Toward the end of the 19th century scientists believed they were close to a complete description of the universe. They imagined that space was filled everywhere by a continuous medium called the ether. Light rays and radio signals were waves in this ether just as sound is pressure waves in air. All that was needed to complete the theory was careful measurements of the elastic properties of the ether; once they had those nailed down, everything else would fall into place.
Soon, however, discrepancies with the idea of an all-pervading ether began to appear. You would expect light to travel...
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