The Michelson-Morley experiment was as important to physics as was Semmelweis and Pasteur's research into the germ theory of disease. Performed in 1887 by Albert Michelson and Edward Morley, the experiment attempted to measure the motion of the earth through the aether and failed to do so. This unexpected failure lead to the Theory of relativity. Michelson got the Nobel Prize for it in 1907.
The experiment was performed in the days when technology had just become sufficiently advanced to accurately measure the speed of light. 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 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.
Results of the Experiment
The experiment gave unexpected results. The 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.
- 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. 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.”