Emitter theory
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Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the
special theory of 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 ...
, explaining the results 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 188 ...
of 1887. Emission theories obey the principle of relativity by having no preferred frame for
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 te ...
transmission, but say that light is emitted at speed "c" relative to its source instead of applying the invariance postulate. Thus, emitter theory combines
electrodynamics In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
and
mechanics Mechanics (from Ancient Greek: μηχανική, ''mēkhanikḗ'', "of machines") is the area of mathematics and physics concerned with the relationships between force, matter, and motion among physical objects. Forces applied to object ...
with a simple Newtonian theory. Although there are still proponents of this theory outside the scientific mainstream, this theory is considered to be conclusively discredited by most scientists.


History

The name most often associated with emission theory is
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...
. In his ''
corpuscular theory In optics, the corpuscular theory of light states that light is made up of small discrete particles called " corpuscles" (little particles) which travel in a straight line with a finite velocity and possess impetus. This was based on an alternate ...
'' Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of ''c'' with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (''c'' ± ''v''). In the 20th century,
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 ...
was created by
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 ...
to solve the apparent conflict between
electrodynamics In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
and the principle of relativity. The theory's geometrical simplicity was persuasive, and the majority of scientists accepted relativity by 1911. However, a few scientists rejected the second basic postulate of relativity: the constancy of the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
in all inertial frames. So different types of emission theories were proposed where the speed of light depends on the velocity of the source, and the Galilean transformation is used instead 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 velo ...
. All of them can 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 188 ...
, since the speed of light is constant with respect to the interferometer in all frames of reference. Some of those theories were: *Light retains throughout its whole path the component of velocity which it obtained from its original moving source, and after reflection light spreads out in spherical form around a center which moves with the same velocity as the original source. (Proposed by
Walter Ritz Walther Heinrich Wilhelm Ritz (22 February 1878 – 7 July 1909) was a Swiss theoretical physicist. He is most famous for his work with Johannes Rydberg on the Rydberg–Ritz combination principle. Ritz is also known for the variational method na ...
in 1908). This model was considered to be the most complete emission theory. (Actually, Ritz was modeling Maxwell–Lorentz electrodynamics. In a later paper Ritz said that the emission particles ''in his theory'' should suffer interactions with charges along their path and thus waves (produced by them) would not retain their original emission velocities indefinitely.) *The excited portion of a reflecting mirror acts as a new source of light and the reflected light has the same velocity ''c'' with respect to the mirror as has original light with respect to its source. (Proposed by Richard Chase Tolman in 1910, although he was a supporter of special relativity). *Light reflected from a mirror acquires a component of velocity equal to the velocity of the mirror image of the original source (Proposed by Oscar M. Stewart in 1911). *A modification of the Ritz–Tolman theory was introduced by
J. G. Fox John Gaston Fox (March 5, 1916 – July 24, 1980) was an American nuclear physicist. He earned his PhD from Princeton in 1941 and was soon recruited to work on the Manhattan Project. He later moved to Pittsburgh where he spent the rest of h ...
(1965). He argued that the extinction theorem (i.e., the regeneration of light within the traversed medium) must be considered. In air, the extinction distance would be only 0.2 cm, that is, after traversing this distance the speed of light would be constant with respect to the medium, not to the initial light source. (Fox himself was, however, a supporter of special relativity.)
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 ...
is supposed to have worked on his own emission theory before abandoning it in favor of his
special theory of 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 ...
. Many years later R.S. Shankland reports Einstein as saying that Ritz's theory had been "very bad" in places and that he himself had eventually discarded emission theory because he could think of no form of differential equations that described it, since it leads to the waves of light becoming "all mixed up".


Refutations of emission theory

The following scheme was introduced by de Sitter to test emission theories: :c'=c\pm kv\, where ''c'' is the speed of light, ''v'' that of the source, ''c' '' the resultant speed of light, and ''k'' a constant denoting the extent of source dependence which can attain values between 0 and 1. According to special relativity and the stationary aether, ''k''=0, while emission theories allow values up to 1. Numerous terrestrial experiments have been performed, over very short distances, where no "light dragging" or extinction effects could come into play, and again the results confirm that light speed is independent of the speed of the source, conclusively ruling out emission theories.


Astronomical sources

In 1910
Daniel Frost Comstock Daniel Frost Comstock ; (August 14, 1883 – March 2, 1970) was an American physicist and engineer. Biography Comstock attained a B.S. from the Massachusetts Institute of Technology in 1904. He also studied in Berlin, Zürich, and Basel, where h ...
and in 1913 Willem de Sitter wrote that for the case of a double-star system seen edge-on, light from the approaching star might be expected to travel faster than light from its receding companion, and overtake it. If the distance was great enough for an approaching star's "fast" signal to catch up with and overtake the "slow" light that it had emitted earlier when it was receding, then the image of the star system should appear completely scrambled. De Sitter argued that none of the star systems he had studied showed the extreme optical effect behavior, and this was considered the death knell for Ritzian theory and emission theory in general, with k < 2\times10^. The effect of
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
on de Sitter's experiment has been considered in detail by Fox, and it arguably undermines the cogency of de Sitter type evidence based on binary stars. However, similar observations have been made more recently in the x-ray spectrum by Brecher (1977), which have a long enough extinction distance that it should not affect the results. The observations confirm that the speed of light is independent of the speed of the source, with k < 2\times10^.
Hans Thirring Hans Thirring (March 23, 1888 – March 22, 1976) was an Austrian theoretical physicist, professor, and father of the physicist Walter Thirring. He won the Haitinger Prize of the Austrian Academy of Sciences in 1920. Together with the mathemat ...
argued in 1924, that an atom which is accelerated during the emission process by thermal collisions in the sun, is emitting light rays having different velocities at their start- and endpoints. So one end of the light ray would overtake the preceding parts, and consequently the distance between the ends would be elongated up to 500 km until they reach Earth, so that the mere existence of sharp
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s in the sun's radiation, disproves the ballistic model.


Terrestrial sources

Such experiments include that of Sadeh (1963) who used a time-of-flight technique to measure velocity differences of photons traveling in opposite direction, which were produced by positron annihilation. Another experiment was conducted by Alväger et al. (1963), who compared the time of flight of gamma rays from moving and resting sources. Both experiments found no difference, in accordance with relativity. Filippas and Fox (1964) did not consider Sadeh (1963) and Alväger (1963) to have sufficiently controlled for the effects of extinction. So they conducted an experiment using a setup specifically designed to account for extinction. Data collected from various detector-target distances were consistent with there being no dependence of the speed of light on the velocity of the source, and were inconsistent with modeled behavior assuming c ± v both with and without extinction. Continuing their previous investigations, Alväger et al. (1964) observed π0-
meson In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, ...
s which decay into photons at 99.9% light speed. The experiment showed that the photons didn't attain the velocity of their sources and still traveled at the speed of light, with k =(-3\pm13)\times10^. The investigation of the media which were crossed by the photons showed that the extinction shift was not sufficient to distort the result significantly. Also
measurements of neutrino speed Measurements of neutrino speed have been conducted as tests of special relativity and for the determination of the mass of neutrinos. Astronomical searches investigate whether light and neutrinos emitted simultaneously from a distant source are ar ...
have been conducted. Mesons travelling nearly at light speed were used as sources. Since neutrinos only participate in the
electroweak interaction In particle physics, the electroweak interaction or electroweak force is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very differe ...
, extinction plays no role. Terrestrial measurements provided upper limits of k\leq10^.


Interferometry

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 interferomet ...
demonstrates that one beam on a rotating platform covers less distance than the other beam, which creates the shift in the interference pattern.
Georges Sagnac Georges Sagnac (14 October 1869 – 26 February 1928) was a French physicist who lent his name to the Sagnac effect, a phenomenon which is at the basis of interferometers and ring laser gyroscopes developed since the 1970s. Life and work Sagna ...
's original experiment has been shown to suffer extinction effects, but since then, the Sagnac effect has also been shown to occur in vacuum, where extinction plays no role. The predictions of Ritz's version of emission theory were consistent with almost all terrestrial interferometric tests save those involving the propagation of light in moving media, and Ritz did not consider the difficulties presented by tests such as the
Fizeau experiment The Fizeau experiment was carried out by Hippolyte Fizeau in 1851 to measure the relative speeds of light in moving water. Fizeau used a special interferometer arrangement to measure the effect of movement of a medium upon the speed of light. ...
to be insurmountable. Tolman, however, noted that a Michelson–Morley experiment using an extraterrestrial light source could provide a decisive test of the Ritz hypothesis. In 1924, Rudolf Tomaschek performed a modified Michelson–Morley experiment using starlight, while Dayton Miller used sunlight. Both experiments were inconsistent with the Ritz hypothesis. Babcock and Bergman (1964) placed rotating glass plates between the mirrors of a
common-path interferometer A common-path interferometer is a class of interferometers in which the reference beam and sample beams travel along the same path. Examples include the Sagnac interferometer, Zernike phase-contrast interferometer, and the point diffraction interf ...
set up in a static Sagnac configuration. If the glass plates behave as new sources of light so that the total speed of light emerging from their surfaces is ''c'' + ''v'', a shift in the interference pattern would be expected. However, there was no such effect which again confirms special relativity, and which again demonstrates the source independence of light speed. This experiment was executed in vacuum, thus extinction effects should play no role.
Albert Abraham Michelson Albert Abraham Michelson FFRS HFRSE (surname pronunciation anglicized as "Michael-son", December 19, 1852 – May 9, 1931) was a German-born American physicist of Polish/Jewish origin, known for his work on measuring the speed of light and espe ...
(1913) and
Quirino Majorana Quirino Majorana (28 October 1871 – 31 July 1957) was an Italian experimental physicist who investigated a wide range of phenomena during his long career as professor of physics at the Universities of Rome, Turin (1916–1921), and Bologna (192 ...
(1918/9) conducted interferometer experiments with resting sources and moving mirrors (and vice versa), and showed that there is no source dependence of light speed in air. Michelson's arrangement was designed to distinguish between three possible interactions of moving mirrors with light: (1) "the light corpuscles are reflected as projectiles from an elastic wall", (2) "the mirror surface acts as a new source", (3) "the velocity of light is independent of the velocity of the source". His results were consistent with source independence of light speed. Majorana analyzed the light from moving sources and mirrors using an unequal arm Michelson interferometer that was extremely sensitive to wavelength changes. Emission theory asserts that Doppler shifting of light from a moving source represents a frequency shift with no shift in wavelength. Instead, Majorana detected wavelength changes inconsistent with emission theory. Beckmann and Mandics (1965) repeated the Michelson (1913) and Majorana (1918) moving mirror experiments in high vacuum, finding ''k'' to be less than 0.09. Although the vacuum employed was insufficient to definitively rule out extinction as the reason for their negative results, it was sufficient to make extinction highly unlikely. Light from the moving mirror passed through a Lloyd interferometer, part of the beam traveling a direct path to the photographic film, part reflecting off the Lloyd mirror. The experiment compared the speed of light hypothetically traveling at ''c + v'' from the moving mirrors, versus reflected light hypothetically traveling at ''c'' from the Lloyd mirror.


Other refutations

Emission theories use the Galilean transformation, according to which time coordinates are invariant when changing frames ("absolute time"). Thus the
Ives–Stilwell experiment The Ives–Stilwell experiment tested the contribution of relativistic time dilation to the Doppler shift of light. The result was in agreement with the formula for the transverse Doppler effect and was the first direct, quantitative confirmati ...
, which confirms relativistic
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 ...
, also refutes the emission theory of light. As shown by
Howard Percy Robertson Howard Percy "Bob" Robertson (January 27, 1903 – August 26, 1961) was an American mathematician and physicist known for contributions related to physical cosmology and the uncertainty principle. He was Professor of Mathematical Physics at the C ...
, the complete Lorentz transformation can be derived, when the Ives–Stillwell experiment is considered together with the Michelson–Morley experiment and the Kennedy–Thorndike experiment. Furthermore,
quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and spec ...
places the propagation of light in an entirely different, but still relativistic, context, which is completely incompatible with any theory that postulates a speed of light that is affected by the speed of the source.


See also

*
History of special relativity The history of special relativity consists of many theoretical results and empirical findings obtained by Albert A. Michelson, Hendrik Lorentz, Henri Poincaré and others. It culminated in the theory of special relativity proposed by Albert Eins ...
*
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 justifi ...


References

* Isaac Newton, Philosophiæ Naturalis Principia Mathematica * Isaac Newton,
Opticks ''Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light'' is a book by English natural philosopher Isaac Newton that was published in English in 1704 (a scholarly Latin translation appeared in 1706). (''Opti ...


External links


de Sitter (1913) papers
on binary stars as evidence against Ritz's emission theory. {{DEFAULTSORT:Emission Theory Special relativity Obsolete theories in physics