Unusual Effects Of Motion
Imagine a rocket ship traveling close to the speed of light. A number of unusual effects occur: Lorentz contraction, time dilation, and mass increase. These effects are as seen by an outside observer at rest. To the pilot in the reference frame of the rocket ship all appears normal. These effects will occur for objects other than rocket ships and do not depend on there being someone inside the moving object. Additionally, they are not the result of faulty measuring devices (clocks or rulers); they result from the fundamental properties of space-time.
A rocket moving close to the speed of light will appear shorter as seen by an outside observer at rest. All will appear normal to an observer such as the pilot moving close to the speed of light inside the rocket. As the speed gets closer to the speed of light, this effect increases. If the speed of light were attainable the object would appear to have a length of zero to an observer at rest. The length of the rocket (or other moving object) measured by an observer at rest in the reference frame of the rocket, such as the pilot riding in the rocket, is called the proper length. This apparent contraction of a moving object as seen by an outside observer is called the Lorentz contraction.
A similar effect, time dilation, occurs for time. As seen by an outside observer at rest, a clock inside a rocket moving close to the speed of light will move more slowly. The same clock appears normal to the pilot moving along with the rocket. The clock is not defective; the rate at which time flows changes. Observers in different reference frames will measure different time intervals between events. The time interval between events measured both at rest in the reference frame of the events and with the events happening at the same place is called the proper time. This time dilation effect increases as the rocket gets closer to the speed of light. Traveling at the speed of light or faster is not possible according to special relativity, but if it were, time would appear to the outside observer to stop for an object moving at the speed of light and to flow backward for an object moving faster than light. The idea of time dilation is amusingly summarized in a famous limerick: "There was a young lady named Bright, Whose speed was much faster than light. She set out one day, In a relative way, And returned on the previous night." As seen by an outside observer, the mass of the rocket moving close to the speed of light increases. This effect increases as the speed increases so that if the rocket could reach the speed of light it would have an infinite mass. As for the previous two effects to an observer in the rocket, all is normal. The mass of an object measured by an observer in the reference frame in which the object is at rest is called the rest mass of the object.
These three effects are usually thought of in terms of an object, such as a rocket, moving near the speed of light with an outside observer who is at rest. But it is important to remember that according to relativity there is no preferred or absolute reference frame. Therefore the viewpoint of the pilot in the reference frame of the rocket is equally valid. To the pilot, the rocket is at rest and the outside observer is moving near the speed of light in the opposite direction. The pilot therefore sees these effects for the outside observer. Who is right? Both are.