Emission theory
Encyclopedia
Emission theory was a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment
. Emission theories obey the principle of relativity
by having no preferred frame for light
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 and mechanics
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..
. In his Corpuscular theory
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
was created by Albert Einstein
to solve the apparent conflict between electrodynamics 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
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
. All of them can explain the negative outcome of the Michelson-Morley experiment
, since the speed of light is constant with respect to the interferometer in all frames of reference. Some of those theories were:
Albert Einstein
is supposed to have worked on his own emission theory before abandoning it in favor of his special theory of relativity. Many years later R.S. Shankland reports Einstein as saying that Ritz' 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".
.
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. Emission theories are considered as disproved for the following reasons:
In addition, 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. For example:
Furthermore, quantum electrodynamics 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.
Michelson-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...
. Emission theories obey the principle of relativity
Principle of relativity
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....
by having no preferred frame for light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
transmission, but say that light is emitted at speed "c"
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
relative to its source instead of applying the invariance postulate. Thus, emitter theory combines electrodynamics and mechanics
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....
with a simple Newtonian theory. Although there are still proponents of this theory outside the scientific mainstream
Mainstream
Mainstream is, generally, the common current thought of the majority. However, the mainstream is far from cohesive; rather the concept is often considered a cultural construct....
, this theory is considered to be conclusively discredited by most scientists..
History
The name most often associated with emission theory is Isaac NewtonIsaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
. In his Corpuscular theory
Corpuscular theory
In optics, corpuscular theory of light, set forward by Sir Isaac Newton, states that light is made up of small discrete particles called "corpuscles" which travel in a straight line with a finite velocity and possess kinetic energy....
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
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...
was created by 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...
to solve the apparent conflict between electrodynamics and the principle of relativity
Principle of relativity
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....
. 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
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
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
Galilean transformation
The Galilean transformation is used to transform between the coordinates of two reference frames which differ only by constant relative motion within the constructs of Newtonian physics. This is the passive transformation point of view...
is used instead 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...
. All of them can explain the negative outcome of the Michelson-Morley experiment
Michelson-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...
, 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 Walther Ritz in 1908). This model was considered to be the most complete emission theory.
- 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 Fox (1965). He argued, that also Extinction (i.e., absorption, scattering, and emission 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
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...
is supposed to have worked on his own emission theory before abandoning it in favor of his special theory of relativity. Many years later R.S. Shankland reports Einstein as saying that Ritz' 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: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. Emission theories are considered as disproved for the following reasons:
- In 1910 Daniel Frost ComstockDaniel Frost ComstockDaniel Frost Comstock was an American physicist and engineer....
and in 1913 Willem de SitterWillem de SitterWillem de Sitter was a Dutch mathematician, physicist and astronomer.-Life and work:Born in Sneek, De Sitter studied mathematics at the University of Groningen and then joined the Groningen astronomical laboratory. He worked at the Cape Observatory in South Africa...
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 arguedDe Sitter double star experimentThe de Sitter effect was described by de Sitter in 1913 and used to support the special theory of relativity against a competing 1908 emission theory by Walter Ritz that postulated a variable speed of light...
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
. The idea that perhaps the speed of light only has an effective value of cEMITTER while it is local to the emitter, as a "light-dragging" or "proximity" effect has been considered in detail by Fox. This can be expressed in terms of the "extinction effect", and it arguably undermines the cogency of de Sitter type evidence based on optical 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 
.
In addition, 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. For example:
- The Sagnac effectSagnac effectThe Sagnac effect , 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 ring interferometry. A beam of light is split and the two beams are made to follow a trajectory in...
demonstrates that one beam on a rotating platform covers less distance than the other beam, which creates the shift in the interference pattern. As Georges SagnacGeorges SagnacGeorges Sagnac 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. Sagnac died at Meudon-Bellevue....
stated, his experiment directly shows that the speed of light is independent of the velocity of the source. And since the Sagnac effect also occurs in vacuum, extinction effects play no role. - Babcock et al (1964) placed rotating glass plates between the mirrors of a Michelson-Morley-type experiment. If the velocity of the glass plates is added up to the photons during the absorption/emission process, a shift in the interference pattern would be expected. However, there was no such effect which again confirms special relativity, i.e. the source independence of light speed. This experiment was executed in vacuum, thus extinction effects play no role.
- Alväger et al. (1964) observed π0-mesonMesonIn particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s which decay into photons at 99.9% light speed. According to emission theory the meson velocity is added up to the photon velocity. However, the experiment showed that the photons still traveled at the speed of light, with
. The investigation of the media which were crossed by the photons showed that the extinction shift was not sufficient to distort the result significantly. - Hans ThirringHans ThirringHans Thirring was an Austrian theoretical physicist, professor, and father of the physicist Walter Thirring....
argued in 1926, 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 lineSpectral lineA spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s in the sun's radiation, disproves the ballistic model.
Furthermore, quantum electrodynamics 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.
Recent non-mainstream models
Some authors still promote ideas similar to emission theories, although none of them gained support by the mainstream scientific community. For example, Waldron (1977), Fritzius (1997), Devasia (2009).External links
- de Sitter (1913) papers on binary stars as evidence against Ritz's emission theory.