Einstein's Gedanken Experiment that lead to the Special Theory of Relativity Once, I read that Einstein founded the special relativity theory by imagining how an observer moves at the speed of light.
How does this thought experiment work? How to reach from this imagination to the relativity of time?
 A: John Norton at Pitt relates the story quite nicely. 
In Einstein's own words:

After ten years of reflection such a principle resulted from a paradox
  upon which I had already hit at the age of sixteen: If I pursue a beam
  of light with a velocity c (velocity of light in a vacuum), I should
  observe such a beam of light as a spatially oscillatory
  electromagnetic field at rest. However, there seems to be no such
  thing. . . on the basis of experience. . . . From the very beginning
  it appeared to me intuitively clear that, judged from the standpoint
  of such an observer, everything would have to happen according to the
  same laws as for an observer who, relative to the earth, was at rest.
  For how, otherwise, should the first observer know, that is, be able
  to determine, that he is in a state of fast uniform motion?

In other words, assuming both  

(1) that all motion is relative and
  (2) that it's possible for an observer to travel at $C$

leads to an impossibility:  

(3) that there is a reference frame in which a beam of light is just a "spatially oscillatory electromagnetic field at rest" i.e, a motionless electromagnetic wave.

Since he judged (3) to be impossible, then either (1) or (2) or both must be wrong. His insight was that (2) is wrong, that the speed of light is not attainable.
A: http://www.ivorcatt.co.uk/x41w.pdf
This idea is undermined by the pulse, or step, travelling unchanged down a transmission line (a circular loop of coaxial cable) at the speed of light. It happens in a USB cable, which Einstein did not know about.
Ivor Catt
