Fizeau experiment

The Fizeau experiment was carried out by Hippolyte Fizeau in 1851 to measure the relative speeds of light in moving water. Fizeau used a special interferometer arrangement to see how the movement of water affects the speed of light.

At the time, scientists thought that light moving through a moving medium would be pulled along by that medium. This meant they expected the speed of light to add up with the speed of the medium. Fizeau did find that the water pulled on the light, but not as much as they thought. When he tried the same experiment with air instead of water, he found no effect at all.

His results supported the idea known as the partial aether-drag hypothesis of Augustin-Jean Fresnel, which surprised many scientists. It wasn’t until more than fifty years later that Albert Einstein explained these results with his theory of special relativity. Einstein showed that Fizeau’s experiment matched what his theory predicted for adding speeds together when they are not too fast.

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

Background

Main article: History of electromagnetism

In the 1700s, scientists were trying to understand light and electromagnetism. They thought there was a special substance called luminiferous aether that helped light waves travel. They wondered how this aether moved and how it interacted with objects. One idea was that the movement of stars in the sky, called astronomical aberration, happened because of light moving through this aether. Later, a scientist named Fresnel suggested that the aether moved with objects depending on how the object let light pass through it. After measuring how fast light traveled in air and water, Fizeau wanted to find out how fast light traveled when water was moving.

Experimental setup

In 1851, Hippolyte Fizeau created an experiment to study how light moves through water that is flowing. He split a beam of light into two parts. One part traveled with the flow of water, and the other part traveled against the flow. After the light passed through the water, the two beams were brought back together. This created 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 create 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 travels faster in water moving in the same direction and slower when the water moves opposite to the light. However, the change in the speed of light was only a small part of the water's speed. This suggested that water partially pulls along the light 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, even though it didn’t fully explain why.

Confirmation

Wilhelm Veltmann's colors of light

In 1870, Wilhelm Veltmann 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 suggested a new idea about how light moves. He thought that a special substance called the aether stayed still while water moved through it. He found a way to explain how fast light would seem to go using something he called "local time."

Later, in 1895, Lorentz used this idea of local time to explain more about how light behaves. But his idea still had a problem because it didn’t match what the Michelson–Morley experiment showed. To fix this, Lorentz suggested in 1892 that objects get shorter when they move very fast. This idea was later called the FitzGerald-Lorentz contraction. He kept working on these ideas until 1904, and the math he created is now known as the Lorentz transformations. These equations look the same as ones Einstein would later create, but Lorentz’s work was made 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. Along with other important experiments, like the moving magnet and conductor problem, the negative aether drift experiments, and the aberration of light, Fizeau's work showed Einstein important clues about how space and time work. Einstein talked about how Fizeau's experiment helped him understand key ideas, especially about how fast light travels when water moves. He said these experiments, including Fizeau's, were very important to his thinking.

Modern interpretation

Max von Laue showed in 1907 that the way light moves when water flows can be explained using special ideas about how speeds add up. When light travels through water that isn’t moving, it goes slower than when it’s in air. But when the water moves, the speed of light changes in a special way.

Scientists can explain this using rules about how different speeds combine, or by using important math ideas called Maxwell’s equations. Both ways give the same result, matching what Hippolyte Fizeau found when he did his experiment long ago.