Back to Special Relativity

Table of Contents

1. It’s Common Sense that Speed is Relative
2. Special Relativity
3. Einstein’s Two Theories of Relativity
4. Two Formulations of Special Relativity

It’s Common Sense that Speed is Relative

• Aircraft
  • An aircraft’s speed differs relative to the
    • Air Around It (True Airspeed)
    • Ground (Ground Speed)
• Moving Walkway
  • The speed of a person running 5 mph on a moving walkway going 3 mph in the same direction differs relative to the
    • Moving Walkway
      • 5 mph
    • Ground
      • 8 mph
• Speed of Earth through Space
  • The speed of Earth through space differs relative to:
    • Sun
      • The Earth orbits the Sun about 67,000 mph 
    • Milky Way Galaxy
      • The Solar System orbits the center of the Milky Way Galaxy around 448,000 mph

Special Relativity

• In the last half of the nineteenth century questions arose regarding the speed of light:
  • Since speed is relative,
    • What is the speed of light relative to?
    • Why does the speed of light appear the same, regardless of the observer’s motion?
• Some physicists proposed that the speed of light is relative to a hypothetical substance through which light waves travel, the ether.  But the hypothesis ran into difficulties. In 1905 Einstein put forth a simple, counterintuitive solution: Special Relativity.
• Einstein’s theory has been overwhelmingly confirmed and is accepted today as established fact.  Physicists see Special Relativity in action in cosmic rays and particle accelerators.

Einstein’s Two Theories of Relativity

• Special Relativity (1905)
  • Einstein’s theory that the speed of light is not relative.
  • Postulates:
    • Principle of Relativity
    • Constancy of the Speed of Light
• General Relativity (1915)
  • Einstein’s theory of gravitation
  • Postulates:
    • Field Equation
    • Geodesic Postulate

Two Formulations of Special Relativity

• Einstein’s Special Relativity (1905)
  • Einstein’s theory that the speed of light is not relative.
  • Postulates:
    • Principle of Relativity
    • Constancy of the Speed of Light
• Minkowski’s Spacetime Geometry (1908)
  • Minkowski’s geometric formulation of Einstein’s theory
  • Postulates
    • Line Element for Spacetime
    • Invariance of the Spacetime Interval