Michelson–Morley Experiment introduction | Special relativity | Physics | Khan Academy

Summary

The Michelson-Morley experiment, conducted in the 19th century, sought to detect the luminiferous ether—a hypothetical medium through which light waves were believed to travel. The experiment involved measuring light's speed in different directions to observe any changes caused by the 'ether wind.' Despite various attempts to measure the difference, the interference pattern remained unchanged. This result cast doubt on the existence of the ether and suggested that light might travel through a vacuum. This groundbreaking experiment helped shift the understanding of light's behavior, leading to further scientific inquiries into relativity and the nature of space.

Takeaways

• 😀 The 19th-century assumption was that light, like other waves, travels through a medium called the luminiferous ether.
• 😀 The luminiferous ether was thought to define an absolute frame of reference for light, similar to how sound and water waves need a medium to propagate.
• 😀 The idea was that the Earth's movement through the ether should cause the speed of light to change depending on direction (faster in the direction of motion, slower against it).
• 😀 Michelson and Morley set up an experiment to detect differences in light speed based on Earth's motion relative to the ether.
• 😀 They used a setup with a light source, half-silvered mirrors, and detectors to create an interference pattern that could reveal changes in light speed due to the ether wind.
• 😀 The experiment was designed to measure the speed of light in different directions and check if it changed based on Earth's motion relative to the ether.
• 😀 If the luminiferous ether existed, the interference patterns should change depending on the orientation of the apparatus (due to the relative motion of the ether).
• 😀 However, no matter how the apparatus was oriented or when the experiment was conducted, Michelson and Morley always observed the same interference pattern.
• 😀 The result of the experiment implied that the speed of light was constant, no matter the Earth's motion, contradicting the idea of a luminiferous ether.
• 😀 This experiment is considered one of the most famous failed experiments in physics because it led to the questioning of the existence of the luminiferous ether and played a key role in developing the theory of relativity.

Q & A

What was the 19th-century assumption about light and its medium?

In the 19th century, scientists assumed that light, like other waves (e.g., sound or water waves), required a medium called the 'luminiferous ether' to travel through. They believed that light's wave-like properties were disturbances moving through this unseen medium.

What was the concept of the luminiferous ether?

The luminiferous ether was a hypothetical medium believed to exist throughout space, through which light waves were thought to propagate. It was assumed to be undetectable directly, but necessary for light to travel, just as sound needs air.

Why did scientists believe there was a 'relative velocity' in relation to the ether?

Scientists believed there was a relative velocity with respect to the ether because Earth was moving in multiple ways: rotating on its axis, orbiting the Sun, and the entire solar system moving through the galaxy. If the ether existed, Earth's motion relative to it would affect the speed of light in different directions.

What was the goal of measuring light's speed in different directions during the 19th century?

The goal was to measure the speed of light in different directions to determine whether light was affected by the motion of Earth relative to the luminiferous ether. Variations in speed would indicate that the ether was influencing light's travel.

What challenge did scientists face in measuring the speed of light accurately?

The challenge was that scientists did not have precise enough tools to measure the speed of light accurately enough to detect small variations caused by Earth's movement through the ether.

What experiment is cited as the turning point in questioning the luminiferous ether?

The Michelson-Morley Experiment is often cited as the turning point. They attempted to measure the speed of light in different directions, but the results did not show any expected variations, leading to doubts about the existence of the ether.

How did the Michelson-Morley Experiment work?

The experiment involved splitting a light beam with a half-silvered mirror, reflecting the two split beams off mirrors, and then recombining them. By observing the interference patterns, they hoped to detect changes in the speed of light depending on the ether's direction.

What did Michelson and Morley observe when they rotated the apparatus?

Michelson and Morley observed that, no matter how they oriented the apparatus (in different directions and seasons), the interference pattern remained unchanged. This suggested that the speed of light was not affected by motion through the hypothesized ether.

Why was the Michelson-Morley Experiment considered a 'failed' experiment?

The Michelson-Morley Experiment was considered a 'failed' experiment because it did not detect any changes in the interference pattern, which contradicted the expectations based on the ether theory. However, this failure was pivotal in pushing forward new ideas in physics.

What was the significance of the results from the Michelson-Morley Experiment?

The experiment's results were significant because they led to the questioning of the luminiferous ether. The constant interference pattern suggested that light might not need an ether to travel, which eventually contributed to the development of the theory of special relativity.