The Michelson-Morley experiment
is often presented as the archetype
of the “crucial experiment
.” Symbolic of the shift of physics from the Euclidean
, and from the absolute
to the relativistic
, those who wish to place a signpost at the point where the road of physics veered off into a new direction plant it firmly in 1887, when Albert Abraham Michelson
and Edward Williams Morley
performed their aether drift experiment
. But rarely are borders so clean, edges so clear, and upheavals so sudden. School textbooks may record the Michelson-Morley
experiment as a decisive break with the past, but history records more conflict, less certainty, and an experimental program that never fully convinced its experimenters the way it does students today.
19th Century Physics
The state of 19th century physics appears almost as a work of Victorian art viewed from the perspective of modern physics. Mass was unchanging, time and space were independent, and electricity and magnetism were being harnessed and understood. Newtonian physics stood at the center of the world. Published more than one hundred fifty years before Edward Morley’s birth, the Philosophiae Naturalis Principia Mathematica was symbolic of the mathematical precision and metaphysical simplicity that physics strove toward. Newtonian physics contained several fundamental assumptions that would later be challenged by Einstein and his theories of relativity. First, classical mechanics assumes the independence of mass, length, and velocity. In none of the equations of classical mechanics, will mass or length ever be found as a function of time. Because this is a notion so obvious to common sense, Newton probably never even considered that a relationship might exist. Barring the then recently discovered electric and magnetic phenomenon, Newtonian physics was sufficient to describe almost all that was seen in the physical world.
Other new discoveries had been made in the intervening years. Beginning with the work of Christiaan Huygens, who was contemporary with Newton, the wave explanation of light had gained popularity. In a debate, which in some ways continues to this day, physicists argued if the true nature of light was particle or wave. Thomas Young in the 18th century further gave strength to the wave theory when he demonstrated wave-like properties of light, like interference in a series of experiments. (Thomas Young Wiki) By Michelson’s time, the wave theory of light was well established.
One other major discovery that had been made, nearly contemporaneously with Michelson and Morley was James Clerk Maxwell’s discovery of equations, later dubbed Maxwell’s laws, that described the behavior of electricity and magnetism in 1873. The peculiar characteristics of these laws would later become of importance.
Of the most relevance to the Michelson-Morley experiment is the concept of the aether, which was widely accepted in the 19th century. Many thinkers to explain effects that traveled through space postulated aethers of various types. René Descartes, believing that a vacuum was impossible, thought the entire universe to be full of matter, though he never used the term “aether”. (Cottingham 1993, 159) To him, gravity and light were artifacts of mechanical effects within it. Robert Hooke referred to the “luminiferous aether” as what carried the vibrations of light, and this idea influenced Newton. (Koyré 1965, pg 47. Also, Chapman 1996) The wave/particle dispute continued through the 18th century, but at the beginning of the 19th century, Thomas Young presented a paper which explained Newton’s rings (where thin films display different colors) using the principle of interferencein what was called the first major theoretical discovery in optics in a century, and presented the following four hypotheses:
- A Luminiferous Ether pervades the Universe, rare and elastic in a high degree.
- Undulations are excited in this Ether whenever a Body becomes luminous.
- The Sensation of different Colours depends on the different frequency of Vibrations, excited by Light in the Retina.
- All material bodies have an Attraction for the ethereal Medium, by means of which it is accumulated within their substance, and for a small Distance around them, in a State of greater Density, but not of greater Elasticity.
(Young in Swenson 1972, pg 16)
By the time of Michelson and Morley, the existence of a medium necessary for the propagation of light, the luminiferous aether
, was taken almost as truth.
Michelson and Light
Albert Abraham Michelson entered the Naval Academy young, but soon proved he was a better scientist than a sailor. His first experiments with light were performed in 1879 when he built an experimental apparatus similar to that of Léon Foucault’s rotating mirror for the purpose of measuring the speed of light. (Swenson 1972, pg 38) This gained him notice from naval astronomer Simon Newcomb, and set him on a path that would have him investigating light for most of the rest of his life. In 1880, he conceived the design for the interferometer that would later come to be associated with his name and arrived at the idea that it might be used to make measurements of the aether. By this time he was in Europe, and he wrote to Newcomb about his experiment idea:
I had quite a long conversation with Dr. Helmholtz concerning my proposed method for finding the motion of the earth relative to the ether, and he said he could see no objection to it, except the difficulty of keeping a constant temperature. (Swenson 1975, pg 68)
In addition to foreseeing the difficulties he would have in the future with his experimental apparatus, this statement shows that Michelson was very much invested in the idea of the aether
at this time. His idea is not to confirm or deny the existence of the aether, but to measure the earth’s motion through it. Under the patronage of Alexander Graham Bell
, Michelson performed the first of his aether drift experiments in 1881, but was forced to report negative results to Bell. Here again, one statement in his report is telling:
Thus the question is solved in the negative, showing that the ether in the vicinity of the earth is moving with the earth. (Swenson 1975, pg 70)
Despite this initial setback, he would realize an oversight on his part that would make the observed effect much smaller than he had anticipated, necessitating further experiments. These would take place in 1887, with the help of Edward Morley.
1887: Michelson and Morley
The preparations for the 1887 experiment were meticulous. Temperature was monitored, vibrations controlled, and all conceivable sources of noise were minimized. Michelson had begun his collaboration with Morley because the facilities of the elder Morley’s laboratory were better suited to the experiments to be conducted. The optical parts of the interferometer they constructed were mounted on a block of sandstone, which floated on a pool of mercury. (Miller 1933, pg 205) In addition to precautions to reduce the effects of vibration and temperature, the new interferometer was built so that the beams of light were reflected several times to increased the effective distance traveled. This would amplify the effect they were hoping to observe. Over several days from July 8 to 12 (resting on the Sunday), Michelson and Morley made a series of observations. Michelson followed the apparatus as it slowly rotated, and called out the observed changes in the interference fringes to Morley. Again, their results were negative. They wrote in an article published in the American Journal of Science: “It appears, from all that precedes, reasonably certain that if there be any relative motion between the earth and the luminiferous ether, it must be small” (Michelson & Morley 1887, pg 341) Although it is indicated in that article that they will repeat the experiment at different times of the year to eliminate the possibility that the earth’s motion was such that the aether would appear at rest, they lost interest and never performed subsequent experiments as partners.
At the time, the negative results of the Michelson-Morley experiment were explained in various ways. This excerpt from the 11th edition of the Encyclopaedia Britannica summarizes the general mood:
The results of the Michelson-Morley experiment are inconsistent with the aether remaining at rest, unless we assume that the dimensions of the moving system depend, though to an extent so small as to be not otherwise detectable, on its orientation. It is, however, in complete accordance with a view that would make the aether near the earth fully partake in its orbital motion. (“Aether” 1910)
This shows two distinct interpretations of the results of the Michelson-Morley experiment. The first comes from the Lorentz-Fitzgerald contraction
. Both George Francis Fitzgerald
and Hendrik Lorentz
independently put forth the idea that the dimensions of a system might change depending on its velocity through the aether. In 1910, when this article was published, Einstein
’s special relativity
was known, but not fully accepted, and the article offers it as a possible explanation. The other interpretation contains what is known as the Stokes’ hypothesis
, in which the aether near the earth’s surface was carried with the earth, making measurements of aether drift at ground level impossible. Dayton Miller
, who worked with Edward Morley on later aether drift experiments, interpreted the 1887 results positively compared to Michelson and Morley:
However, and this fact must be emphasized, the indicated effect was not zero; the sensitivity of the apparatus was such that the conclusion, published in 1887, stated that the observed relative motion of the earth and ether did not exceed one-fourth of the earth’s orbital velocity. This is quite different from a null effect now so frequently imputed to this experiment by writers on Relativity. (Miller 1933, 206)
As Miller writes, the experiment was not taken by all at the time to be conclusive evidence of aether drift. There were still competing interpretations, none of which had clearly become dominant.
Looking back at the Michelson-Morley experiment
of 1887, the scene looks quite different. While the interceding years have seen other significant developments that have given weight to one theory over another, accounts of the Michelson-Morley experiment have very much been re-written. This excerpt from a current edition of the Encyclopaedia Britannica shows a clear example:
This null result of the Michelson-Morley experiment seriously discredited the ether theories and ultimately led to the proposal by Albert Einstein in 1905 that the speed of light is a universal constant. (“Michelson-Morley Experiment” 2002)
Ignoring the fact that Albert Einstein
was not led to the constancy of the speed of light by the Michelson-Morley experiment (see Swenson 1972, pg 227. Also Collins & Pinch 1993, pg 30) it is clear that this account of the interpretation of the experiment does not fit the interpretation given by the experimenters themselves, or by previous editions of the Encyclopaedia Britannica.
How should this discrepancy be understood? History is literally re-written, but for what reason? The shift from classical theory
of the aether to the theory of relativity
would seem to be a paradigm case of a Kuhnian paradigm shift
. The result of the Michelson-Morley experiment, especially viewed from the modern context, could easily be seen to be an anomaly
preceding a scientific revolution
. The change in text could be explained by a alteration in viewpoint across that change. Thomas Kuhn writes in the preface to the third edition of The Structure of Scientific Revolutions
if I am right that each scientific revolution alters the historical perspective of the community that experiences it, then that change of perspective should affect the structure of postrevolutionary textbooks and research publications. (1996, pg xi)
Later in the book Kuhn suggests “what were ducks in the scientist’s world before the revolution are rabbits afterwards.” (1996, pg 111) The Michelson-Morley experiment seen from this viewpoint would appear to be a victim of an unconscious sort of revisionism
undertaken by relativist revolutionaries. The theory of relativity, already in a strong position, went back into history to reclaim the Michelson-Morley experiment as one of its own, re-interpreting it in post-revolution terms, and re-integrating it into the complex mythology of relativity as further evidence for the wisdom of Einstein. Collins and Pinch put forth a similar interpretation: “Once the seed crystal has been offered up, the crystallization of the new scientific culture happens at breathtaking speed.” (1993, pg 53) Although they assert that the change occurs on a more conscious level than Kuhn does, they share the same basic idea.
Bruno Latour offers an alternative way in which the change can be understood. Latour divides science into two states: science is either in the making, or “black-boxed.” The quotes above from the two eras might be grouped into two Latourian categories: positive and negative modalities. (1987, pg 23) Positive modalities are those sentences that take the subject away from its origins, and away from its machinery and allow science to ask new questions and consider novel consequences. In contrast, negative modalities bring science back towards its beginnings, forcing science to look at what makes it believable, or what can be doubted. The sentence from the current edition of the Encyclopaedia Britannica clearly falls into the first category. The machinery of the Michelson-Morley experiment is inconsequential, and the diversity of interpretations that came out of it are unimportant. The sentence emphasizes the results that arose from the experiment. Having placed the experiment in the same box as relativity, science is free to consider its implications. Miller’s statement about the experiment falls into the second category. It considers the experimental apparatus, and its accuracy. The result was not null, but only less than expected; the result may still be significant. The main implication of this type of statement is that more needs to be done to reach a conclusion; it is still science in the making.
The Michelson-Morley experiment is a case where an interesting result became a crucial experiment. It shows how science does not come pre-packaged, but as a mess of numbers, figures, and equations that can mean different things to different people. Whether the science fell into a black box, or was the victim of a violent revolution, it is apparent that science is not a clean, directed affair.
(2002). Michelson-Morley Experiment. In The New Encyclopaedia Britannica (Vol. 8, p. 98). Chicago: Encyclopaedia Britannica, Inc.
Cottingham, John. (1993). A Descartes Dictionary. Oxford: Blackwell Reference.
Collins, Harry & Pinch, Trevor. (1993). The Golem: what everyone should know about science. Cambridge: Cambridge University Press.
Feynman, Richard P., Leighton, Robert B., and Sands, Matthew. (1963) The Feynman Lectures on Physics: Volume I. Reading, Massachusetts: Addison-Wesley Publishing Company.
Koyré, Alexandre. (1965) Newtonian Studies. London: Chapman & Hall.
Kuhn, Thomas S. (1996) The Structure of Scientific Revolutions,Third Edition. Chicago: The University of Chicago Press.
Larmor, Sir Joseph. (1910). Aether. In The Encyclopaedia Britannica, 11th edition. (Vol. 1, pp. 292-297) New York: The Encyclopaedia Britannica Company.
Latour, Bruno. (1987) Science in Action: how to follow scientists and engineers through society. Cambridge, Mass.: Harvard University Press.
Michelson, Albert A. & Morley, Edward W. (1887). On the Relative Motion of the Earth and the Luminiferous Ether. American Journal of Science, 34(203), 333-345.
Michelson-Morley experiment. Retrieved February 5, 2004, from Wikipedia: the Free Encyclopedia: http://en.wikipedia.org/wiki/Michelson-Morley_experiment
Miller, Dayton C. (1933) The Ether-Drift Experiment and the Determination of the Absolute Motion of the Earth. Reviews of Modern Physics, 5(7), 203-242.
Swenson, Jr., Loyd S. (1972) The Ethereal Aether: A History of the Michelson-Morley-Miller Aether-Drift Experiments, 1880-1930. Austin & London: University of Texas Press.
Thomas Young. Retrieved March 25, 2004, from Wikipedia: the Free Encyclopedia: http://en.wikipedia.org/wiki/Thomas_Young_(scientist)
This was submitted to a class on science in society. Node your homework.