Most waves need a medium to travel through. But the way that light and gravitational waves travel show that space can’t be a medium at all.
“Well! I’ve often seen a cat without a grin,” thought Alice, “but a grin without a cat! It’s the most curious thing I ever saw in my life.” In Lewis Carroll’s imagination, the now-common picture of a Cheshire Cat took shape: where a cat’s grin could persist independent of the cat itself. It sounds like a complete absurdity, but in the world of physics, intuitively absurd notions are sometimes the only explanation that fits what the Universe presents to us.
For a long time, we understood waves as entities that propagate through some sort of medium. Water waves require a body of water to travel through. Seismic waves require a planetary object to travel through. Sound waves require something (normally air, but rock, bone, and other media work too) to travel through. If you take the medium away — no water, no planet, no air or other material, etc. — there can be no wave at all.
This held true up until the 1800s, when scientists first demonstrated the wave-like nature of light. While most assumed there was a medium that light traveled through, even naming it the luminiferous aether, experiment after…
