I know that there are several experiments (conducted after Einstein's formulation of Special Relativity) that confirm to some extend Einstein's postulate that nothing can travel faster than light. But my question is what motivated Einstein to make this assumption on the first place back in 1905.

A possible answer is due to the wave-like nature of light and the fact that a moving wave-source does not change the speed of the wave as long as the frequency is constant. But is this enough?

Apparently, within the theory, if this postulate is violated, several paradoxes appear. But my question is what motivated the postulate at the first place in order to build the theory based on the assumption that the speed of light is constant in all frames of reference.

I would really appreciate any light on that, as well as any resources or quotes from Einstein himself.

  • $\begingroup$ It comes from Maxwell's equations. Maxwell predicted the existence of electromagnetic waves, and he predicted that they always would be obsevered to travel at a fixed speed, C, in any frame of reference. Many physicists wondered whether light was explained by Maxwell's electromagnetic waves, and they wondered about the consequences of that fixed speed. Einstein was the first one who was able to tie all of their various ideas into one coherent theory. $\endgroup$ Mar 26, 2019 at 12:26
  • $\begingroup$ Check out the answers in physics.stackexchange.com/q/248499 $\endgroup$
    – jim
    Mar 26, 2019 at 12:27
  • 2
    $\begingroup$ This belongs on the history of science stack, where it has already been answered. The only thing those answers don't seem to cover very well is that Einstein has a comment somewhere where he also mentions that he was impressed by Maxwell’s eqs (thus took Lorentz more seriously than he took himself), because you could calculate the current in a coil of wire when that wire was moving towards a magnet, and when the magnet was moving towards the wire. The equations described different physics, but the same current. $\endgroup$
    – CR Drost
    Mar 26, 2019 at 12:34
  • 4
    $\begingroup$ I'm voting to close this question as off-topic because it is a history question that belongs on History of Science and Mathematics. See these guidelines for more details. (Though given the existing duplicate there, I'm unsure about the appropriateness of migration.) $\endgroup$ Mar 26, 2019 at 13:10
  • $\begingroup$ Why is no one mentioning the Michelson-Morley experiment? $\endgroup$
    – D. Halsey
    Mar 26, 2019 at 13:36

1 Answer 1


There were several motivations for the postulate that the speed of light is constant in every frame of reference. First and foremost was Maxwell's work on Electromagnetism. Maxwell found that the vacuum solutions to his equations gives rise to waves which he recognized as light. He gave formula for the speed of light $$c=\sqrt{\frac{1}{\epsilon_0 \mu_0}}$$ Since for all observers the value of $\epsilon_0$ and $\mu_0$ is same, the speed of light also must remain constant. Then there were also experiments like Michelson Morley experiment and Fizeau experiment that had confirmed constancy of the speed of light (though they did not intend to). There was enough scientific data for the postulate. In fact Henrik Lorentz had done the same two years before Einstein, that is why the relativistic transformations are named after him.


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