# Overview of Special Relativity

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