Simple Nature - Light and Matter

t / The Michelson-Morley experiment, shown in photographs, anddrawings from the original 1887 paper. 1. A simplified drawing of theapparatus. A beam of light from the source, s, is partially reflected andpartially transmitted by the half-silvered mirror h 1 . The two half-intensityparts of the beam are reflected by the mirrors at a and b, reunited, andobserved in the telescope, t. If the earth’s surface was supposed to bemoving through the ether, then the times taken by the two light waves topass through the moving ether would be unequal, and the resulting timelag would be detectable by observing the interference between the waveswhen they were reunited. 2. In the real apparatus, the light beams werereflected multiple times. The effective length of each arm was increasedto 11 meters, which greatly improved its sensitivity to the small expecteddifference in the speed of light. 3. In an earlier version of the experiment,they had run into problems with its “extreme sensitiveness to vibration,”which was “so great that it was impossible to see the interference fringesexcept at brief intervals . . . even at two o’clock in the morning.” Theytherefore mounted the whole thing on a massive stone floating in a poolof mercury, which also made it possible to rotate it easily. 4. A photo ofthe apparatus.Light travels at c.Now consider a beam of light. We’re used to talking casuallyabout the “speed of light,” but what does that really mean? Motionis relative, so normally if we want to talk about a velocity, we haveto specify what it’s measured relative to. A sound wave has a certainspeed relative to the air, and a water wave has its own speed relativeto the water. If we want to measure the speed of an ocean wave, forexample, we should make sure to measure it in a frame of referenceSection 7.2 Distortion of Space and Time 395