Experiments of Rayleigh and Brace
Encyclopedia
The experiments of Rayleigh and Brace (1902, 1904) were aimed to show whether length contraction
leads to birefringence
or not. They were some of the first optical experiments measuring the relative motion of Earth and the luminiferous aether
which were sufficiently precise to detect magnitudes of second order to v/c. The results were negative, which was of great importance for the development of the Lorentz transformation
and consequently of the theory of relativity
. See also Tests of special relativity.
, George FitzGerald
(1889) and Hendrik Lorentz
(1892) introduced the contraction hypothesis
, according to which a body is contracted during its motion through the stationary aether
.
Lord Rayleigh (1902) interpreted this contraction as a mechanical compression which should lead to optical anisotropy of materials, so the different refraction indices
would cause birefringence
. To measure this effect, he installed a tube of 76 cm length upon a rotatable table. The tube was closed by glass at its ends, and was filled with carbon bisulphide or water, and the liquid was between two nicol prism
s. Through the liquid, light (produced by an electric lamp and more importantly by limelight
) was sent to and fro. The experiment was sufficiently precise to measure retardations of
of a half wavelength
, i.e. of the order . Depending on the direction relative to Earth's motion, the expected retardation due to birefringence was of order 10−8, which was well within the accuracy of the experiment. Therefore it was, besides the Michelson-Morley experiment and the Trouton-Noble experiment
, one of the few experiments by which magnitudes of second order in v/c could be detected. However, the result was completely negative. Rayleigh repeated the experiments with layers of glass plates (although with a diminished precision by a factor of 100), and again obtained a negative result.
However, those experiments were criticized by DeWitt Bristol Brace
(1904). He argued that Rayleigh hadn't properly considered the consequences of contraction ( instead of 10−8) as well as of the refraction index, so that the results were in no way conclusive. Therefore, Brace conducted experiments of much higher precision. He employed an apparatus that was 4.13 m long, 15 cm wide, and 27 cm deep, which was filled with water, and which could be rotated (depending on the experiment) about a vertical or a horizontal axis. Sun light was directed into the water through a system of lenses, mirrors and reflexion prisms, and was reflected 7 times so that it traversed 28.5 m. In this way, a retardation of order was observable. However, also Brace obtained a negative result. Another experimental installation with glass instead of water (precision: ), also yielded no sign of birefringence.
The absence of birefringence was initially interpreted by Brace as a refutation of length contraction. However, it was shown by Lorentz (1904) and Joseph Larmor
(1904) that when the contraction hypothesis is maintained and the complete Lorentz transformation is employed (i.e. including the time transformation), then the negative outcome can be explained. Furthermore, if the relativity principle is considered as valid from the outset, as in Albert Einstein
's theory of special relativity
(1905), then the result is quite clear, since an observer in uniform translational motion can consider himself as at rest, and consequently won't experience any effect of his own motion. Length contraction is thus not measurable by a co-moving observer, and has to be supplemented by time dilation
, which was subsequently also confirmed by the Trouton-Rankine experiment
(1908) and the Kennedy-Thorndike experiment
(1932).
Length contraction
In physics, length contraction – according to Hendrik Lorentz – is the physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer...
leads to birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
or not. They were some of the first optical experiments measuring the relative motion of Earth and the luminiferous aether
Luminiferous aether
In the late 19th century, luminiferous aether or ether, meaning light-bearing aether, was the term used to describe a medium for the propagation of light....
which were sufficiently precise to detect magnitudes of second order to v/c. The results were negative, which was of great importance for the development of the Lorentz transformation
Lorentz transformation
In physics, the Lorentz transformation or Lorentz-Fitzgerald transformation describes how, according to the theory of special relativity, two observers' varying measurements of space and time can be converted into each other's frames of reference. It is named after the Dutch physicist Hendrik...
and consequently of the theory of relativity
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....
. See also Tests of special relativity.
The experiments
To explain the negative outcome of the Michelson-Morley experimentMichelson-Morley experiment
The Michelson–Morley experiment was performed in 1887 by Albert Michelson and Edward Morley at what is now Case Western Reserve University in Cleveland, Ohio. Its results are generally considered to be the first strong evidence against the theory of a luminiferous ether and in favor of special...
, George FitzGerald
George FitzGerald
George Francis FitzGerald was an Irish professor of "natural and experimental philosophy" at Trinity College in Dublin, Ireland, during the last quarter of the 19th century....
(1889) and Hendrik Lorentz
Hendrik Lorentz
Hendrik Antoon Lorentz was a Dutch physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for the discovery and theoretical explanation of the Zeeman effect...
(1892) introduced the contraction hypothesis
Length contraction
In physics, length contraction – according to Hendrik Lorentz – is the physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer...
, according to which a body is contracted during its motion through the stationary aether
Luminiferous aether
In the late 19th century, luminiferous aether or ether, meaning light-bearing aether, was the term used to describe a medium for the propagation of light....
.
Lord Rayleigh (1902) interpreted this contraction as a mechanical compression which should lead to optical anisotropy of materials, so the different refraction indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
would cause birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
. To measure this effect, he installed a tube of 76 cm length upon a rotatable table. The tube was closed by glass at its ends, and was filled with carbon bisulphide or water, and the liquid was between two nicol prism
Nicol prism
A Nicol prism is a type of polarizer, an optical device used to produce a polarized beam of light from an unpolarized beam. See polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol of Edinburgh...
s. Through the liquid, light (produced by an electric lamp and more importantly by limelight
Limelight
Limelight is a type of stage lighting once used in theatres and music halls. An intense illumination is created when an oxyhydrogen flame is directed at a cylinder of quicklime , which can be heated to 2572 °C before melting. The light is produced by a combination of incandescence and...
) was sent to and fro. The experiment was sufficiently precise to measure retardations of
of a half wavelengthWavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
, i.e. of the order . Depending on the direction relative to Earth's motion, the expected retardation due to birefringence was of order 10−8, which was well within the accuracy of the experiment. Therefore it was, besides the Michelson-Morley experiment and the Trouton-Noble experiment
Trouton-Noble experiment
The Trouton–Noble experiment attempted to detect motion of the Earth through the luminiferous aether, and was conducted in 1901–1903 by Frederick Thomas Trouton and H. R. Noble...
, one of the few experiments by which magnitudes of second order in v/c could be detected. However, the result was completely negative. Rayleigh repeated the experiments with layers of glass plates (although with a diminished precision by a factor of 100), and again obtained a negative result.
However, those experiments were criticized by DeWitt Bristol Brace
DeWitt Bristol Brace
DeWitt Bristol Brace was an US-American physicist who was known for his optical experiments, especially as regards the relative motion of Earth and the luminiferous aether.-Life and work:...
(1904). He argued that Rayleigh hadn't properly considered the consequences of contraction ( instead of 10−8) as well as of the refraction index, so that the results were in no way conclusive. Therefore, Brace conducted experiments of much higher precision. He employed an apparatus that was 4.13 m long, 15 cm wide, and 27 cm deep, which was filled with water, and which could be rotated (depending on the experiment) about a vertical or a horizontal axis. Sun light was directed into the water through a system of lenses, mirrors and reflexion prisms, and was reflected 7 times so that it traversed 28.5 m. In this way, a retardation of order was observable. However, also Brace obtained a negative result. Another experimental installation with glass instead of water (precision: ), also yielded no sign of birefringence.
The absence of birefringence was initially interpreted by Brace as a refutation of length contraction. However, it was shown by Lorentz (1904) and Joseph Larmor
Joseph Larmor
Sir Joseph Larmor , a physicist and mathematician who made innovations in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter...
(1904) that when the contraction hypothesis is maintained and the complete Lorentz transformation is employed (i.e. including the time transformation), then the negative outcome can be explained. Furthermore, if the relativity principle is considered as valid from the outset, as in Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
's theory of special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
(1905), then the result is quite clear, since an observer in uniform translational motion can consider himself as at rest, and consequently won't experience any effect of his own motion. Length contraction is thus not measurable by a co-moving observer, and has to be supplemented by time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
, which was subsequently also confirmed by the Trouton-Rankine experiment
Trouton-Rankine experiment
The Trouton–Rankine experiment was an experiment designed to measure if the Lorentz–FitzGerald contraction of an object according to one frame produced a measurable effect in the rest frame of the object, so that the ether would act as a "preferred frame"...
(1908) and the Kennedy-Thorndike experiment
Kennedy-Thorndike experiment
The Kennedy–Thorndike experiment first conducted in 1932, is a modified form of the Michelson–Morley experimental procedure, and tests special relativity....
(1932).