Michelson-Morley Experiment

Introduction

The Michelson-Morley experiment was a pivotal moment in the history of physics. Conducted by Albert A. Michelson and Edward W. Morley in 1887, it aimed to detect the relative motion of matter through the hypothetical “luminiferous aether” which was believed to carry light waves. The null result of this experiment set the stage for the development of Einstein’s special theory of relativity.

Background: The Luminiferous Aether

In the 19th century, it was commonly believed that just as sound waves require a medium to travel through, so too must light waves. This hypothetical medium was called the “luminiferous aether”. If this aether existed, the Earth’s motion through it would result in an “aether wind” which should be detectable by comparing the speed of light in the direction of Earth’s motion to the speed of light perpendicular to Earth’s motion.

The Experiment

The Michelson-Morley experiment used an interferometer to compare the speed of light in two perpendicular directions. Light from a single source was split into two beams traveling at right angles to each other. After reflecting off mirrors, the two beams were recombined and their interference pattern was observed.

If the speed of light was constant in all directions, the interference pattern would remain unaltered as the apparatus was rotated. If the speed of light varied with direction (due to the aether wind), the path length of one of the light beams would change relative to the other as the apparatus rotated, leading to a shift in the interference pattern.

Key Equation: Fringe Shift

The expected fringe shift due to the aether wind can be calculated using the equation

where is the length of each arm of the interferometer, is the wavelength of the light, is the speed of the Earth through the aether, and is the speed of light.

Results and Implications

Despite careful measurements and repeated trials, Michelson and Morley observed no significant fringe shift as they rotated the interferometer, indicating that the speed of light was the same in all directions to within the experimental uncertainty. This result was inconsistent with the existence of the luminiferous aether.

The null result of the Michelson-Morley experiment was a major puzzle for physicists of the time. It wasn’t until 1905, with Albert Einstein’s publication of the special theory of relativity, that a satisfactory explanation was provided. Einstein postulated that the speed of light is a constant in all inertial frames of reference, which naturally accounted for the null result of the Michelson-Morley experiment without the need for an aether. This marked the beginning of the modern understanding of space and time.

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