Yes, that ether that thinkers assumed to pervade otherwise empty space right into the modern era:
In the 18th and 19th centuries, as physicists grappled with light, there was some debate over whether it was a wave or a particle. (Trick question—we now know it has properties of both.) Scientists thought that if light were a wave, then it needed a medium to travel through. Waves, after all, aren’t objects themselves, but the effects of movement on a substance like air or water. And so, once again minds turned toward aether.
Dutch scientist Christiaan Huygens first proposed the “luminiferous” or light-bearing aether as a medium for the propagation of light. It was a popular theory. Nikola Tesla held onto it well into the 20th century, writing in 1938 that all attempts to explain the workings of the universe without recognizing the existence of a light-bearing aether are futile.
Yes, Nicholas Tesla said that.
But as scientists learned more about the true nature of light, the properties of aether became increasingly magical. To be consistent with the laws of physics, aether had to be fluid, so it could fill all space, but also solid enough to support light waves. It existed everywhere, yet was invisible, weightless, undetectable, and had no effect on physical objects—almost as if it didn’t exist at all. Meg Neal, “The Eternal Quest for Aether, the Cosmic Stuff That Never Was” at Popular Mechanics
As Neal goes on to note, while Einstein described the non-existence of ether as “very perplexing to physicists,” his relativity theory provided a simpler explanation: Light travels at a constant speed in a vacuum. In any event, his spacetime continuum, where everything is moving relative to everything else, became the new underlying concept, the new “ether.”
Doubtless, some protested, without ether, there would be nothing! But, as astronomer Hugh Ross explains, “nothing” has itself become a multi-layered concept in physics. That may make up in part, philosophically, for the loss of ether.
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See also: What does “nothing” mean in physics? (Hugh Ross)