The Principle of Relativity
Section Five
Einstein states the Principle of Relativity as follows:
If, relative to K, K' s a uniformly moving co-ordinate system devoid of rotation, then natural phenomena run their course with respect to K' according to exactly the same general laws as with respect to K. (p. 16)
How can you get the motion of the raven in K'? "Elementary, my dear Watson!", as Holmes may say. It is easily obtained by the "rule of parallelogram", and for this, see Inertia.
Although there seem to be some reasons that suggest this principle may not be universally valid, there is also a powerful evidence that supports this principle. If this principle were not valid, we might expect that some phenomena would show that the direction of motion affect the law of nature; but we have never seen such phenomena.
the most careful observations have never revealed such anisotropic properties in terrestrial physical space, i.e. a physical non-equivalence of different directions. (p. 18)
In terms of our example of the raven, does it move according to different laws in K and K'? No one has dreamt of that possibility! Remember the distinction between "initial conditions" and "laws"; the former may be quite different in the two systems, but not the latter. Notice also that the raven may move with an acceleration, but still, the laws governing its movement seem to be the same in both systems.
See also Galilean Relativity and Galilean Transformation
Last modified, April 18, 2002. (c) Soshichi Uchii
suchii@bun.kyoto-u.ac.jp
