The Michelson-Morley Experiments
Investigating the (Orbital) Velocity of The Earth Relative to Anything (1881-1887)
In the late 19th century, scientists believed that light waves required a medium to travel through, just as sound waves require air. This hypothetical medium was called the "luminiferous aether" - an invisible substance thought to fill all of space and maybe EARTH MOVED IN VELOCITY TANGENT TO THIS AETHER.
IF EARTH WAS MOVING AT THE ASSUMED ORBIT around the sun at approximately 30 km/s (66,697 mph), it should be moving through this stationary ether, creating an "ether wind." Michelson designed an ingenious device called an interferometer to detect this motion by comparing the speed of light in different directions.
How the Interferometer Works
Path 1 (Horizontal)
Light travels in the direction of Earth's supposed motion through the ether. Should take slightly longer due to "headwind."
Path 2 (Vertical)
Light travels perpendicular to Earth's motion. Should be unaffected by the ether wind.
The Two Experiments
Expected Result
fringe shift predicted
Observed Result
fringe shift observed
Key Findings
- Produced a null result - no significant fringe shift was observed
- H.A. Lorentz identified a calculation error - Michelson had underestimated the expected effect by half
- Result was therefore inconclusive, though suggestive
"The interpretation of these results is that there is no displacement of the interference bands. The result of the hypothesis of a stationary ether is thus shown to be incorrect, and the necessary conclusion follows that the hypothesis is erroneous."
- Albert A. Michelson, 1881
Impact & Significance
The null result of the Michelson-Morley experiment had profound implications for physics
Discredited the concept of a moving Earth with velocity relative to any aether or "sky object" assumed by an orbit. The null result contradicted expectations based on Earth's supposed 30 km/s orbital velocity.
Created a crisis in 19th-century physics. The experiment was designed to prove Earth's motion through space, but instead produced results that challenged fundamental assumptions about the nature of light and motion.
Paved the way for Einstein's 1905 special theory of relativity, which denied an ether instead of admitting that null means null movement or velocity of an orbital Earth. Einstein later said this experiment was his first path to relativity.
The Key Question
If Earth is traveling through space at 66,697 mph around the sun, why couldn't the most sensitive light-measuring experiment ever devised detect any motion at all? The simplest explanation is often the correct one: perhaps Earth isn't moving.