz. We've already seen many of these developments in earlier units. These concepts include the following (you may want to review relevant units if you don't remember these!):
    1. Displacement, velocity, acceleration
    2. Force, Energy, and Momentum
    3. Electromagnetic fields
    4. Thermodynamic uncertainties
    5. Particles and waves
  2. The Michelson-Morley Experiment -- a magnificent failure (see link in science weblecture).
    1. Determination of the speed of light
    2. Attempt to detect the ether
    3. No shift = no ether!
  3. Lorentz-Fitzgerald Contraction -- the dependence of magnitudes on motions (see link in science weblecture).
    1. An object becomes shorter along its axis of motion
      1. Explains results of MM experiment
      2. Consistent with Maxwell's equations for electromagnetic fields
    2. Poincaré proposes relativity:
      1. Laws of physics the same for all observers
      2. Speed of light is the upper limit for all velocities
  4. Einstein's background (web reading above)
    1. Early interest: emphasis on relationships between masses
    2. 1905 papers
      1. Photons as quantized light (explains the Compton effect)
      2. Atomic size
      3. Electrodynamics of moving bodies: special relativity
  5. Special relativity (Hawking's discussion above and the links on the science weblecture)
    1. Primary postulates
      1. Laws of nature are the same in all inertial systems
      2. Speed of light same for all observers
    2. Thought experiments
    3. Time dilation
      1. Space and time are connected (contrary to Newton's claims)
      2. Simultaneous events do not appear simultaneous to all observers
      3. Twin paradox
    4. Physical evidence: muon decay time
    5. Conversion of matter to energy
  6. General relativity
    1. Equivalence of inertial mass and gravitational mass
    2. Interaction of gravity and light: light bent by solar gravitational field
    3. Interaction of gravity and space: Mercury's elliptical orbit affected

Study/Discussion Questions:

Further Study/On Your Own