“The relativity theory arose from necessity, from serious and deep contradictions in the old theory from which there seemed no escape. The strength of the new theory lies in the consistency and simplicity with which it solves all these difficulties, using only a few very convincing assumptions …” Special Relativity

Part 1: The Postulates of Special Relativity

Frames of Reference A frame of reference refers to where an observer is standing. The motion of an object is always measured relative to its frame of reference.

Frames of Reference Imagine that you are riding on a skateboard moving at 5 m/s east when you launch a water balloon at 10 m/s east at your friend who is standing on the side of the road. What is the speed of: The water balloon relative to your friend? Your friend relative to you? You relative to the water balloon?

Frames of Reference All frames of reference are valid. There is no such thing as an absolute frame of reference. There is no place in the universe that is completely stationary. Since everything is moving, all motion is relative.

Inertial frames of reference are frames of reference where Newton’s Laws hold true. Example: Standing on a bus moving at a constant speed on a smooth road. Under what circumstances would the bus be a non- inertial frame?

The 1st Postulate of Special Relativity: The laws of physics hold true for all inertial frames of reference (moving with constant velocities relative to each other). The 2nd Postulate of Special Relativity: The speed of light is constant in all inertial frames of reference. It is independent of the motion of the observer or source.

For a mechanical wave, i. e For a mechanical wave, i.e. one travelling through the vibration or movement of matter, if the source has a velocity, it is added to the velocity of the object when observed by a stationary viewer, like in the example above. Before the theory of relativity was devised it was thought that light needed, therefore, a medium to travel through …

The Michelson-Morley Experiment Scientists called this medium the luminiferous ether. If this were true, then as the Earth moves we would be able to detect an ether wind.

The Michelson-Morley Experiment Light is beamed through a splitting mirror into two beams which are each reflected by mirrors then recombined to form an interference pattern.

The Michelson-Morley Experiment If there is an ether wind, then the light traveling parallel into the wind should travel at a different speed from the light travelling perpendicular to the wind.

The Michelson-Morley Experiment As the apparatus is rotated through the ether wind the speeds of the light beams would change resulting in varying interference fringes … But the fringes remained the same regardless of the rotation. This result held when repeated in different conditions by other scientists. Therefore: There is no luminiferous ether Light travels at the same speed in all frames of reference

Michelson, Morley and other scientists were trying to describe light with Newtonian mechanics and Galilean transformations (adding velocities) with the idea of the ether. This however wasn’t consistent with the theory of electromagnetism which states that the speed of light is a constant 𝜇 0 𝜖 0 − 1 2 ≈3.00× 10 8 m/s. According to Newtonian mechanics with Galilean transformations someone running with a flashlight could make the emitted light exceed this constant value. This is the contradiction Einstein spoke of in the opening quote. His two postulates effectively deal with this contradiction but change the way we look at Newtonian mechanics and results in some very strange phenomena...

Simultaneity Since time can pass at different rates depending on the frame of reference, the order in which events occur is not absolute. Moving and stationary observers will see things happen at different times, and both points of view are valid; simultaneity is not an absolute concept.