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Emission theory (relativity)

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This article is about the emission theory of relativity. For the emission theory of vision, see Emission theory (vision).

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. 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.[1][2].

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:[3][1]

  • 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.[4]
  • 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).[5]
  • 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).[6]
  • 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.)[1]

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".[7] [8] [9].

Refutations of emission theory

The following scheme was introduced by de Sitter[10] 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:

de Sitter's double star argument
  • 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 .[11][12][10] 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 .[2]

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 effect 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 Sagnac 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.[13]
  • 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.[14]
  • Alväger et al. (1964) observed π0-mesons 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.[15]
  • 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.[16]

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)[17], Devasia, [18]

See also

References

  1. ^ a b c Fox, J. G. (1965), "Evidence Against Emission Theories", American Journal of Physics, 33 (1): 1–17, Bibcode:1965AmJPh..33....1F, doi:10.1119/1.1971219.
  2. ^ a b Brecher, K. (1977), "Is the speed of light independent of the velocity of the source", Physical Review Letters, 39 (17): 1051–1054, Bibcode:1977PhRvL..39.1051B, doi:10.1103/PhysRevLett.39.1051.
  3. ^ Tolman, Richard Chace (1912), "Some Emission Theories of Light" , Physical Review, 35 (2): 136–143
  4. ^ Ritz, Walter (1908), "Recherches critiques sur l'Électrodynamique Générale", Annales de Chimie et de Physique, 13: 145–275. See also the English translation.
  5. ^ Tolman, Richard Chace (1910), "The Second Postulate of Relativity" , Physical Review, 31 (1): 26–40
  6. ^ Stewart, Oscar M. (1911), "The Second Postulate of Relativity and the Electromagnetic Emission Theory of Light", Physical Review, 32 (4): 418–428
  7. ^ Shankland, R. S. (1963), "Conversations with Albert Einstein", American Journal of Physics, 31 (1): 47–57, Bibcode:1963AmJPh..31...47S, doi:10.1119/1.1969236
  8. ^ Norton, John D., John D. (2004), "Einstein's Investigations of Galilean Covariant Electrodynamics prior to 1905", Archive for History of Exact Sciences, 59: 45–105, Bibcode:2004AHES...59...45N, doi:10.1007/s00407-004-0085-6
  9. ^ Martínez, Alberto A. (2004), "Ritz, Einstein, and the Emission Hypothesis", Physics in Perspective, 6 (1): 4–28, Bibcode:2004PhP.....6....4M, doi:10.1007/s00016-003-0195-6
  10. ^ a b De Sitter, Willem (1913), "On the constancy of the velocity of light" , Proceedings of the Royal Netherlands Academy of Arts and Sciences, 16 (1): 395–396
  11. ^ Comstock, Daniel Frost (1910), "A Neglected Type of Relativity" , Physical Review, 30 (2): 267
  12. ^ De Sitter, Willem (1913), "A proof of the constancy of the velocity of light" , Proceedings of the Royal Netherlands Academy of Arts and Sciences, 15 (2): 1297–1298
  13. ^ Sagnac, Georges (1913), "Sur la preuve de la réalité de l'éther lumineux par l'expérience de l'interférographe tournant", Comptes Rendus, 157: 1410–1413
  14. ^ Babcock, G. C.; Bergman, T. G. (1964), "Determination of the Constancy of the Speed of Light", Journal of the Optical Society of America, 54 (2): 147–150, doi:10.1364/JOSA.54.000147{{citation}}: CS1 maint: multiple names: authors list (link)
  15. ^ Alväger, T.; Farley, F. J. M.; Kjellman, J.; Wallin, L. (1964), "Test of the second postulate of special relativity in the GeV region", Physics Letters, 12 (3): 260–262, Bibcode:1964PhL....12..260A, doi:10.1016/0031-9163(64)91095-9.{{citation}}: CS1 maint: multiple names: authors list (link)
  16. ^ Thirring, Hans (1924), "Über die empirische Grundlage des Prinzips der Konstanz der Lichtgeschwindigkeit", Zeitschrift für Physik, 31 (1): 133–138, Bibcode:1925ZPhy...31..133T, doi:10.1007/BF02980567.
  17. ^ Richard Arthur Waldron (1977). The wave and ballistic theories of light:a critical review. F. Muller. ISBN 0584101481.
  18. ^ Nonlinear Models for Relativity Effects in Electromagnetism, Zeitschrift fur Naturforschung A, Vol. 64a (5-6), pp. 327-340, May-June, 2009.
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