The Luminiferous Ether
Every wave nineteenth-century physicists knew about needed something to travel through — sound needs air, ripples need water. When Maxwell showed light was a wave too, the obvious next question was: a wave in what? The answer they proposed was a substance filling the whole of space, and it set up one of the most important null results in the history of physics.
Every wave needs a medium — or so it seemed
By the late nineteenth century, Maxwell's equations had firmly established light as an electromagnetic wave. Every other wave known to physics at the time, though, was a disturbance travelling through some physical substance: sound is a pressure wave passing through air (or water, or a solid), ripples are a disturbance passing through the surface of water, and a wave on a guitar string is a disturbance passing through the string itself. In every known case, "wave" meant "a pattern moving through a medium" — remove the medium, and there was nothing left for the wave to be a disturbance in.
Reasoning by this analogy, physicists of the time concluded that light, being a wave, must also need a medium to travel through. Since light reaches us from the Sun and distant stars across seemingly empty space, this medium had to fill the entirety of space itself — including the vacuum between the Earth and the Sun, and indeed between every star in the sky. They named this hypothesised substance the luminiferous ether (literally, "light-bearing ether"): a medium proposed to be completely transparent, weightless, and undetectable by any means other than its role in carrying light waves.
Think of the ether the way you'd think of the air carrying sound. A bell struck in a vacuum chamber makes no sound at all, because there's no air to carry the vibration to your ear — remove the medium, and the wave has nothing to propagate through. Nineteenth-century physicists expected light to work the same way: a wave that, on this picture, could only exist as a disturbance rippling through the ether filling all of space, just as sound is a disturbance rippling through the air filling a room.
The consequence: an "ether wind"
If the ether genuinely exists and fills the whole of space, it follows that the Earth — orbiting the Sun at roughly $30\,\text{km}\,\text{s}^{-1}$ — must be moving through it. Just as walking or cycling through still air creates a noticeable "wind" relative to the moving person, the Earth's motion through a stationary ether should create an analogous ether wind sweeping past every point on the Earth's surface.
This ether wind matters enormously for measuring the speed of light. If light is a wave in the ether, its speed should be fixed relative to the ether itself — the same way the speed of sound is fixed relative to the air it travels through, regardless of how fast the source of the sound happens to be moving. A beam of light sent out in the same direction as the ether wind should, on this picture, be measured travelling slightly faster or slower than a beam sent out perpendicular to the wind, in exactly the same way a sound wave's measured speed depends on whether it's travelling with, against, or across a genuine wind of moving air.
If this directional difference in light's speed could be measured, it would give physicists something no purely mechanical experiment had ever achieved: a way to detect the Earth's absolute motion through space itself, rather than merely its motion relative to some other object. Detecting the ether wind wouldn't just confirm the ether existed — it would nail down a fixed, universal frame of reference against which every other motion in the universe could, in principle, be measured.
The obvious next step was to build an instrument sensitive enough to measure this tiny predicted difference in the speed of light — and that is exactly what the Michelson-Morley experiment, covered in the next lesson, was designed to do.
Key takeaways
- Every wave known to nineteenth-century physics travelled through a physical medium (sound through air, ripples through water), so light — shown by Maxwell to be a wave — was assumed to need one too.
- The hypothesised medium, filling the entirety of space, was named the luminiferous ether: transparent, weightless, and otherwise undetectable except for its assumed role in carrying light.
- If the ether exists, the Earth's orbital motion (roughly $30\,\text{km}\,\text{s}^{-1}$) means the Earth must be moving through it, creating an "ether wind" analogous to the wind felt when moving through still air.
- This predicts that light's measured speed should differ slightly depending on its direction of travel relative to the ether wind — travelling with it versus across it.
- If detectable, this directional difference would give physicists a way, in principle, to measure the Earth's absolute motion through space — something no mechanical experiment had ever managed.
- The Michelson-Morley experiment, covered next, was designed precisely to detect this predicted ether wind.