http://www.ck12.org Chapter 22. The Special Theory of Relativity
22.1 Galilean Relativity
Objectives
The student will:
- Understand the apparent contradiction between Galilean principle of relativity and the invariability of the
speed of light. - Understand that the equations of Galilean relativity must be changed to remain consistent with Maxwell’s
equations.
Vocabulary
- ether
- Galilean relativity:The Galilean equations of motion.
- inertial frames of reference
Introduction
The basic principle of relativity is this:
If you are moving at a constant speed, all physical laws should work the same as if you are at rest, and
everything else is moving relative to you.This was true of classical physics, known asGalilean Relativity. However, it ran up against a very difficult
paradox with the results of experiments in the speed of light. The resolution to that paradox was formulated by
Albert Einstein, and is known as the Special Theory of Relativity, along with the General Theory of Relativity.
Galilean Relativity
Earlier, we discussed the concepts of Galilean relativity and relative motion. A simple example of relative motion is
sound waves. Sound waves travel at a fixed speed relative to the air, roughly 340 meters per second. Consider these
cases:
- A person standing still who shouts will see the sound wave travel off at 340 m/s.
- A person who is driving at 40 m/s will feel a wind blowing at 40 m/s at them. If they honk their horn, the
sound wave travels at 340 m/s with respect to the air, which is only 300 m/s ahead of the car. - An airplane travelling at 350 m/s will create a sonic boom. The sound from the plane travels at 340 m/s
relative to the air, which means that it is slower than the plane.
In Galilean relativity, the result are very intuitive. The velocities are simply added together.