In 1905 Einstein published three papers. The first (dealing with the so-called ``photoelectric effect'') gave a very strong impulse to quantum theory, and got him the Nobel prize in 1921. The second dealt with the movement of small particles in a fluid (Brownian motion).
The third paper (Fig. 6.3) of 1905 was called On the electrodynamics of moving bodies, it changed the face of physics and the way we understand nature.
This paper starts with a very simple (and well known) example: if a magnet is moved inside a coil a current is generated, if the magnet is kept fixed and the coil is moved again the same current is produced (Fig. 6.4) This, together with the difficulties in detecting the motion with respect to the ether, led Einstein to postulate that
which is known as the Principle of Relativity.
In order to understand the implications of the Principle of Relativity we need (again) the concept of an inertial observer (see Sec. 4.3.1). This is a person which, when observing an object on which no forces act, finds that it moves with constant speed in a straight line, or else is at rest. In terms of inertial observers we can restate the Principle of Relativity:
Galileo made a very similar statement but he referred only to the laws of mechanics, Einstein's achievement was not only to provide a generalization, but to derive a host of strange, surprising, unexpected and wonderful consequences from it.