Invariance vs relativity

A physical quantity is said to be

invariant

if different inertial observers would obtain the same result from a measurement of this quantity. Example: the mass of an object.

relative

if different inertial observers would obtain different result from a measurement of this quantity. Example: the speed of an object.

The principle of Galilean relativity states that Newton's law of motion are the same in all inertial frames of reference.

Until about 1860s, the Newtonian physics was considered completed, giving the ultimate and final description of the universe. (Recall, what Aristotle thought about his teaching?)

In 1860s James Clerk Maxwell (1831-1879) developed a theory of electricity and magnetism, which we now call Electromagnetism.

He also discovered electromagnetic waves, which propagate through space all by themselves at the speed of light. Soon it became clear that those waves were indeed light itself.

In Maxwell's theory the speed of light is the same in all reference frames, i.e. it is invariant, rather than relative. However, the principle of Galilean invariance states that the speed of anything, including light, is a relative quantity.

There appeared a contradiction...

The notion of waves propagating all by themselves, rather than in some medium (like sound waves in the air, ocean waves on the surface of the ocean etc) was also unusual to scientists, and thus disturbing.

The conclusion was quickly discovered: light was propagating not by itself, but in a special medium called luminiferous ether. So, the ether came back.

The luminiferous ether had no other reason for its existence than to provide the expected medium for the propagation of light.

Ether was massless and invisible. It could not be directly detected in the lab.

It was generally assumed that the Maxwell equations were valid only in the frame of reference of this ether. Thus, the speed of light was equal to

$$299,792,458{\rm\,m}/{\rm\,s}$$

only in the ether frame of reference. In other inertial frames it would be different according to the principle of Galilean invariance. In particular, it should be different on the Earth which moves through the ether.

In 1887, two American physicists, Albert Michelson and Edward Morley set out to measure the motion of the Earth with respect to the ether.

Michelson-Morley experiment

The result of their experiment was negative. They found that the speed of light along the Earth orbital direction was the same as the speed of light perpendicular the Earth orbital direction to within 5 km/s (the accuracy of their measurement).

Since the Earth orbital speed is about 30 km/s, the experiment clearly demonstrated that the speed of light was the same in all reference frames, in a clear contradiction to the principle of Galilean invariance. The ether was gone!

Another problem with the standard physics was explaining the radiation of a parcel of highly heated gas, the so called blackbody radiation.

From these two small ``clouds'' on a clean horizon of XIX century physics a two hurricanes came: Einstein's theory of relativity and quantum mechanics.