The Michelson-Morley experiment, performed for the first time in 1887 by Albert Michelson and Edward Morley, attempted to measure the motion of the Earth, orbiting the Sun through the aether, that was expected to play a role of absolute frame of reference through which the Newton's laws of motion are to be referenced. However, it failed to detect any such motion at all. This unexpected failure led to the Theory of relativity. Michelson and Morley repeated their experiment many times up until 1929, but always with the same results and conclusions:
- either the Earth is always stationary in the ether,
- or there is something wrong with the fundamental principles on which the interpretation of the experiment relies.
The experiment was as important to physics as was Semmelweis and Pasteur's research into the germ theory of disease. Michelson got the Nobel Prize in 1907 for related optical precision instruments and the spectroscopic and metrological investigations such us these experiments carried out with their aid.
The experiment was performed in the days when technology had just become sufficiently advanced to accurately measure the speed of light. Michelson's experiment could not be performed earlier as it required:
- The general advance in technology, i.e. the aid of delicate apparatus, the Michelson's famous interferometer
- Michelson's experimental genius and skills.
At that time it was known that light behaved like a wave. Scientists believed that light waves (consisting of photons) traveled through a medium referred to as the aether (sometimes spelled ether). The aether was believed to be fixed in space and so was regarded as an absolute frame of reference, relative to which the velocity of any object (moving body) could (in theory), be measured. The concept of the aether was in some ways similar to the modern concept of "space-time" though some differences exist. Thus the idea was that absolute movement existed.
Scientists at the time thought that if they could accurately measure the relative velocity between the Earth and light waves, they might get different results for the speed of the light waves moving in different directions. They believed this would happen because the Earth was constantly moving, and the aether was fixed. By examining the results they could measure the relative velocity between the Earth and the aether, and hence determine the absolute velocity of the Earth through space. The Michelson-Morley experiment was an attempt to do that.
The Fundamental Principle of the Experiment
The Michelson's experiment pertains to the determination of the Earth's motion through the ether and it is based on assumption that light consists of waves of vibration advancing at a fixed rate through the ether in any direction. Michelson's apparatus is moving through the ether with the Earth. In the centre of this apparatus the light is divided so that:
- one half-ray goes in one direction along the apparatus through a given distance, and is reflected back to the centre by a mirror in the apparatus
- the other half-ray goes the same distance across the apparatus in a direction at right angles to the former ray and it also is reflected back to the centre.
The reunited rays are then reflected onto a screen in the apparatus and owing to a minute difference in the length of paths of the two half-rays, the interference bands, i.e. the bands of blackness where the crests of the waves of one ray have filled up the troughs of the other rays, should be seen. These differences in length will be affected by the motion of the earth for it is the lengths of path in the ether which count. Since the apparatus is moving with the Earth, the path of one-half ray will be disturbed by the motion in a different manner than the path of the other half-ray. This effect of the Earth's motion would affect the positions on the screen of the interference bands. Moreover, after turning the apparatus round, through a right angle, the effect of the Earth's motion on the two half-rays will be interchanged, and the position of the interference bands would be shifted. The small shift expected due to Earth's motion round the Sun can be calculated and on top of this effect possibly another one, namely the Sun's motion through the ether, should be reckoned with. The delicacy of the instrument can be tested, and it can be proved that these effects of shifting are large enough to be detected by it and observed by an experimenter.
Results of the Experiment
The experiment gave unexpected results, no interference band shifting has been observed after turning the instrument round. Thus, the result suggested the interpretation that speed of light was found to be the same regardless of the direction the light was traveling in. At first it was assumed that the result was due to poor equipment or some other anomaly, but further work ruled that possibility out.
- See also: Lorentz
Consequences of the results
Scientists then faced the dilemma. They believed the Earth was moving through space, but the Michelson-Morley experiment and similar experiments indicated that the Earth was somehow stationery relative to the aether at all times.
A number of theories were proposed to explain the paradox, including the idea that the aether might in some way be "dragged along" by the Earth. George FitzGerald proposed a length contraction to explain it, as a logical consequence of the speed of light being the same for all observers. Lorentz and Poincare extended this to the theory of special relativity. That became the accepted explanation, and the aether appeared to be unobservable.
Link to Einstein's Special Theory of Relativity
Some authors state that the celebrated Michelson-Morley experiment was the starting point of the Einstein's Theory of Special Relativity while others are taking effort to deny any link and try to assure that his 1905 paper was not written with the Michelson-Morley experiment in mind but with entirely different motives. Einstein was criticized for the omission of any reference to the work of H.A. Lorentz and H. Poincare in his 1905 paper to such degree that Whittaker in his detailed study attributes the presented theory entirely to them.
- Moshe Carmeli (2002). Cosmological Special Relativity, The Large-Scale Structure of Space, Time and Velocity, 2nd Edition. World Scientific Publishing, 38, 43. ISBN 9-789-02-4936-5. “The special theory of relativity was developed by Einstein in 1905 in order to overcome and correct certain basic concepts that were in use at that time, such as asymmetries in relative motion of bodies. Examples of relative motion in electrodynamics, and the unsuccessful attempt to detect the motion of the Earth by the experiment of Michelson and Morley, suggested that the phenomena of electrodynamics and mechanics do not depend on the Newtonian notion of absolute rest. Rather, the laws of electrodynamics should be valid in all frames of references in which the equations of mechanics are valid. ... Lorentz invariance is in accordance with the Michelson-Morley null experiment which showed that on the moving Earth light spreads with the same speed in all directions.”
- Alfred North Whitehead (1926, 2011). Science and the Modern World. Cambridge University Press, 162–168. ISBN 978-0-521-23778-9. “Now the point is that nothing was observed. There was no shifting as you turned instrument round. The conclusion is either that the Earth is always stationary in the ether, or that there is something wrong with the fundamental principles on which the interpretation of the experiment relies.”
- The Nobel Foundation. Relativity:The Postulates of Special Relativity:The Michelson-Morley Experiment. nobelprize.org. Retrieved on July 26, 2013. “In 1887, Albert A. Michelson and Edward W. Morley tried to measure the speed of the ether. The concept of the ether was made in analogy with other types of media in which different types of waves are able to propagate; sound waves can, for example, propagate in air or other materials. The result of the Michelson-Morley experiment was that the speed of the Earth through the ether (or the speed of the ether wind) was zero. Therefore, this experiment also showed that there is no need for any ether at all, and it appeared that the speed of light in vacuum was independent of the speed of the observer! Michelson and Morley repeated their experiment many times up until 1929, but always with the same results and conclusions. Michelson won the Nobel Prize in Physics in 1907.”
- The Nobel Foundation. [https://www.nobelprize.org/nobel_prizes/physics/laureates/1907/ The Nobel Prize in Physics 1907: Albert A. Michelson]. nobelprize.org. Retrieved on July 26, 2013. “The Nobel Prize in Physics 1907 was awarded to Albert A. Michelson "for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid".”
- L.Essen (1971). The Special Theory of Relativity: A Critical Analysis. Oxford Science Research Papers, Clarendon Press, 1–2. Retrieved on July 26, 2103.
- Klaus Hentschel et al. (2005). "7: Testing Relativity", in M.Mamone Capria: Physics Before and After Einstein. IOS Press, 164. ISBN 9781586034627.