Overview

Back to 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