Essay: Michelson and Morley Experiment

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Michelson-Morley experiment of 1887 was not the first of its kind nor was it the last. Michelson had built a prototype half a decade earlier and used it to test the movement of light. And even until 1930, the Michelson-Morley hypothesis was still being tested (Lombardi). However, the conclusion reached in 1887 was not that which was hypothesized, and the impact of the experiment was that a new theory of light movement needed presentation. Albert Einstein presented such a theory in 1905, the theory of special relativity. This paper will examine the Michelson-Morley experiment, detailing its conclusions and its impact on the study of light, and how the lack of support for its hypothesis helped form Einstein's.

First, the discussion must begin with a look at electromagnetism. Studies in the field of electromagnetism had produced several observations. James Clerk Maxwell not only "developed a scientific theory to explain electromagnetic waves [after] noticing that electrical fields and magnetic fields can couple together to form electromagnetic waves," ("Scientists and Electromagnetic Waves") but he also united previous observations concerning electricity, magnetism and light to formulate a theory of wave movement. According to P.J. Nahin, "By showing that the science of light and optics is merely a branch of electromagnetism, Maxwell achieved the second great unification in physics (the first being Newton's unification of terrestrial and celestial mechanics)." Since, however,

All the other waves [scientists] knew about required a medium, [and] since no medium was apparent between the earth and the sun, it was presumed that this medium was transparent and therefore not readily observable -- it was called the "ether." The popular presumption was that this ether was stationary and filled all of space. This involved the presumption that there was an absolute reference frame in the universe, and that all the movement of planets and stars was through this ether. (Rave)

Maxwell had written to the U.S. Nautical Almanac Office in Washington in 1879 to inquire "about the possibility of measuring the velocity of the solar system through the ether by observing the eclipses of Jupiter's moons" (Nave). But Maxwell ultimately decided that what he sought to measure would be too small to observe. Albert Michelson had just moved to the U.S. Nautical Almanac Office and was intrigued by the question. A decade later, and contrary to what Maxwell believed, both Michelson and Edward Morley intended to show that light waves moved through ether-wind, which affected their course.

The theories of wave movement at the end of the 19th century were concerned with the question of medium:

J PhysicsPhysics is a natural science that involves the study of matter and its motion through spacetime, as well as all applicable concepts, including energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the...

ust as water waves must have a medium to move across (water), and audible sound waves require a medium to move through (such as air or water), so also light waves require a medium, the "luminiferous ether." Because light can travel through a vacuum, it was assumed that the vacuum must contain the medium of light. Because the speed of light

The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Light and all other electromagnetic radiation always travel at this speed in empty space, regardless of the motion of the source or the inertial frame of the observer. Its value is exactly...

is so great, designing an experiment to detect the presence and properties of this ether took considerable ingenuity. ("Michelson-Morley Experiment")

Detecting the medium through which light moved was the primary objective of Michelson's experiment. While his initial experiment showed no evidence of medium, the famed Michelson-Morley experiment of 1887 was considered a success, even though Michelson and Morley still found no evidence to support their hypothesis. What it did do was allow Maxwell's unification of light, magnetism, and electricity to serve as the gateway to Einstein's theory of special relativity.

Albert Michelson had first attempted to measure the velocity of light in 1881 after inventing the interferometer "for the purpose of discovering the effect of the Earth's motion on the observed velocity" ("Albert a. Michelson -- Biography"). His initial attempt yielded no evidence of the effect of ether wind on light velocity. Michelson's apparatus, however, was only a prototype, and a much tighter experiment was soon to follow with the help of Edward Morley. Still, the result of the Michelson-Morley experiment would show "that light travels at a constant speed in all inertial systems of reference" ("Albert a. Michelson -- Biography").

The interferometer that Michelson and Morley used in 1887 was comprised of a single source of light that was split "so that one beam strikes a fixed mirror and the other a movable mirror. When the reflected beams are brought back together, an interference pattern results" (Rave). What Michelson attempted to do soon after Maxwell called the U.S. Nautical Almanac Office in Washington was to observe the movement of light through the ether medium:

In trying to measure the speed of the Earth through the supposed "ether," you could depend upon one component of that velocity being known -- the velocity of the Earth around the sun, about 30 km/s. Using a wavelength of about 600 nm, there should be a shift of about 0.04 fringes as the spectrometer was rotated 360°. Though small, this was well within Michelson's capability. Michelson, and everyone else, was surprised that there was no shift. (Rave).

Michelson himself recorded a description of the experiment, noting that the hypothesis appeared to be incorrect: "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" (Michelson).

However, Michelson was determined to try again -- and so were many scientists after him, right down to Charles Rogers and Richard Selvaggi at Texas a&M, who designed a replicate Michelson interferometer in an experiment to better monitor and examine the conclusions of Michelson's second attempt to observe light movement through ether in the Michelson-Morley experiment of 1887. Rogers and Selvaggi concluded that

The unidirectional drift is present in all previously described one-way and two-way light experiments. In both Michelson-Morley and Miller experiments the fringe drift was removed before analyzing the data. The drift of the interference fringes has been observed by others with considerable speculation as to their cause. We believe that corrections for environmental variations can be applied to the data to accurately remove this effect. (Rogers, Selvaggi)

Michelson -- encouraged by such proponents of the ether theory as Lord Rayleigh (Rave) -- attempted to account for such variables in his next experiment with Morley. Together the two men constructed a new interferometer with multiple mirrors and a pathlength about 10 times longer. This device should have given a fringe shift of about 0.4, but they observed less than 0.005 fringe. Although repeated over the next 40 years with ever greater precision and the same negative result, this 1887 experiment is pointed to as one of the experimental foundations of relativity, and earned Michelson the Nobel Prize in 1907. (Rave)

Such was the success of the experiment -- despite its lack of support for any "ether" medium -- that Michelson's thoroughly crafted experiment seemed to confirm in the eyes of many that no such medium existed and that light waves had to be explained by some other phenomena.

Since there was "no direct experimental evidence for the existence of the ether [and] everything can be explained without it," (Rave) new explanations were formulated. One such theory was developed by Albert Einstein in 1905 -- and it referred to the "constancy of the speed of light" which was motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous ether but not, contrary to widespread belief, the null result of the Michelson -- Morley experiment. However the null result of the Michelson -- Morley experiment helped the notion of the constancy of the speed of light gain widespread and rapid acceptance. ("Michelson-Morley Experiment")

Einstein's theory of special relativity was therefore able to gain legitimacy in the eyes of many because the experiment which was supposed to produce evidence of the ether medium through which light traveled had failed to produce any evidence whatsoever.

Einstein's theory was ultimately based upon two premises, which the Michelson-Morley experiment did not in anyway disprove:

1) the speed of light is the same for all observers, no matter what their relative speeds. 2) the laws of physics are the same in any inertial (that is, non-accelerated) frame of reference. This means that the laws of physics observed by a hypothetical observer traveling with a relativistic particle must be the same as those observed by an observer who is stationary in the laboratory.

Einstein's famous scientific equation, energy equals mass times the velocity of light squared, was… [END OF PREVIEW]

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