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I couldn't help but notice all you guys getting spooked up over here.
Back around the period from the 19th to the 20th century measuring standards/instruments and experimental procedures were just starting to really get super duper precise. So physicists naturally took a liking to playing around with and experimenting with light because of the level of precision required to do so.
A strange observation was then made. Light emitted from a moving object did not seem to gain any speed from it's moving point of origin. A lantern on a train going 50mph should emit light at C+50mph. But that simply wasn't the case. No matter which way you were attempting to cut the measurement.
This is where the "Aether Wind" idea was proposed. That the Universe and all of it's vast emptiness was actually a sort of fluid-like subtance as of yet undetectable known as aether. The strange behavior exhibited by light propagating through a vacuum (having the same speed in all frames of reference) merely a property of the aether itself. The "Aether wind" if you will. As with any as of yet untestable hypothesis it continued as a topic of intense debate well toward the end of the 19th century and even into the 20th century when the modern physics gang started to form. The most well known name in this gang of course being Albert Einstein...