When people still thought space was absolute—that is, that every observer anywhere could measure space in the same way regardless of their position or velocity— they needed to explain what light traveled through so that all observers’ measurements of its speed would be the same. They thought there was an ether everywhere in the universe, even in empty space, that light and other waves traveled through. Albert Einstein pointed out this wasn’t necessary if people got rid of the concept of absolute time.
Ether Term Timeline in A Brief History of Time
The timeline below shows where the term Ether appears in A Brief History of Time. The colored dots and icons indicate which themes are associated with that appearance.
...Swiss patent office clerk Albert Einstein suggested there was no need for the idea of ether if you accepted time was not absolute. Henri Poincaré, a French mathematician, made a similar... (full context)