Fizeau experiment

The Fizeau experiment was done by Hippolyte Fizeau in 1851. He wanted to measure how fast light travels in moving water.

Fizeau used a special tool called an interferometer to see how moving water changes the speed of light.

At the time, scientists thought that when light moves through something that is moving, like water, the light would be pulled along with it. They thought the speed of the light would add up with the speed of the water. Fizeau found that the water did pull on the light, but not as much as they expected. When he tried the same test with air instead of water, he found no effect at all.

His results agreed with an idea called the partial aether-drag hypothesis by Augustin-Jean Fresnel. This surprised many scientists. It was more than fifty years later that Albert Einstein explained these results with his theory of special relativity. Einstein showed that Fizeau’s experiment fit with what his theory predicted.

Even though it is called the Fizeau experiment, Fizeau was a busy scientist who did many different experiments to measure the speed of light in many situations.

Background

Main article: History of electromagnetism

In the 1700s, scientists were trying to learn more about light and electromagnetism. They believed there was a special substance called luminiferous aether that helped light waves travel. They were curious about how this aether moved and how it interacted with objects. One idea was that stars appear to move because of light traveling through this aether, called astronomical aberration. Later, a scientist named Fresnel thought the aether moved with objects depending on how the object allowed light to pass through it. After measuring the speed of light in air and water, Fizeau wanted to find out how fast light traveled when water was moving.

Experimental setup

In 1851, Hippolyte Fizeau did an experiment to see how light moves through water that is flowing. He split a beam of light into two parts. One part went with the flow of water, and the other went against the flow. After the light went through the water, the two beams came back together. This made a pattern of light and dark bands called an interference pattern. By looking at this pattern, scientists could learn about the speed of light in moving water.

Fizeau used a special setup to make sure both beams of light traveled almost the same distance. This helped make clear patterns even when using sunlight instead of a special light source. The light beams traveled back and forth through tubes of water, meeting again to show interference patterns that could be measured.

Result

Fizeau's experiment showed that light moves faster in water that flows in the same direction and slower when the water flows opposite to the light. But the change in light speed was only a tiny part of how fast the water was moving. This hinted that water can pull light a little as it moves.

Impact

Main article: Aether drag hypothesis

When Fizeau did his experiment, scientists had two ideas about how light and moving water worked together. One idea, from Augustin-Jean Fresnel, said that water would only pull light along a little bit. The other idea, from George Stokes, said water would pull light along completely.

Fizeau’s results showed that light was pulled along by the moving water, but not as much as Stokes’ idea predicted. This helped scientists see that Fresnel’s idea worked better.

Confirmation

Wilhelm Veltmann's colors of light

In 1870, Wilhelm Veltman showed that a theory about how light travels could explain results for different colors of light. This theory suggested that different colors might act a little differently when moving through certain materials.

Hoek experiment

Main article: Hammar experiment

In 1868, Martin Hoek tested an idea about how light moves through water. He used a special setup to compare how light travels in water and in air. His results helped support an older theory about light's behavior.

Mascart's birefringence experiment

In 1872, Éleuthère Mascart studied how light behaves when it passes through special materials. His work showed that light moves differently depending on its direction and type, matching predictions from an earlier theory.

Michelson and Morley confirmation

In 1886, Albert A. Michelson and Edward W. Morley repeated an earlier experiment with better tools. They made changes to avoid problems in the first test and confirmed the earlier results, showing how light behaves in both water and air.

Zeeman and Lorentz's improved formula

In 1895, Hendrik Lorentz suggested a new idea to improve an older formula about light. Later, in 1915, Zeeman tested this idea using special tools and water in Amsterdam, confirming Lorentz's suggestion.

Later confirmations

In 1910, Franz Harress used a spinning device to test how light moves and found results that matched an older theory, though he also noticed something new. Since then, many more tests have been done using different materials and methods to learn more about how light travels.

t 1 = A B c + v + D E c n − v   , {\displaystyle t_{1}={\frac {AB}{c+v}}+{\frac {DE}{{\frac {c}{n}}-v}}\ ,} t 2 = A B c − v + D E c n + v   . {\displaystyle t_{2}={\frac {AB}{c-v}}+{\frac {DE}{{\frac {c}{n}}+v}}\ .}

Lorentz's interpretation

Main articles: Lorentz ether theory and History of Lorentz transformations

In 1892, Hendrik Lorentz had a new idea about how light moves. He thought there was a special thing called the aether that stayed still even when water moved through it. He used something called "local time" to explain how fast light seemed to go.

Later, in 1895, Lorentz used this idea to explain more about light. But his idea did not match what the Michelson–Morley experiment showed. To fix this, Lorentz in 1892 suggested that things get shorter when they move very fast. This idea was later called the FitzGerald-Lorentz contraction. Lorentz kept working on these ideas until 1904. The math he made is now called the Lorentz transformations. These equations look like ones Einstein made later, but Lorentz made his work to fit what he saw, not from basic rules.

Einstein's use of Fizeau's experiment

Main article: Special relativity

Einstein used ideas from Fizeau's experiment to help develop his theory of special relativity. Other important experiments, like the moving magnet and conductor problem, the negative aether drift experiments, and the aberration of light, also gave Einstein clues about space and time. Fizeau's work helped Einstein understand how fast light travels when water moves. He thought these experiments, including Fizeau's, were very important for his ideas.

Modern interpretation

In 1907, Max von Laue showed that light's speed changes when water flows. When light moves through still water, it is slower than in air. But when the water moves, the light's speed changes in a special way.

Scientists explain this using rules about how speeds add together or with important math ideas called Maxwell’s equations. Both methods give the same result as Hippolyte Fizeau’s experiment long ago.