Michelson-morley Experiment

Type of physical science: Relativity

Field of study: Special relativity

The Michelson-Morley experiment was intended to detect the motion of the earth through "the ether." The failure to detect the presence of an ether drift was a severe blow to the classical physics concept of absolute space and time. The idea of the ether as a fixed frame of reference by which to measure all motion in the universe became meaningless. Eventually, the special theory of relativity came out of this confusion.

Overview

By the 1860's, it had been established by various researchers in the fields of optics and electromagnetism that light consists of electromagnetic waves that move through space at a speed of 300,000 kilometers per second. A major problem with this wave theory of light was that all waves require some type of conducting medium. For example, sound could not travel without air to conduct the sound waves. For the same reason, light could not travel through space from the sun or the stars without some kind of medium. To solve this problem, scientists postulated the existence of an ether. This all-penetrating substance was thought to fill the space between and within all material bodies. The ether had to be frictionless, transparent, incompressible, of uniform density, and rigid enough to conduct high-velocity waves.

A notable implication of the ether hypothesis was that the ether might serve as an absolute frame of reference. Light could then be described as moving at 300,000 kilometers per second relative to the ether. In fact, all the motions of the universe could be compared to this absolute frame of reference. No matter how desirable the concept of an ether was, however, it still had to be detected by scientific experiment.

The most famous experiment conducted for this purpose was the Michelson-Morley experiment of 1887. In principle, the theory behind this experiment was identical to the theory behind the experiment that Albert Abraham Michelson had conducted when he was a graduate student in physics at the University of Berlin in 1881. Michelson's idea was to set up a system of mirrors and a light source on a bench in his laboratory and project beams at various angles. The first beam was projected in the direction in which the earth travels in its orbit, and a second beam was projected at a right angle to the first one. Each beam would then strike a mirror an equal distance from the source and be reflected to an observer's eyepiece. Michelson believed that the first beam would be retarded by the flow of ether passing the earth and that the second beam, which crossed this current at right angles, should arrive ahead of the first, even though the distance traveled by both beams was the same. The difference in their arrival would be a length of time determined by the velocity of the earth. Michelson explained the experiment by comparing it to a race between two swimmers, one struggling upstream and back, and the other, covering the same distance, swimming across the river and back. The second swimmer would always win the contest if there were any current in the river.

It was crucial to the success of the experiment to split a single light beam, send the two parts off at right angles, and by means of mirrors, rejoin them to observe the fringes of interference and thus determine which beam returned first and how much sooner. If a single beam were split, the frequency of each newly created beam would be equal to that of the original beam. If two different beams had been used, it might have been argued that the beams had different frequencies at the beginning of the experiment, and the data would therefore have been invalid.

In the figure below, the source of light, S, is a lantern. The lightly silvered mirror, A (the beam splitter), reflects some of the light to mirror C, while the remainder passes through it to mirror D. These two beams are reunited at A and transmitted to O, where the fringes may be examined in the eyepiece to see if the waves are in phase. The two optical paths are equal. If the arm AC is parallel to the earth's motion in orbit, the beam traveling with it might be affected to a greater degree than the beam moving at right angles to it. If the instrument is turned 90 degrees, the path AC will be the one experiencing the greater change. Michelson hoped to find a total shift of the interference fringes of 0.08 of a fringe, a distance that he knew he could measure. Because the beam of light that passes along the path SACA must pass through mirror A three times, a compensating plate of glass, B, was inserted between A and D to equalize the journey.

Throughout the winter of 1880-1881, Michelson worked on his experiment. He believed that he would be able to detect a difference in the speed of light along the arms of his device--a very small difference perhaps, but enough to verify his supposition. After six months of careful work, his data showed no evidence of any shift in the fringes.

In order to explain the negative results of his experiment, Michelson embraced a theory that had been proposed by George Stokes. This so-called ether drag theory suggested that the earth carried the ether along with itself. If this theory were correct, no interference patterns could be detected with a device such as Michelson's.

After concluding his graduate work, Michelson accepted a teaching position at Case School of Applied Science in Cleveland, Ohio. As he began his new assignment, he became more interested in his early experiments on the speed of light than in his experiment on the ether drift. He told his students that "the luminiferous ether is to some extent a hypothetical substance and if it consists of matter at all, must be very rare and elastic. It entirely escapes all our senses of perception." He never mentioned his 1881 experiment; however, those who knew of it were very concerned about its failure to detect the ether.

While attending a conference in Montreal in 1884, Michelson met an old acquaintance, Edward Williams Morley of Western Reserve University. The two discussed the problems that Michelson had encountered with the first experiment and decided to attempt the experiment jointly.

In the spring of 1887, Michelson and Morley were ready to conduct what has become one of the most famous experiments in the history of physics. They started with a device that was essentially the same as the one that Michelson had used in his 1881 experiment, but that had a few improvements that increased the complexity of the apparatus. Initially, they were confronted by the same problems that had been encountered by Michelson in 1881. The first of these problems was the inability to rotate the apparatus without producing distortion; the second was the device's extreme sensitivity to vibration. In order to solve these problems, Michelson and Morley mounted the optical parts on a sandstone slab 1.5 meters square and 0.3 meters thick. The slab rested on a wooden float and was supported by mercury, which was contained by a cast-iron trough. This arrangement eliminated vibrations, kept the apparatus horizontal, and permitted it to be easily rotated around a central pin.

Instead of the two-mirror system used in the 1881 experiment, the new interferometer contained sixteen mirrors, four at each corner of the slab. The beam splitter was placed near the center of the apparatus in such a position that light from the source, passing through a lens, would strike it at an angle of approximately 45 degrees. As in the first interferometer, a compensator plate was used. The mirrors themselves were made of speculum metal carefully worked to optically plane surfaces 5 centimeters in diameter with a thickness of 1.25 centimeters.

Each mirror could be adjusted by means of screws set in the mirror-support brackets. One mirror could be moved by means of a precision screw. The beam splitter and the compensator plate were of the same thickness as the mirrors; their surfaces measured 5.0 x 7.5 centimeters. The entire apparatus was enclosed within a wooden cover to prevent sudden changes of temperature and air currents from invalidating the data.

Prior to running the experiment, Michelson and Morley painstakingly adjusted the mirrors until they were parallel. The length of the arms of the interferometer was approximately equal, having been set by means of a steel scale, to tenths of millimeters. The telescope at the observer's station was adjusted for distinct vision of the range of expected interference bands, the sodium light source was installed, and the movable mirror was adjusted until interference bands appeared. The apparatus was now ready for observations.

Around the base of the apparatus were sixteen equidistant marks. Michelson and Morley rotated the device very slowly while making observations. The data were taken by setting the cross wire of a micrometer on the clearest of the interference fringes at the passing of one of the sixteen marks. The motion was so slow that this could be done quite accurately. A reading on the micrometer was taken, and the apparatus was rotated to the second mark, where the process was repeated. This continued until the apparatus had completed six revolutions.

No matter how Michelson and Morley turned their apparatus, they found no sign of the ether wind. The only difference measured was, according to them, less than one-fortieth of the amount of shift that they had expected. Again, Michelson was astounded and disappointed. His astonishment was shared by physicists all over the world. The initial reaction of most physicists was to question whether the experiment had been performed properly and its results interpreted correctly. Others speculated on possible reasons for discounting the results of the experiment and, at the same time, saving the ether wind theory. It was, however, left to Albert Einstein to suggest a bold, remarkable way out of the extraordinary confusion caused by the results of the Michelson-Morley experiment.

Applications

The null result of the Michelson-Morley experiment came as a shock to the scientific community. Those who were aware of the efforts of Michelson and Morley believed that their experiment and the apparatus were well designed for the task at hand. The natural reaction was to question whether the experimental data had been interpreted properly. There was also some concern that Michelson and Morley failed to follow up with additional tests and measurements.

They had discussed the possibility of taking additional measurements at different times during the year when the earth was in different positions relative to the sun and the proposed ether.

These measurements, however, were not made. Apparently, Michelson and Morley believed that their attempt to detect the ether wind was a complete failure, and they decided to pursue more promising scientific research. Years later, Michelson was to write that the invention of the device used in their experiment, the interferometer, made up for the failure of the experiment. Since its invention, the interferometer has found many uses in science and technology.

Some prominent scientists attempted to defend the results of the experiment by suggesting reasons why the ether drift could not be detected. The ether drag theory, which had been proposed by Stokes in 1845, was brought up as a possibility. This theory, which was accepted by Michelson and Morley as the explanation for the failure of their experiment, proposed that there is no relative motion between the earth and the ether. As the earth moves through space in its orbit around the sun, it drags the ether in its immediate vicinity along with it.

Short of going back to the idea of an Earth-centered solar system with the earth in a state of complete rest, this was the only proposal that seemed to explain the problem. Most scientists, however, rejected the ether drag theory because it was shown to disagree with other observations.

One notable explanation for the results of the Michelson-Morley experiment was proposed soon after the experiment by the Irish physicist George Francis FitzGerald. It was FitzGerald's contention that any body that has a velocity with respect to the ether contracts along its axis of motion. FitzGerald concluded that if the arm of Michelson and Morley's interferometer were shrunk by just the right amount, their experiment could be reconciled with the existence of the ether drift.

In 1892, the Dutch physicist Hendrik Antoon Lorentz arrived at the same conclusion and published his work. This theory is often referred to as the Lorentz-FitzGerald contraction.

Lorentz and FitzGerald agreed that the so-called molecular forces that bind solid bodies together act by the intervention of the ether. Lorentz had stated in his paper that it was conceivable that there might have been some change in the length of the arms of Michelson and Morley's device.

This change in length would be responsible for the null result of the experiment.

In 1904, the French mathematician and theoretical physicist Jules-Henri Poincare proposed still another solution to the problem. It was his idea that a new science of mechanics should be created, one in which the velocity of light could not be exceeded. One year later, a then unknown employee in a Swiss patent office was to do just that. In his special theory of relativity, Einstein rejected the ether drag along with absolute space and time. His theory held that the laws of physics are valid in any inertial frame of reference. This special case of relativity deals with frames of reference that are not being accelerated, but are in uniform motion along a straight line.

The theory goes on to propose that the speed of light is the same for all observers, regardless of the motion of the observer or the source.

The special theory of relativity explains exactly why Michelson and Morley were unable to detect the presence of the ether wind. There is no ether wind. Einstein identified the limits of the mechanics of Sir Isaac Newton and Galileo and created the new mechanics of relativity.

Context

Throughout the history of science, the nature of light has been a topic of much debate.

By the latter part of the seventeenth century, however, two major theories were being advanced to explain the nature of light. The first of these was the particle or corpuscular theory, which was proposed by Newton. According to Newton's theory, a beam of light consists of vast numbers of tiny particles. These particles travel outward, from sources such as the sun or flames, in straight lines at enormous speeds. The wave theory was advocated by Robert Hooke and particularly by Christiaan Huygens, both of whom were contemporaries of Newton. According to the wave theory, light is the motion of a disturbance through the distance between a source and the eye.

As time went on, followers of Newton and Huygens divided into factions, passionately supporting one theory or the other. Experiments with interference and diffraction conducted in the early 1800's swung the debate solidly toward the wave theory of light. In the 1860's, James Clerk Maxwell brought the science of optics into line with electromagnetism with his conclusion that light is a form of electromagnetic wave. Maxwell's theory was proved correct when his calculated velocity of light matched the known velocity of light in a vacuum.

The wave theory of light was now virtually unchallenged, but one problem remained.

All wave phenomena were thought to require the existence of a medium, a substance that carried the disturbance. For example, it would not be possible for sound to travel without air, a conducting medium. Accordingly, how could light waves travel through space (a vacuum) if there were nothing to vibrate? How is it possible that light can reach the earth from the sun and the stars through millions of kilometers of empty space? To avoid this problem, scientists postulated the existence of an ether. This all-penetrating universal medium was thought to fill the space between and within all material bodies. The ether had to be frictionless, transparent, incompressible, rigid enough to conduct high-velocity waves, and of uniform density. It must be frictionless or else the planets would experience friction as they orbit the sun and would slow down. The ether must also be incompressible and uniformly spread throughout the universe, since otherwise the velocity of light would be different at various places in the universe. If the ether were not transparent, stars would be invisible, and darkness would prevail over the universe. The ether was, indeed, a very unusual kind of substance.

A notable implication of the ether theory was that the ether might serve as an absolute frame of reference. Light could then be described as moving 300,000 kilometers per second relative to the ether frame of reference. In fact, the ether could be used as a frame of reference for all motion in the universe. No matter how desirable the concept of the ether was, however, and how many problems its existence might solve, the issue still remained to be resolved by scientific investigation. It was necessary to detect the ether by means of an experiment.

Shortly before his death in 1879, Maxwell had been considering such an experiment.

Intrigued by the problem, Michelson began planning his own experiment. The results were Michelson's experiment of 1881 and the more famous Michelson-Morley experiment of 1887.

Since 1887, this experiment has been repeated many times, with the same results.

Principal terms

ELECTROMAGNETIC WAVES: forms of radiant energy, such as light, which are transmitted through space as two transverse waves that are perpendicular to each other; the electric and magnetic components, which are represented by the two waves, are in phase with each other and have the same velocity

ETHER: a hypothetical substance that was believed to fill the entire universe and serve as a medium for conducting waves

FRAME OF REFERENCE: a point of reference from which motion is measured

INERTIAL FRAME OF REFERENCE: a frame of reference that moves at constant velocity and in which Newton's laws are valid

INTERFEROMETER: a device that uses interference fringes to measure length or a change in length

Bibliography

Casper, Barry M., and Richard J. Noer. REVOLUTIONS IN PHYSICS. New York: W. W. Norton, 1972. This volume, which covers topics in general physics, was intended as a college-level text for nonscience majors. The reader should have at least a high school level physics background.

Gardner, Martin. RELATIVITY FOR THE MILLIONS. New York: Macmillan, 1962. This well-written, well-illustrated volume on special and general relativity is ideally suited for the layperson.

Hoffman, Banesh. RELATIVITY AND ITS ROOTS. New York: W. H. Freeman, 1983. This volume examines both the special and general theories of relativity as well as the history of their development. This volume is considerably more technical than the Gardner work. The reader should have some understanding of basic physics.

Inglis, Stuart J. PHYSICS: AN EBB AND FLOW OF IDEAS. New York: John Wiley & Sons, 1970. This volume traces the development of physics from the ancient Greeks through relativity and quantum theories. Although this book has been used as a college freshman text, it is accessible to the informed reader.

Kim, S. A. PHYSICS: THE FABRIC OF REALITY. New York: Macmillan, 1975. This volume covers topics related to relativity and quantum physics. The reader should have some understanding of basic physics.

Magie, William F. A SOURCE BOOK IN PHYSICS. Cambridge, Mass.: Harvard University Press, 1965. This fairly technical volume contains extracts from the writings of important contributors to physics prior to 1900. The reader should have some college-level training in mathematics and physics.

Tipler, Paul A. MODERN PHYSICS. New York: Worth, 1978. A highly technical volume covering relativity theory, quantum theory, and topics in nuclear physics, this book was intended as an advanced undergraduate text.

Michelson-Morley split-beam experiment