The Michelson-Morley Experiment and Special Relativity

Introduction

The Michelson-Morley experiment is a cornerstone in the development of Einstein’s special theory of relativity. Its null result challenged the prevailing understanding of light and the universe, ultimately paving the way for revolutionary new physics.

The Aether Hypothesis

• In the 19th century, physicists believed that light, being a wave, required a medium to propagate, similar to how sound waves travel through air.
• This hypothetical medium was called the aether (or luminiferous aether).
• Properties attributed to the aether:
  • It filled all of space.
  • It was massless and transparent.
  • It provided an absolute frame of reference for the universe.

KEY TAKEAWAY: The aether was proposed as a medium for light propagation and as an absolute frame of reference.

The Michelson-Morley Experiment

• Designed by Albert Michelson and Edward Morley in 1887.
• Purpose: To detect the Earth’s motion through the aether by measuring the difference in the speed of light in different directions relative to the Earth’s presumed motion through the aether.
• Hypothesis: If the aether existed, the speed of light would be different depending on whether it was traveling:
  • In the same direction as the Earth’s motion through the aether (“downstream”).
  • Opposite to the Earth’s motion through the aether (“upstream”).
  • Perpendicular to the Earth’s motion.
• Apparatus: The Michelson interferometer, a device that splits a beam of light into two beams traveling along paths at right angles to each other. These beams are then reflected back and recombined, creating an interference pattern.
  • Diagram Description: A light source emits a beam that is split by a half-silvered mirror. One beam travels to a mirror along one arm of the interferometer, and the other beam travels to a mirror along the perpendicular arm. The beams are reflected back, recombine at the half-silvered mirror, and are observed as an interference pattern.
• Expected Result: A shift in the interference pattern as the interferometer was rotated, indicating a difference in the speed of light in different directions.

EXAM TIP: Understand the purpose, setup, and expected outcome of the Michelson-Morley experiment.

The Null Result

• The Michelson-Morley experiment detected no significant shift in the interference pattern.
• This meant that the speed of light appeared to be the same in all directions, regardless of the Earth’s motion.
• This unexpected result became known as the null result.

COMMON MISTAKE: Students often confuse the expected result with the actual (null) result.

Implications of the Null Result

• The null result of the Michelson-Morley experiment presented a major problem for classical physics and the aether hypothesis.
• Possible explanations considered:
  • The Earth was not moving through the aether (highly improbable).
  • The aether was being dragged along by the Earth (contradicted astronomical observations).
  • The aether hypothesis was incorrect.

STUDY HINT: Create a timeline of the experiment, the result, and the subsequent interpretations.

Einstein’s Special Theory of Relativity

• In 1905, Albert Einstein published his special theory of relativity, which provided a revolutionary explanation for the null result.
• Two Postulates:
  1. The laws of physics are the same for all observers in uniform motion (inertial frames of reference).
  2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
• How it Explains the Null Result:
  • Einstein’s second postulate directly contradicts the idea that the speed of light should vary depending on the observer’s motion relative to a hypothetical aether.
  • If the speed of light is constant for all observers, then the Michelson-Morley experiment would naturally yield a null result, as there would be no difference in the speed of light in different directions.

REMEMBER: Einstein’s postulates are the foundation of special relativity and explain the Michelson-Morley result.

Support for Special Relativity

• The null result of the Michelson-Morley experiment is considered strong evidence in support of Einstein’s special theory of relativity.
• It demonstrated that the classical understanding of space, time, and light was incomplete and that a new framework was needed.
• Further experimental evidence and theoretical developments have continued to validate special relativity.

APPLICATION: Special relativity has numerous real-world applications, including GPS technology and particle physics.

Comparison with Classical Physics

VCAA FOCUS: VCAA often asks about the differences between classical physics and special relativity.