Postulates of Special Relativity

• Inertial and Non-Inertial Frames of Reference
• Definition:
• An inertial frame of reference is one in which Newton's Laws of Motion are valid. (not accelerating)
• A non-inertial frame is one in which Newton's Laws of Motion are not valid.
• Example:

• consider an accelerating boxcar
• a mass suspended from the ceiling will hang as shown
• look at things from two point of views
• Observer outside of boxcar
• Apply Newton's Second Law in Horizontal and vertical directions



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• Observer inside boxcar
• Apply Newton's Second Law in Horizontal and vertical directions



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• Newton's Laws do not appear valid unless we introduce some fictitious force.



• Classical Principle of Relativity
• The Laws of Physics should be the same to all observers in inertial frames of reference
• What about light?

• Michelson- Morley Experiment

• goal to measure the speed of the Earth through the Ether
• Derivation
• "complete failure"

• Postulates of Special Relativity
• The Laws of Physics should be the same to all observers in inertial frames of reference.
• The speed of light is the same for all inertial observers.