Introduction

In the late 19th century, physicists faced a profound question: How does light travel through space? The prevailing theory suggested the existence of a mysterious medium called the luminiferous aether. The groundbreaking Michelson-Morley experiment of 1887 would challenge this understanding and lay the groundwork for Einstein's special relativity.

The Aether Hypothesis

Scientists in the 1800s believed that light, like all known waves, required a medium for propagation. They proposed the luminiferous aether, theorized to possess several unique properties:

According to Newtonian mechanics, Earth's motion through this aether should create an "aether wind" - similar to the wind felt when moving through still air. This wind was expected to affect light's speed depending on its direction of travel.

The Michelson-Morley Experiment

Experimental Setup

The experiment utilized several crucial components:

The Method

The experimental design was ingenious in its simplicity:

The mathematical prediction for the time difference ($\Delta t$) between the two beams, if aether existed, would be:

$$\Delta t = \frac{2L}{c} \cdot \frac{v^2}{c^2}$$

Where:

Results and Implications

The experiment produced a null result - no interference pattern changes were observed regardless of the apparatus's orientation. This unexpected outcome led to several possible interpretations:

Binary Star Evidence

Additional evidence for light speed constancy came from binary star systems.

In a binary star system, two stars orbit their common center of mass. If light's speed depended on the source's motion (emission theory), we would observe:

$$v_{observed} = c \pm v_{star}$$

This would result in:

Such variation would cause observed orbital motions to deviate from Kepler's laws. However, all observed binary star systems follow Keplerian orbits perfectly, supporting Einstein's postulate of constant light speed.

Impact on Modern Physics

The Michelson-Morley experiment's null result, combined with binary star observations, provided crucial evidence supporting Einstein's special relativity postulates:

While these results didn't definitively disprove the aether theory, they made it unnecessary, leading to its eventual abandonment in favor of Einstein's more elegant explanation.