Maxwell Vs. Relativity

People already knew about the principle of relativity well before Einstein. (Click here for 'Relativity before Einstein). Galileo introduced relativity and Newton’s equations confirmed it. Newton’s equations of motion had shown that every event in space is relative to the observer and that every motion in space or velocity of anything is not absolute, but are relative to something else. In other words Newton’s equations of motion eradicated the notion of an absolute space which was till then the accepted norm. The lack of an absolute space seemed bizarre (even Newton found it difficult to accept it because he believed that it would indicate towards lack of an absolute god), but had to be accepted.

Ether

The principle of relativity originally predicted that everything moved in space relative to something else. But soon it was realized that this statement applied to everything except one that is light. Light was believed to be moving in vacuum, or simply, it was moving relative to nothing. This was no doubt contradictory for the principle of relativity. So, to save the theory, scientists assumed that the whole universe was filled with a substance known as ether and that everything including light travelled relative to this universal ether. Ether was just assumed to be there, it was never detected.

Now, the possibility of ether introduced a new hypothesis. Imagine the universe filled with an infinite sea of ether and all celestial bodies including earth moved through it. So this meant the ether moved relative to the motion of the celestial bodies. And as light also moved relative to the ether, the velocity of light would seem to be different in different parts of the universe. Different observers moving relative to ether would see light coming towards them at different. speeds, but the speed of light relative to ether would remain fixed. So, this meant that the speed of light was also relative to the observer. 

A SIMPLE ANALOGY-

This can be understood by a simple analogy. Imagine two observers observing a certain event at a certain place in space. So, now as space is relative, both the observers would get different values for the distance at which the event occurred, but as time was believed to be absolute, both of them will get the same value for the time taken for the light coming from the event to reach them. Now, as speed simply means distance divided by time, the observers would get different values for the speed of light. Well this fact about light was readily accepted (though not proved) but only until 1861 when Maxwell's theory of electromagnetism came up.

Maxwell’s theory & Michelson-Morley Experiment

According to Maxwell’s theory of electromagnetism, light was an electromagnetic wave which moved at a constant speed irrespective to the observer. This theory also got its proof when the famous Michelson-Morley experiment was conducted. In 1887, two scientists named Albert Michelson and Edward Morley conducted the first experiment to test whether light's speed did vary. It was expected that as earth was moving through the ether, there would be ether wind blowing all over the earth in a direction opposite to the direction of motion of earth through it. This meant that light would travel faster along the direction of the ether wind and it would travel slower against the direction of the ether wind. But, in which ever direction they measured, the speed of light appeared to be exactly the same! This result was quite unexpected and shocking for the scientists themselves. Michelson was later awarded the Nobel Prize in physics for his astounding experimental work.

The experiment was conducted by a device known as the interferometer invented by Albert Michelson. It could precisely measure changes in the speed of light along two arms set perpendicularly with mirrors at ends.

So the Michelson-Morley experiment proved Maxwell’s
electromagnetic theory but at the same time, posed a threat against the theory of relativity. This incompatibility of principle of relativity and Maxwell’s theory was quite a great problem. 
But in 1905 Einstein’s special relativity came for the rescue. (Click Here for SPECIAL RELATIVITY)

Albert Abraham Michelson

Edward Williams Morley