Luminiferous aether

The luminiferous aether or ether was an old idea about an invisible stuff that filled all of space. People thought it helped light travel. They knew light acted like waves, and waves usually need something to move through, like water or air. Since space looks empty, they wondered how light could move through it. So, they thought the ether was the special material that let light move even in a vacuum, which is empty space with nothing in it.

For many years, scientists argued about whether the ether really existed. They thought it was an endless, invisible material that didn’t affect anything else. But as they studied light more, especially in the 1800s, the idea of the ether got confusing. By the late 1800s, many scientists were starting to wonder if the ether was even real.

A very important test called the Michelson–Morley experiment in 1887 showed no sign of the ether. More tests later agreed with this. This helped scientists develop new ideas about how light works. One big breakthrough was the special theory of relativity. The Michelson–Morley experiment, along with other important studies like the blackbody radiator and photoelectric effect, helped create what we now call modern physics. This includes ideas like relativity and quantum theory, which help us understand light as both a wave and a particle.

History of light and aether

Particles vs. waves

In the 1600s, a scientist named Robert Boyle thought a special substance called the aether helped explain how light and magnetism worked. Another scientist, Christiaan Huygens, suggested that light was a wave moving through this aether. But they could only imagine these waves moving in one way, which couldn’t explain how crystals split light into different colors.

Isaac Newton believed that light was made of tiny particles. This idea could explain why light travels in straight lines and bounces off surfaces. However, it couldn’t fully explain how light bends when it passes through different materials.

Bradley suggests particles

In the early 1700s, James Bradley tried to measure the distance to stars by watching their positions change as Earth orbits the Sun. He didn’t find what he expected, but he did discover that stars appeared to shift slightly in position because of Earth’s movement. Bradley explained this using Newton’s idea of light as particles.

Wave-theory triumphs

Later, scientists Thomas Young and Augustin-Jean Fresnel brought back the idea that light was a wave. They showed that light could be a wave moving in two directions, which explained how crystals split light. They thought that, like other waves, light needed a medium to travel through, so they imagined aether filling all space.

Electromagnetism

In the mid-1800s, scientists found links between light and electric and magnetic forces. James Clerk Maxwell showed that light could be a wave made by electric and magnetic fields. He suggested that these fields moved at the same speed as light, supporting the idea that light was a form of electromagnetic wave.

Problems

As time went on, the idea of aether became harder to support. The aether was thought to be a special substance that filled all space, but experiments trying to detect it failed. By the early 1900s, new ideas about how light and space worked made the aether idea unnecessary.

Relative motion between the Earth and aether

Aether drag

Main article: Aether drag hypothesis

Two ideas tried to explain how the Earth moves through the aether. One was by Augustin-Jean Fresnel in 1818. He thought the aether was almost still but moved a little when Earth passed through it. Another was by George Gabriel Stokes in 1844. He thought the aether moved completely with Earth. But this idea did not match what we see with stars, so it was not accepted.

An important test of Fresnel's idea came from Fizeau's experiment in 1851. It showed that light moving through water changes speed in a way Fresnel had predicted.

Negative aether-drift experiments

A big problem with Fresnel's idea came from two theories: Newton's rules for motion and Maxwell's rules for electricity and magnetism. These two did not always match up when thinking about light moving through the aether.

One simple way to think about this is with sound. Sound moves faster in water than in air. If you hear a splash underwater and then hear it again through the air, you’ll hear it later the second time. Similarly, if you're in a moving airplane, you can still talk to someone next to you because the sound moves with the air inside the plane.

But for light, things were different. Maxwell’s work said that light always moves at the same speed no matter where you are.

The Michelson–Morley experiment compared the time for light to reflect from mirrors in two orthogonal directions.

Because of this, scientists thought there should be one special place where the aether was still. They tried many experiments to see if Earth was moving through this aether, but most of these early tests gave no clear answer.

First-order experiments

Even though Fresnel thought the aether was almost still, his idea only expected very small effects in these tests. Many early tests looked for these small effects but found nothing.

These tests included experiments by François Arago, George Biddell Airy, Éleuthère Mascart, Fizeau, Martin Hoek, Wilhelm Klinkerfues, Ketteler, and others. Most of these found no sign that Earth was moving through the aether.

There were also tests using electricity and magnetism, like those by Wilhelm Röntgen, Theodor des Coudres, Königsberger, and Frederick Thomas Trouton. These also showed no clear sign of movement through the aether.

Second-order experiments

Later, more careful tests were done to look for even smaller effects. The famous Michelson–Morley experiment in 1887 was one of these. It looked for changes in light when sent in different directions but found almost no difference, which was a big problem for the idea of aether. Since then, many more tests have been done, and they all agreed closely with the idea that there is no aether.

Many scientists tried to fix the aether idea by adding new rules, but none worked well. Even tests that looked for twists or changes in light caused by Earth’s movement found nothing.

During the 1920s, Dayton Miller said he saw small effects, but others could not repeat his results. Since then, tests have become much more exact, and they all agree that there is no aether.

Lorentz aether theory

Main article: Lorentz ether theory

Between 1892 and 1904, Hendrik Lorentz created a theory about how light travels. In his idea, a special material called the aether existed. He explained that changes in this material could only happen at the speed of light. Lorentz also introduced ideas like length contraction to explain experiments and created a math rule called the Lorentz transformation. Later, Henri Poincaré improved Lorentz's work and talked about how the speed of light stays the same.

End of aether

Special relativity

The idea of aether was challenged when Albert Einstein introduced his special theory of relativity. This theory changed how scientists understood space and time. Instead of needing a special substance called aether for light to travel through, Einstein showed that space and time could change depending on how you move. This made the idea of aether unnecessary.

Einstein also explained that light could behave like both a wave and a particle, which meant it didn’t need a medium to travel through. Over time, most scientists stopped using the idea of aether because Einstein’s theories worked better and explained many observations.

Other models

Some scientists later suggested new versions of the aether idea, but these were very different from the old concept. For example, Paul Dirac, a famous physicist, wondered if there might still be something like aether, but he didn’t create a full theory to prove it.

Einstein's views on the aether

When Einstein was young, he was very interested in the aether. Later, after developing his theory of general relativity, he talked about a “new aether.” This wasn’t like the old aether but rather a way to describe how space and time have properties that affect matter. However, most scientists didn’t follow Einstein’s use of the word “aether,” and it didn’t change how physics developed.