The concept of relativity has been known since the days of Galileo. The idea is that:
- The laws of physics are the same for all observers independent of how any given observer may be moving through space
- There is no experiment one can do that will distinguish one observers frame-of-reference as somehow 'preferred' (closer to being at rest with respect to the universe as a whole) over that of any other observer.
This applies to both Newtonian and Einstianian physics.
So, if I am on the surface of the earth and I measure everything relative to the surface of the earth, then I will see that the objects obey laws such as newtons laws, conservation of energy, momentum, etc. If I then measure these quantities from a moving train, the quantities themselves may be different but, the laws will still be obeyed. Of course, for the laws to be obeyed we must be consistent and measure all quantities from the same frame of reference.
This only applies when the frame-of-reference is moving at constant velocity, not accelerating or rotating. For example, if the train starts or stops moving, the people in it may be thrown forward or backward.
So, these principles can apply to the quantities marked relative in the following table:
| position | relative |
| orientation | relative |
| linear velocity | relative |
| angular velocity | absolute |
| linear acceleration | absolute |
| angular acceleration | absolute |
| momentum | relative |
| energy | relative |
Next
| special relativity page | do these principles apply to light? |
| Lorentz rotation | what happens when we apply these ideas in Euclidean space? |
