The results of the Michelson-Morley experiment caused a new wave of research on the behavior of light. The result was Einstein's Special Theory of Relativity published in 1905. Because there is no aether, he knew then that there was no absolute frame of reference through which all laws of nature could be referenced. In other words, you can't measure the speed of an object relative to the aether, so you must measure the speed based on other objects. Motion is always relative.
Out of this discovery and further research, Einstein made two postulates that formed the basis for the Special Theory of Relativity.
Einstein's First Postulate
The laws of physics are the same in all frames of reference that are moving uniformly.
Einstein's Second Postulate
The speed of light is constant and is independent of the motion of the source or the observer.
Read through the tabs below to learn a bit more about each postulate.
First Postulate
Second Postulate
Think of a passenger on a train who is looking out the window. If they are moving at a constant speed and look out the window and see another train move passed their window, there is no way for that observer to know whether it is them that is moving or the other train or both. An observer may see the same relative motion if they are stationary and the other train is moving, the other train is stationary and they are moving, or they may both be moving at the same time. Note that if the train had no windows and the observer was moving at a constant speed (no bumps along the way) there would be no way to tell if they were moving. It is the non-uniformity of motion that can be felt and observed (i.e. acceleration).
Einstein's first postulate boils down to the fact that there are no experiments that can be done to detect uniform motion, only accelerated motion. You do not have to adjust the way you do things if you are moving at different constant speeds, because the laws of physics are constant at all different constant speeds. For example, if you were playing pool on a smoothly moving boat, you don't have to adjust how you play to accommodate different laws of physics!
Light behaves unlike most particles or objects, as we have discovered in our studies of the wave-particle duality of light. The second postulate is a little difficult to understand, but it has been verified through experimentation over and over again. Basically, it does not matter how quickly an observer is moving (even close to the speed of light) if they turned on a light, they would observe that light moving at 3.00 x 108 m/s.
Consider a car moving away from you and someone in that car turns on the light. The motionless observer (you) can measure the speed of that light to be exactly as expected: 3.00 x 108 m/s. It is a little counter-intuitive because relative velocities would seem to make that speed increase because of the source moving away (remember the Doppler effect with sound), but it doesn't happen. It wouldn't even happen if you were to have a source moving away from you at speeds close to the speed of light. All observers regardless of the speed of the source or the observer will ALWAYS measure the speed of light to be the same.
Question
Which of the postulates does not line up with what was known in classical physics?
The second postulate did not line up with classical physics, which says that the speed of something is relative to the speed of the observer and speed of the source. Light does not obey this rule of classical physics.
