Back to Special Relativity

**Table of Contents**

- It’s Common Sense that Speed is Relative
- Special Relativity
- Einstein’s Two Theories of Relativity
- 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)

- An aircraft’s speed differs relative to the
**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

- 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
**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

- Sun

- The speed of Earth through space differs relative to:

##### 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?

- Since speed is relative,
- 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**

**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