Emission theory, also called emitter theory
or ballistic theory of light, was a competing
theory for the special theory of relativity,
explaining the results of the Michelson–Morley
experiment of 1887. 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.
== History ==
The name most often associated with emission
theory is Isaac Newton. 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:
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 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 (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 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'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
±
k
v
{\displaystyle 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 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
×
10
−
3
{\displaystyle k<2\times 10^{-3}}
.The effect of extinction 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
×
10
−
9
{\displaystyle k<2\times 10^{-9}}
.Hans 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 lines 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-mesons
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
±
13
)
×
10
−
5
{\displaystyle k=(-3\pm 13)\times 10^{-5}}
. 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 have been conducted. Mesons travelling
nearly at light speed were used as sources.
Since neutrinos only participate in the electroweak
interaction, extinction plays no role. Terrestrial
measurements provided upper limits of
k
≤
10
−
6
{\displaystyle k\leq 10^{-6}}
.
=== Interferometry ===
The Sagnac effect 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'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 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
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 (1913) and Quirino Majorana (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, which confirms
relativistic time dilation, also refutes the
emission theory of light. As shown by Howard
Percy Robertson, 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
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
Tests of special relativity
