S6. Einstein's Postulates of Special theory of Relativity

There was no absolute standard of rest. There exists a relativity principle that applies to all of physics, but it's not Galilean relativity. Laws of mechanics must be revised. Perhaps laws of electrodynamics can survive intact.”

In the year 1905, Albert Einstein published the theory of special relativity.
Einstein explained that when two objects are moving at a constant speed, relative motion between the two objects was essential, instead of appealing to the ether as an absolute frame of reference that defined what was going on. For instance, if you and some astronaut, Amber, are moving in different spaceships and want to compare your observations, all that matters is how fast you and Amber are moving with respect to each other and nothing more.
What Einstein does is to  extend the concept of the old Galilean relativity to include light too. He asserts that when this be done the laws of mechanics has to be modified. We must note here, that this holds only as long as the system being considered is moving with relative velocity. We thus have the special case (hence the name Special Relativity) .
The motion it explains is only if you’re travelling in a straight line at a constant speed. As soon as you accelerate or curve — or do anything that changes the nature of the motion in any way — special relativity ceases to apply. That’s where Einstein’s general theory of relativity comes in, because it can explain the general case of any sort of motion.
My apologies that, I presented the notes on basics of General Theory of Relativity first.
Anyway, Einstein’s theory has two key postulates:
The principle of relativity:
The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.
The principle of the speed of light:
·    As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.
The genius of Einstein’s discoveries is that he looked at the experiments and assumed the findings were true. This was the exact opposite of what other physicists seemed to be doing. Instead of assuming the theory was correct and that the experiments failed, he assumed that the experiments were correct and the theory had failed.
The belief in ether had caused a mess of things. In the latter part of the 19th century, physicists were searching for the mysterious thing called ether — the medium they believed existed for light waves to wave through. However, Einstein just removed the ether entirely and assumed that the laws of physics, including the speed of light, worked the same regardless of how you were moving  and that was the genius of the Einstein.
In the next notes we will be discussion on the concept of the LorentzTransformation.