For people who believed that light must travel through an ether, the result of the
Michelson-Morley experiment was like taking a ride in a boat and discovering that
the boat crossed the wave crests at the same rate when it was driving against the waves as
when it was driving in the same direction as the waves.
No one was sure what to make of the Michelson-Morley experiment until 1905, when
Albert Einstein offered the two basic postulates of special relativity and changed forever
the way we think about space and time. He asked all sorts of unconventional questions,
such as, “What would I see if I were traveling at the speed of light?” and came up with all
sorts of unconventional answers that experiment has since more or less confirmed.
The Basic Postulates of Special Relativity
Special relativity is founded upon two basic postulates, one a holdover from Newtonian
mechanics and the other a seeming consequence of the Michelson-Morley experiment. As
we shall see, these two postulates combined lead to some pretty counterintuitive results.
First Postulate
The laws of physics are the same in all inertial reference frames.
An inertial reference frame is one where Newton’s First Law, the law of inertia, holds.
That means that if two reference frames are moving relative to one another at a constant
velocity, the laws of physics in one are the same as in the other. You may have
experienced this at a train station when the train is moving. Because the train is moving
at a slow, steady velocity, it looks from a passenger’s point of view that the station is
moving backward, whereas for someone standing on the platform, it looks as if the train
is moving forward.