The experiments of

Lord Rayleigh John William Strutt, 3rd Baron Rayleigh ( ; 12 November 1842 – 30 June 1919), was an English physicist who received the Nobel Prize in Physics in 1904 "for his investigations of the densities of the most important gases and for his discovery ...

and

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 Wilso ...

(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, also called double refraction, 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 described as birefrin ...

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 Transmission medium, medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empt ...

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 transformation, linear coordinate transformation, transformations from a Frame of Reference, coordinate frame in spacetime to another frame that moves at a constant vel ...

and consequently of the

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

. 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 ...

Description

To explain the negative outcome of the

Michelson–Morley experiment The Michelson–Morley experiment was an attempt to measure the motion of the Earth relative to the luminiferous aether, a supposed medium permeating space that was thought to be the carrier of light waves. The experiment was performed between ...

, George FitzGerald (1889) and

Hendrik Lorentz Hendrik Antoon Lorentz ( ; ; 18 July 1853 – 4 February 1928) was a Dutch theoretical physicist who shared the 1902 Nobel Prize in Physics with Pieter Zeeman for their discovery and theoretical explanation of the Zeeman effect. He derive ...

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

aether Aether, æther or ether may refer to: Historical science and mythology * Aether (mythology), the personification of the bright upper sky * Aether (classical element), the material believed to fill the universe above the terrestrial sphere ** A ...

(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 Carbon disulfide (also spelled as carbon disulphide) is an inorganic compound with the chemical formula and structure . It is also considered as the anhydride of thiocarbonic acid. It is a colorless, flammable, neurotoxic liquid that is used as ...

or water, and the liquid was between two

nicol prism A Nicol prism is a type of polarizer. It is an optical device made from calcite crystal used to convert ordinary light into plane polarized light. It is made in such a way that it eliminates one of the rays by total internal reflection, i.e. ...

s. 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 non-electric type of stage lighting that was once used in theatres and music halls. An intense illum ...

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

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

, ''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 tha ...

, 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

(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 the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically perc ...

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 mathematician and physicist who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influential work was ...

(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 Relativity may refer to: Physics * Galilean relativity, Galileo's conception of relativity * Numerical relativity, a subfield of computational physics that aims to establish numerical solutions to Einstein's field equations in general relativity ...

is considered as valid from the outset, as in

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

's theory of

special relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between Spacetime, space and time. In Albert Einstein's 1905 paper, Annus Mirabilis papers#Special relativity, "On the Ele ...

(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 Time dilation is the difference in elapsed time as measured by two clocks, either because of a relative velocity between them (special relativity), or a difference in gravitational potential between their locations (general relativity). When unsp ...

for non-comoving observers, which was subsequently also confirmed by the Trouton–Rankine experiment (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 clas ...

(1932).

Primary sources

Secondary sources

{{Tests of special relativity Tests of special relativity