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We can find that no object can have speed more than light from Einstein's equation of relativity because if anything have speed more than light then we get -ve value within square root. But why did Einstein took speed of light as unit or constant?

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    $\begingroup$ If you derive the speed of light from Maxwell's equations, you get an answer that is independent of reference frame. This was widely regarded as a shortcoming of the theory, but Einstein accepted it and ran with it... $\endgroup$
    – lemon
    Apr 9, 2016 at 17:58
  • $\begingroup$ Physics does not change by calling 299792458 $\frac{m}{sec}$ a constant equal to 1. Even better, all the equations and analysis become convenient if we ignore $c$ altogether until the very end and then only then to put it back into the result. $\endgroup$
    – Benjamin
    Apr 9, 2016 at 17:58
  • $\begingroup$ The speed was interpreted to be wrt a stationary medium pervading the universe, the aether. $\endgroup$
    – jim
    Apr 9, 2016 at 18:06
  • $\begingroup$ It's a bit rude to ask a question like this without first using google. What about the wikipedia page en.wikipedia.org/wiki/… is unsatisfactory? $\endgroup$
    – user12029
    Apr 9, 2016 at 22:56
  • $\begingroup$ Maxwell's equations gave Einstein a clue about $c$ as a limit. $\endgroup$ Apr 11, 2016 at 14:05

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The speed of light was first measured by

Ole Christensen Rømer (Danish pronunciation: [ˈo(ː)lə ˈʁœːˀmɐ]; 25 September 1644 – 19 September 1710) was a Danish astronomer who in 1676 made the first quantitative measurements of the speed of light.

When Maxwell formed what is the classical electromagnetic theory it was evident that the speed of light would be a constant in the equations.

The thinking out of the box by Einstein is that he imposed as a limit the velocity of light for massive objects also and proposed special relativity. The theory has been validated innumerable times in nuclear physics and particle physics.

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I think it was partially motivated by the following: With Maxwell's equations, a plane wave is a sinusoidal wave that varies in space in time and moving with speed $c$. These variations are linked by Maxwell's equations. What would happen if you could travel along with a plane wave at the speed c? You would observe fields that would be fixed in space and this would contradict Maxwell's equations. (See http://www.pitt.edu/~jdnorton/Goodies/Chasing_the_light/)

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