We now examine the two postulates of relativity, on which Einstein’s theory is based
1. The Relativity Postulate: The laws of physics are the same for observers in all
inertial reference frames. No frame is preferred.
Galileo assumed that the laws of mechanics were the same in all inertial reference frames. (Newton’s first law of motion is one important consequence.) Einstein tended that idea to include a/l the laws of physics, especially electromagnetism and optics. This postulate does not say that the measured values of all physical quantities are the same for all inertial observers: most are not the same. It is the laws of physics. which relate these measurements to each other, that are the same.
2. The Speed of Light Postulate: The speed of light in vacuum has the same value c in all directions and in all inertial reference frames. We can also phrase this postulate to say that there is in nature an ultimate speed c. the same in all directions and in all inertial reference frames. Light happens to travel at this ultimate speed. as do any massless particles (neutrinos might be an example). However. no entity that carries energy or information can exceed this limit. Moreover, no particle that does have mass can actually reach speed c. no matter how much or how long it is accelerated. Both postulates have been exhaustively tested. and no exceptions have ever been found.