Statement of the Principle of Relativity

statement of the principle of relativity is needed, a statement that can be tested by experiment. The following is the definition we will use in this text.

In the above definition the capsule can use anything you want as an example—a jet plane, a car, or a room in a building. Generally, think of it as a sealed capsule like the jet plane where the moving picture is being shown and all of the window shades are shut. Of course you can look outside, and you may see things going by. But, as shown in Figure (1), seeing things outside go by does not prove that you and the capsule are moving. That cannot be used as evidence of your own uniform motion. Think about what kind of experiments you might perform in the sealed capsule to detect your uniform motion.
One experiment is to drop a coin on the floor. If you are at rest, the coin falls straight down. But if you are in a jet travelling 500 miles per hours and the flight is smooth, and you drop a coin, the coin still falls straight down. Dropping a coin does not distinguish between being at rest or moving at 500 miles per hour; this is one experiment that does not violate the principle of relativity.
There are many other experiments you can perform. You could use gyroscopes, electronic circuits, nuclear reactions, gravitational wave detectors, anything you want. The principle of relativity states that none of these will allow you to detect your uniform motion.