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Dr. Albert Einstein: Theoretical Physics, Relativity and the Photoelectric Effect, 1935

Galileo and Relativity

Though Einstein is the scientist most frequently associated with the theory of relativity, there are several thinkers who are responsible for its formulation. The first known person to theorize about relativity was Galileo, who articulated the first "relativity principle" in the seventeenth century. In generating his relativity principle, Galileo removed the distinction between stationary and moving observers, arguing that people on earth cannot tell if they are really at rest or if they are moving with the rotation of the earth each day. To demonstrate this, Galileo used the example of a cannonball falling from the top of a ship's mast. He noted that the cannonball will land at the base of the mast whether the ship is moving steadily through the ocean, or whether it is at rest in a dock. Even if they observe the falling ball, people on the ship cannot tell if they are really at rest or if they are moving with the ship. They cannot distinguish their state of rest from the ship's state by observing motion that takes place within the "reference frame" of the ship. In other words, a person at rest on the deck of a ship cannot determine whether the ship is at rest or moving at a steady speed through the ocean by observing actions that happen on the ship itself. That person must observe the ship relative to its surrounding environment in order to make such a determination.