Einstein's Theory of Relativity

Aether (also called ether): The substance which supposedly pervaded the universe and which transmitted electromagnetic waves.

Michelson-Morley experiment: The experiment was designed to calculate the speed of the "aether drift" in which the earth drifted. They proposed to calculate the expected differences in the speed of light from different vantage points with respect to the aether (when the earth is in different positions in its orbit). They found no difference in speed in any orientation or position that they tried.

Interferometer: the device used by Michelson and Morley to measure the speed of light by using the interference patterns of light waves.

Galilean Transformation: the rules that were used before the theory of relativity to go from one observer of measurements to a that of another observer who is moving at a constant speed with respect to the first.

Lorentz Transformation: the rules that need to be used according to the theory of relativity to go from one observer of measurements to a that of another observer who is moving at a constant speed with respect to the first.

Albert Einstein.

Had a super productive year in 1905 while working at a Swiss patent office.
- Photoelectric Effect.
- Brownian Motion.
- Special Theory of Relativity.
Left Germany in 1933 to come to US.
Added his signature to a letter to Roosevelt about the possibility of an atomic bomb.

Assumptions that Einstein used to develop his Special Theory of Relativity:

The laws of nature are the same in different inertial frames of reference.
The speed of light is constant in a vacuum.

Some properties of measurement which result from relativity. Assume that one observer O is moving with respect to another observer O'.

Length contraction; lengths shrink. If O measures the length of an object moving with O' then he will measure it smaller than O' will.
Time Dilation, time slows. If O measures the length of time between two events that happened in O' moving frame then it will appear to take less time than O' measurement of the time between the same two events.
Events which appear simultaneous in the O frame of reference may not be simultaneous in the O' frame of reference.

Assumptions that Einstein used to develop his General Theory of Relativity:

The laws of nature are the same in different frames of reference (where acceleration is taken into consideration).
Gravitational mass is equivalent to inertial mass.

Some properties that follow from General Relativity.

Light will be bent due to gravity (confirmed during a solar eclipse in 1919). Recall the elevator thought experiment.
Massive bodies bend space-time (confirmed by the anomalies of the orbit of Mercury).
Gravity will slow down time.

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