De Sitter Double Star Experiment
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The de Sitter effect was described by
Willem de Sitter Willem de Sitter (6May 187220November 1934) was a Dutch mathematician, physicist, and astronomer. He is known for the de Sitter universe is a cosmological model, which was named after him. Life and work Born in Sneek, Netherlands, de Sitte ...
in 1913 (as well as by
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 ...
in 1910) and used to support the
special theory of relativity In physics, the special theory of relativity, or special relativity for short, is a scientific theory of the relationship between space and time. In Albert Einstein's 1905 paper, "On the Electrodynamics of Moving Bodies", the theory is presen ...
against a competing 1908 emission theory by
Walther 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 n ...
that postulated a variable speed of light dependent on the velocity of the emitting object. De Sitter showed that Ritz's theory would have predicted that the orbits of binary stars would appear more eccentric than consistent with experiment and with the laws of
mechanics Mechanics () is the area of physics concerned with the relationships between force, matter, and motion among Physical object, physical objects. Forces applied to objects may result in Displacement (vector), displacements, which are changes of ...
, and would have been inconsistent with astronomical observations. This was confirmed by Kenneth Brecher in 1977 by observing
x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
spectra. For other experiments related to special relativity, see tests of special relativity.


The effect

According to simple emission theory,
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
thrown off by an object should move at a speed of c with respect to the emitting object. If there are no complicating dragging effects, the light would then be expected to move at this same speed until it eventually reached an observer. For an object moving directly towards (or away from) the observer at v, this light would then be expected to still be travelling at (c+v) ( or (c-v) ) at the time it reached us. In 1913,
Willem de Sitter Willem de Sitter (6May 187220November 1934) was a Dutch mathematician, physicist, and astronomer. He is known for the de Sitter universe is a cosmological model, which was named after him. Life and work Born in Sneek, Netherlands, de Sitte ...
argued that if this was true, a star orbiting in a double-star system would usually, with regard to us, alternate between moving towards us and away from us. Light emitted from different parts of the orbital path would travel towards us at different speeds. For a nearby star with a small orbital velocity (or whose orbital plane was almost perpendicular to our line of view) this might merely make the star's orbit seem erratic, but for a sufficient combination of orbital speed and distance (and inclination), the "fast" light given off during approach would be able to catch up with and even overtake "slow" light emitted earlier during a recessional part of the star's orbit, and the star would present an image that was scrambled and out of sequence. That is, Kepler's laws of motion would apparently be violated for a distant observer. De Sitter made a study of double stars and found no cases where the stars' computed orbits appeared non-Keplerian. Since the total flight-time difference between "fast" and "slow" light signals would be expected to scale linearly with distance in simple emission theory, and the study would (statistically) have included stars with a reasonable spread of distances and orbital speeds and orientations, de Sitter concluded that the effect ''should'' have been seen if the model was correct, and its absence meant that the emission theory was almost certainly wrong.


Notes

* Modern experiments of the de Sitter type refute the idea that light might travel at a speed that was partially dependent on the velocity of the emitter (''c'=c + kv''), where the emitter's velocity ''v'' can be positive or negative, and ''k'' is a factor between 0 and 1, denoting the extent to which the speed of light depends on the source velocity. De Sitter established an upper limit of ''k'' < 0.002, but extinction effects make that result suspect. * De Sitter's experiment was criticized because of extinction effects by J. G. Fox. That is, during their flight to Earth, the light rays would have been absorbed and re-emitted by interstellar matter nearly at rest relative to Earth, so that the speed of light should become constant with respect to Earth, regardless of the motion of the original source(s). * In 1977, Kenneth Brecher published the results of a similar double-survey, and reached a similar conclusion - that any apparent irregularities in double-star orbits were too small to support the emission theory. Contrary to the data cited by de Sitter, Brecher observed the x-ray spectrum, thereby eliminating possible influences of the extinction effect. He established an upper limit of k < 2\times10^. * There are also terrestrial experiments that speak against such theories, see experiments testing emission theories.


References

{{Tests of special relativity Physics experiments 1913 in science