Special Theory of Relativity

Galilean-Newtonian Relativity

Motion Viewed in Different Reference System

Galileo asked if observers could decide whose description was “correct”. He concluded that they couldn’t. In fact, each observer’s description was correct!

Galilean Principle of Relativity

The laws of physics are the same for all inertial reference systems.

You wake up in a windowless room on a train, which rides along particularly smooth tracks. Imagine that you have a collection of objects and measuring devices in your room. What experiment could you do to determine whether the train is stopped at the train station or moving horizontally at a constant velocity?

Michelson-Morley Experiment

Postulates of the Special Theory of Relativity

The idea of an absolute frame of reference was gone with the idea of stationary ether.

First Postulate

All laws of nature are the same in all uniformly moving frames of reference.

What would the light beam look if you traveled along beside it?

Second Postulate

The speed of light in the free space has the same measured value for all observers, regardless of the motion of the source or the motion of the observer; that is, the speed of light is a constant.

Simultaneity

Two events that are simultaneous in one frame of reference need not be simultaneous in a frame of reference moving relative to the first frame.

This non simultaneity of events in one frame that are simultaneous in another is a purely relativistic effect – a consequence of light always having the same speed for all observers.

Time Dilation

Clocks moving relative to an observer are measured by that observer to run more slowly.

Lorentz Factor,

2

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1

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Twin Paradox

Length Contraction

The length of the object is measured to be shorter when it is moving relative to the observer than when it is at rest.