لخّصلي

Early 19th-century scientists hypothesized an invisible, massless "ether" permeating all space, existing solely to transmit light waves. In 1887, physicists Albert A. Michelson and E. W. Morley devised an experiment using a specially designed interferometer to detect Earth's motion through this medium.

Their apparatus split a monochromatic light beam into two paths, then recombined them, creating bright and dark "fringes" from constructive and destructive interference due to phase differences. These differences would stem from path variations and, critically, time differences caused by motion through the ether. It was predicted that rotating the apparatus 90 degrees would alter the fringe pattern as the phase difference changed. However, Michelson and Morley's experiments consistently found no observable change in the fringe pattern, thereby failing to detect the Earth's motion through the hypothesized ether.

• In the early 19th century, scientist believed there was some invisible, massless
medium called “ether” that permeated all space, was undetectable by any
mechanical means and exists for the purpose of transmitting wave light only.
• Another important experiment was developed in 1887 by the American physicist
Albert A. MICHELSON and his associate E. W. MORLEY, who designed an
apparatus consisting of a specially designed Michelson interferometer, A
monochromatic beam of light is split in two; the two beams travel different paths,
and are recombined.
• Phase differences between the beams that combine lead to bright and dark bands,
or "fringes", due to constructive and destructive interference, respectively.
• There are 2 contributions to the phase difference between the beams:

1. The first contribution comes from the path difference AB − AC; one of the beams
   may travel a longer distance.


2. The second contribution, which would still be present even if the path lengths
   were equal, comes from the time difference between the upstream-downstream
   and cross-stream paths.
   • As the apparatus is rotated through 90◦, the fringes should change from bright to
   dark and back again as the phase difference changes. However, when Michelson
   and Morley performed their experiment, there was no observable change in the
   fringe pattern!
   • Moreover, in no experiment were Michelson and Morley able to detect the
   motion of the Earth through the ether!