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The speed of light in a vacuum has been shown both in experimental observatons and in direct observations in the real world to agree to a very high degree with  Einstein's equation.  It has been measured for over 150 years, and has never varied more than the statistical error of the measurements.  It's not like he made this stuff up out of thin air. It is so acccurate, in fact, that we know the distance from the Earth to the moon at any point in it's orbit to a very high degree of accuracy by bouncing lasers off reflectors left there by the Apollo astronauts.