Galileo's Ship - Background

Background

Galileo's 1632 book Dialogue Concerning the Two Chief World Systems considered (the Second Day) all the common arguments then current against the idea that the Earth moves. One of these is that if the Earth were spinning on its axis, then we would all be moving to the East at hundreds of miles an hour so a ball dropped straight down from a tower would land West of the tower which would have moved some distance East in the interim. Similarly, the argument went, a cannon ball fired to the East would land closer to the cannon than one fired to the West because the cannon moving East would partly catch up with the ball. To counter such arguments the book observes that a person on a uniformly moving ship has no sense of movement and so a cannon ball dropped from the top of the mast would fall directly to the foot. To prove the point Galileo's fictional advocate Salviati proposed the experiment described below to show the classical principle of relativity according to which there is no internal observation (i.e. without, as it were, looking out the window) by which one can distinguish between a system moving uniformly from one at rest. Hence, any two systems moving without acceleration are equivalent, and unaccelerated motion is relative. The principle was stated only for mechanical motion. Later its application to the behavior of light led Albert Einstein to formulate the special theory of relativity.