# Special Relativity

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##### Scientific Theories
• scientific theory is an axiom system
• designed to explain certain kinds of phenomena
• defined by its postulates
• supported or disproved by its predictions
##### Introduction

It’s common sense that speed is relative.

• So, what is the speed of light relative to?
• And why does the speed of light appear the same, regardless of the observer’s motion?

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##### The Postulates of Special Relativity
• Principle of Relativity
• The laws of physics are the same in every non-accelerating reference frame.
• Constancy of the Speed of Light
• The speed of light is 299,792,458 m/s in a vacuum in every non-accelerating reference frame.

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##### Minkowski’s Spacetime Relativity

In 1908 Hermann Minkowski formulated Einstein’s Special Relativity as a geometry of four-dimensional spacetime, providing a useful and illuminating framework for Einstein’s theory and serving as the basis for Einstein’s 1915 theory of gravity, General Relativity.

• One of two identical twins blasts off into space, travels for several years near the speed of light, and returns. Special Relativity predicts she will be years younger than her stay-at-home sister.
• But motion is relative: if an object moves relative to a second, the second moves relative to the first.  Since the spacecraft moves relative to Earth, the Earth moves relative to the spacecraft.
• Therefore, Special Relativity should also predict that the Earth-bound twin will be younger than the traveling twin, making each twin younger than the other.

##### Derivations
• From Postulates
• From Lorentz Transformations
##### Modifications to Mechanics, and E = mc2

Classical Mechanics and Quantum Mechanics presuppose that distances and intervals are the same across inertial reference frames. The theories had to be modified to make them compatible with Special Relativity.

##### Historical Development
• 1865 Maxwell’s Electromagnetism
• 1865 Ether Hypothesis
• 1887 Michelson-Morley Experiment
• 1889 Contraction Hypothesis
• 1905 Annus Mirabilis
• 1905 Special Relativity
• 1908 Minkowski Spacetime