Imperceptible motion

Galileo presented the main idea behind Galilean relativity in the Dialogue about the Two Chief World Systems (1632). In this work, a character named Salvatius describes two scenarios connecting a ship's cabin. In both scenarios, two friends are in the cabin, along with butterflies and other small flying animals, fish swim in a bowl, a bottle from which drops of water fall into a further container, and a ball. The cabin is below deck, so neither person can see outside.

In the first scenario, the ship is at rest. The animals move about obviously, and the two friends throw the ball to each other and leap about. The friend’s view that the flying nature fly with equal speed to all side of the cabin, the fish swim in all directions and the drops of water fall straight downward. When one friend throws the ball to the other, the effort required for the throw does not depend on the direction of the throw. When either person jumps forward, the effort required for the jump does not depend on the direction of the jump.

In the second scenario, the ship is roving at a constant velocity. That is, both the speed and direction of the ship are fixed. All the events that occur in the first scenario happen again: The small creatures fly and swim, the water drips, and the two associates throw the ball and leap. The motion of the ship has no effect on any of these events. Salvatius explains why this is so: All the objects in the cabin, as well as the living things, share in the motion of the ship.

Because the ship's motion has no effect on the events in the cabin, neither friend can tell by observing those events whether the ship is at rest or sad. This is the main idea last Galilean relativity.

Strictly speaking, an actual ship would not tour at a constant rate. For example, the ship would travel in a curve because Earth's surface -- as well as the surface of the water -- is curved. The ship would also curve due to Earth's rotary motion on its axis and its revolution around the sun. During periods of a few seconds, however, the ship's velocity could be almost entirely constant

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