The negative result of the Michelson-Morley experiment is often quoted in physics books.

At that time, it was generally expected that a difference in the speed of light would be detected. After all, if you run after car, the car has a speed of 60 km/h with respect to the ground, and you run with 10 km/h, you would expect the relative velocity to be 60-10 = 50 km/h. (Think about mph if you prefer; the idea is the same.) That is, after an hour, the distance between you and the car would increase by 50 km.

But no such difference was found, despite the fact that the Michelson-Morley experiment, or similar ones, have been repeated with much greater precision in more recent times.

Einstein apparently wasn't aware of the Michelson-Morley experiment, or so he claimed.

>Hi Hilmar,
>
>I found this on the Gravity Probe B web site under [Project Timeline].
>
>http://einstein.stanford.edu/content/timeline/timeline.html
>
>The fact that the speed of light is constant regardless of the speed of the observer changed everything.
>
>
>#------------------------------------------------
>
>1887
>Michelson and Morley Experiment showing the speed of light remains constant - Einstein's theories sprang from a ground of ideas prepared by decades of experiments. One of the most striking, in retrospect, was done in Cleveland, Ohio, by Albert Michelson and Edward Morley in 1887. Their apparatus was a massive stone block with mirrors and crisscrossing light beams, giving an accurate measurement of any change in the velocity of light. Michelson and Morley expected to see their light beams shifted by the swift motion of the earth in space. To their surprise, they could not detect any change. It is debatable whether Einstein paid heed to this particular experiment, but his work provided an explanation of the unexpected result through a new analysis of space and time.
>
>#-------------------------------------------------
>
>Regards,
>
>LelandJ