The experiments of Rayleigh and Brace (1902, 1904) were aimed to show whether

length contraction Length contraction is the phenomenon that a moving object's length is measured to be shorter than its proper length, which is the length as measured in the object's own rest frame. It is also known as Lorentz contraction or Lorentz–FitzGerald ...

leads to

birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...

or not. They were some of the first optical experiments measuring the relative motion of Earth and the

luminiferous aether Luminiferous aether or ether ("luminiferous", meaning "light-bearing") was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space (a vacuum), so ...

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 In physics, the Lorentz transformations are a six-parameter family of linear transformations from a coordinate frame in spacetime to another frame that moves at a constant velocity relative to the former. The respective inverse transformation i ...

and consequently of the

theory of relativity The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity, proposed and published in 1905 and 1915, respectively. Special relativity applies to all physical phenomena in ...

. See also

Tests of special relativity Special relativity is a physical theory that plays a fundamental role in the description of all physical phenomena, as long as gravitation is not significant. Many experiments played (and still play) an important role in its development and justific ...

The experiments

To explain the negative outcome of the

Michelson–Morley experiment The Michelson–Morley experiment was an attempt to detect the existence of the luminiferous aether, a supposed medium permeating space that was thought to be the carrier of light waves. The experiment was performed between April and July 1887 ...

, George FitzGerald (1889) and

Hendrik Lorentz Hendrik Antoon Lorentz (; 18 July 1853 – 4 February 1928) 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. He also derived the Lorentz t ...

(1892) introduced the contraction hypothesis, according to which a body is contracted during its motion through the stationary aether.

Lord Rayleigh John William Strutt, 3rd Baron Rayleigh, (; 12 November 1842 – 30 June 1919) was an English mathematician and physicist who made extensive contributions to science. He spent all of his academic career at the University of Cambridge. Amo ...

(1902) interpreted this contraction as a mechanical compression which should lead to optical anisotropy of materials, so the different refraction indices would cause

. To measure this effect, he installed a tube of length 76 cm 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 prisms. Through the liquid, light (produced by an electric lamp and more importantly by

limelight Limelight (also known as Drummond light or calcium light)James R. Smith (2004). ''San Francisco's Lost Landmarks'', Quill Driver Books. is a type of stage lighting once used in theatres and music halls. An intense illumination is created when ...

) was sent to and fro. The experiment was sufficiently precise to measure retardations of of a half

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

, ''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 The Trouton–Noble experiment was an attempt to detect motion of the Earth through the luminiferous aether, and was conducted in 1901–1903 by Frederick Thomas Trouton and H. R. Noble. It was based on a suggestion by George FitzGerald that ...

, 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 (January 5, 1859 – October 2, 1905) was an American physicist who was known for his optical experiments, especially as regards the relative motion of Earth and the luminiferous aether. Life and work Brace was born in Wilson ...

(1904). He argued that Rayleigh hadn't properly considered the consequences of contraction ( instead of ) as well as of the refraction index, so that the results were not 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 Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when ...

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. Brace1904

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 Sir Joseph Larmor (11 July 1857 – 19 May 1942) was an Irish and British physicist and mathematician who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influent ...

(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 In physics, the principle of relativity is the requirement that the equations describing the laws of physics have the same form in all admissible frames of reference. For example, in the framework of special relativity the Maxwell equations have ...

is considered as valid from the outset, as in

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

's theory 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 ...

(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 comoving observer, and has to be supplemented by

time dilation In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...

for non-comoving observers, which was subsequently also confirmed by the

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 (as defined by the luminiferous aether) produced a measurable effect in the rest frame of the object, ...

(1908) and the

Kennedy–Thorndike experiment The Kennedy–Thorndike experiment, first conducted in 1932 by Roy J. Kennedy and Edward M. Thorndike, is a modified form of the Michelson–Morley experimental procedure, testing special relativity. The modification is to make one arm of the class ...

(1932).

Primary sources

Secondary sources

{{Tests of special relativity Tests of special relativity