De Sitter double star experiment
Encyclopedia
The 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
. De Sitter showed that Ritz's theory predicted that the orbits of binary stars would appear more eccentric than consistent with experiment and with the laws of mechanics
.
A similar effect was already described by Daniel Frost Comstock
in 1910.
See also Tests of special relativity.
, light
thrown off by an object should move at a speed of
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
metres per second, this light would then be expected to still be travelling at 
( or 
) metres per second at the time it reached us.
Willem de Sitter
argued that if this was true, a star in a double-star system would usually have an orbit that caused it to have alternating approach and recession velocities, and light emitted from different parts of the orbital path would then 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.
De Sitter made a study of double stars (1913) and found no cases where the stars' images appeared scrambled.
Since the total flight-time difference between "fast" and "slow" lightsignals 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, deSitter 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.
Emission theory
Emission theory was a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment...
by Walter Ritz
Walter Ritz
Walther Ritz was a Swiss theoretical physicist.His father, Raphael Ritz, a native of Valais, was a well-known landscape and interior scenes artist. His mother was the daughter of the engineer Noerdlinger of Tübingen. Ritz studied in Zurich and Göttingen...
that postulated a variable speed of light
Variable speed of light
The variable speed of light concept states that the speed of light in a vacuum, usually denoted by c, may not be constant in most cases. In most situations in condensed matter physics when light is traveling through a medium, it effectively has a slower speed...
. De Sitter showed that Ritz's theory predicted that the orbits of binary stars would appear more eccentric than consistent with experiment and with the laws of 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....
.
A similar effect was already described by Daniel Frost Comstock
Daniel Frost Comstock
Daniel Frost Comstock was an American physicist and engineer....
in 1910.
See also Tests of special relativity.
The effect
According to simple emission theoryEmission theory
Emission theory was a competing theory for the special theory of relativity, explaining the results of the Michelson-Morley experiment...
, 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...
thrown off by an object should move at a speed of
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...
with respect to the emitting objectEmitter
-In general:*A device used to exude any signal, beacon, light, odor, liquid, fragrance, ionizing particles or any other type of signal.-In horticulture:*A device used in drip irrigation.-In electronics and instrument physics:...
.
If there are no complicating dragging effects
Light-dragging effects
In physics, there are several situations in which the motion of matter might be said to drag light.Under special relativity's simplified model it is assumed that these light-dragging effects do not happen, and that the speed of light is independent of the speed of a body's motion...
, 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
metres per second, this light would then be expected to still be travelling at ( or ) metres per second at the time it reached us.Willem de Sitter
Willem de Sitter
Willem 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...
argued that if this was true, a star in a double-star system would usually have an orbit that caused it to have alternating approach and recession velocities, and light emitted from different parts of the orbital path would then 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.
De Sitter made a study of double stars (1913) and found no cases where the stars' images appeared scrambled.
Since the total flight-time difference between "fast" and "slow" lightsignals 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, deSitter 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.