Michelson, Morley and the interferometer
In 1887 Albert Michelson and Edward Morley determined if there was any difference in the speed of light, based on the direction it travelled in.
They built an interferometer that send out two light beams: one beam of light was pointed along the direction of the earth's motion, the other light beam was pointed at a 90 degree angle relative to the first light beam. The beams were reflected by mirrors and focussed at a common point where they could interfere with each other. The light beam travelling along the direction of the earth's motion around the sun was expected to travel at a different speed compared to the light beam travelling at 90 degrees compared to this light beam.
They did not encounted any interference that suggested these two beams of light travelled at even slightly different speeds. They would have been able to measure a difference in speed if there was one. They could not detect any difference in speed between these two light beams.
In 1958 Froome used a microwave interferometer and a Kerr cell shutter to measure the speed of light to be 299,792.5 km/s.
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