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Michelson-Morley experiment (view source)
Revision as of 12:51, July 27, 2013
, 12:51, July 27, 2013extending article: The principle of experiment
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 through the [[aether]] as it orbited the Sun and failed to do so.<ref name="CSR">{{cite book
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]] towards which the Newton's laws of motion are to be referenced. However, it failed to detect any such motion at all.<ref name="CSR">{{cite book
|author=Moshe Carmeli
|author=Moshe Carmeli
|title=Cosmological Special Relativity, The Large-Scale Structure of Space, Time and Velocity, 2nd Edition
|title=Cosmological Special Relativity, The Large-Scale Structure of Space, Time and Velocity, 2nd Edition
|publisher=Cambridge University Press
|publisher=Cambridge University Press
|year=1926, 2011
|year=1926, 2011
|pages=162-168
|url=http://www.cambridge.org/gb/knowledge/isbn/item6461182/?site_locale=en_GB
|url=http://www.cambridge.org/gb/knowledge/isbn/item6461182/?site_locale=en_GB
|isbn=978-0-521-23778-9
|isbn=978-0-521-23778-9
Albert A. Michelson|author= The Nobel Foundation|publisher=nobelprize.org|url=http://www.nobelprize.org/nobel_prizes/physics/laureates/1907/|accessdate=July 26, 2013|quote=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".}}</ref>
Albert A. Michelson|author= The Nobel Foundation|publisher=nobelprize.org|url=http://www.nobelprize.org/nobel_prizes/physics/laureates/1907/|accessdate=July 26, 2013|quote=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".}}</ref>
== Historical Background ==
== Historical Background ==
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 (science)|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.
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 science|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 (science)|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.
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.<ref name="Whitehead"/>
== Results of the Experiment ==
== 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.
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: [[Hendrik Lorentz|Lorentz]].
See also: [[Hendrik Lorentz|Lorentz]].