The Michelson-Morley experiment and the death of the luminiferous aether

The aether hypothesis

• In the 19th century, physicists believed that light, like sound, needed a medium to travel through. This hypothetical medium was called the luminiferous aetherA hypothetical invisible substance that was proposed to fill all of space and act as the medium through which light waves propagate. The Michelson-Morley experiment showed it does not exist..
• The aether was supposed to fill all of space and be perfectly transparent, massless, and rigid enough to support waves travelling at $3.0 \times 10^8$ m s$^{-1}$.
• As the Earth orbits the Sun at about 30 km s$^{-1}$, it should move through the aether, creating an "aether wind" that affects the measured speed of light depending on the direction of measurement.

The Michelson-Morley interferometer (1887)

• Michelson and Morley designed an interferometer to detect the aether wind by comparing the speed of light in two perpendicular directions.
• A beam of monochromatic light is split into two beams by a beam splitterA half-silvered mirror that transmits half the incident light and reflects the other half, creating two separate beams from one source. (a half-silvered mirror set at 45 degrees).
• One beam travels parallel to the expected aether wind and reflects off a mirror back to the beam splitter. The other beam travels perpendicular to the wind and also reflects back.
• The two returning beams recombine at the beam splitter and produce an interference pattern.
• If the aether exists, light travelling parallel to the wind should take a different time from light travelling perpendicular to it. When the apparatus is rotated by 90 degrees, the path lengths effectively swap, and the fringe pattern should shift.

The null result

• Michelson and Morley observed no fringe shift when the apparatus was rotated. The experiment was repeated at different times of year (when the Earth's velocity through the supposed aether would be in different directions) and the result was always the same.
• This null result meant there was no detectable aether wind, which implied either that the aether does not exist or that the speed of light is the same in all directions regardless of the observer's motion.
• Crucially, this experiment set the stage for Einstein's special relativity, which abandoned the aether entirely and took the constancy of the speed of light as a fundamental postulate.