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The Michelson–Gale–Pearson experiment (1925) is a modified version of the Michelson–Morley experiment and the Sagnac-Interferometer. It measured the
Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer ...
due to
Earth's rotation Earth's rotation or Earth's spin is the rotation of planet Earth around its own Rotation around a fixed axis, axis, as well as changes in the orientation (geometry), orientation of the rotation axis in space. Earth rotates eastward, in retrograd ...
, and thus tests the theories of
special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates: # The laws o ...
and
luminiferous ether Luminiferous aether or ether ("luminiferous", meaning "light-bearing") was the postulated Transmission medium, medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty ...
along the rotating frame of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
.


Experiment

The aim, as it was first proposed by Albert A. Michelson in 1904 and then executed in 1925 by Michelson and Henry G. Gale, was to find out whether the rotation of the Earth has an effect on the propagation of light in the vicinity of the Earth. The Michelson-Gale experiment was a very large ring interferometer, (a perimeter of 1.9 kilometers), large enough to detect the angular velocity of the Earth. Like the original Michelson-Morley experiment, the Michelson-Gale-Pearson version compared the light from a single source (carbon arc) after travelling in two directions. The major change was to replace the two "arms" of the original MM version with two
rectangle In Euclidean plane geometry, a rectangle is a quadrilateral with four right angles. It can also be defined as: an equiangular quadrilateral, since equiangular means that all of its angles are equal (360°/4 = 90°); or a parallelogram containi ...
s, one much larger than the other.
Light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
was sent into the rectangles, reflecting off
mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
s at the corners, and returned to the starting point. Light exiting the two rectangles was compared on a screen just as the light returning from the two arms would be in a standard MM experiment. The expected fringe shift in accordance with the stationary aether and special relativity was given by Michelson as: :\Delta=\frac where \Delta is the displacement in fringes, A the area in square kilometers, \phi the latitude (41° 46'), c the speed of light, \omega the angular velocity of Earth, \lambda the effective wavelength used. In other words, this experiment was aimed to detect the
Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer ...
due to Earth's rotation.


Result

The outcome of the experiment was that the angular velocity of the Earth as measured by astronomy was confirmed to within measuring accuracy. The ring interferometer of the Michelson-Gale experiment was not calibrated by comparison with an outside reference (which was not possible, because the setup was fixed to the Earth). From its design it could be deduced where the central interference fringe ought to be if there would be zero shift. The measured shift was 230 parts in 1000, with an accuracy of 5 parts in 1000. The predicted shift was 237 parts in 1000. According to Michelson/Gale, the experiment is compatible with both the idea of a stationary ether and special relativity. As it was already pointed out by Michelson in 1904, a positive result in such experiments contradicts the hypothesis of complete aether drag, as the spinning surface of the Earth experiences an aether wind. The Michelson-Morley experiment shows on the contrary that Earth fully drags the aether in its orbital motion, resulting in a null aether wind opposite the orbital speed. Those two results are not incompatible per se, but in the absence of a model to reconcile them, they are more adhoc than the explanation of both experiments within special relativity.
Georg Joos Georg Jakob Christof Joos (25 May 1894 in Bad Urach, German Empire – 20 May 1959 in Munich, West Germany) was a German experimental physicist. He wrote ''Lehrbuch der theoretischen Physik'', first published in 1932 and one of the most influ ...
: ''Lehrbuch der theoretischen Physik.'' 12. edition, 1959, page 448
The experiment is consistent with relativity for the same reason as all other Sagnac type experiments (see
Sagnac effect The Sagnac effect, also called Sagnac interference, named after French physicist Georges Sagnac, is a phenomenon encountered in interferometry that is elicited by rotation. The Sagnac effect manifests itself in a setup called a ring interferometer ...
). That is, rotation is absolute in special relativity, because there is no inertial frame of reference in which the whole device is at rest during the complete process of rotation, thus the light paths of the two rays are different in all of those frames, consequently a positive result must occur. It's also possible to define rotating frames in special relativity (
Born coordinates In relativistic physics, the Born coordinate chart is a coordinate chart for (part of) Minkowski spacetime, the flat spacetime of special relativity. It is often used to analyze the physical experience of observers who ride on a ring or disk r ...
), yet in those frames the speed of light is not constant in extended areas any more, thus also in this view a positive result must occur. Today, Sagnac type effects due to Earth's rotation are routinely incorporated into
GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
.Extract of page 11
/ref>


References

{{DEFAULTSORT:Michelson-Gale-Pearson experiment Physics experiments Aether theories 1925 in science