Kennedy-Thorndike experiment
Encyclopedia
The Kennedy–Thorndike experiment first conducted in 1932, is a modified form of the Michelson–Morley experimental procedure, and tests special relativity
.
The modification is to make one arm of the classical Michelson–Morley (MM) apparatus shorter than the other one. While the Michelson-Morley experiment showed, that the speed of light is independent of the orientation of the apparatus, the Kennedy–Thorndike experiment showed that it is also independent of the velocity of the apparatus in different inertial frames. It also served as a test to indirectly verify time dilation
: while the negative result of the Michelson-Morley experiment can be explained by length contraction
alone, the negative result of the Kennedy–Thorndike experiment also requires time dilation besides length contraction to explain why no phase shifts will be detected while the earth moves around the sun. The first direct confirmation of time dilation was achieved by the Ives–Stilwell experiment. See also Tests of special relativity.
only. Kennedy had already made several increasingly sophisticated versions of the MM experiment through the 1920s when he struck upon a way to test time dilation
as well. In their own words:
By making one arm of the experiment much shorter than the other, a change in speed of the earth would cause changes in the travel times of the light rays, from which a fringe shift would result except if the frequency of the light source would change to the same degree. In order to determine if such a fringe shift
took place, the interferometer was made extremely stable and the interference patterns were photographed for later comparison. The tests were done over a period of many months. As no significant fringe shift was found, the experimenters concluded that time dilation occurs as predicted by Special relativity.
, maser
, cryogenic optical resonators, etc.. Examples that considerably reduce the possibility of anisotropy, are Hils and Hall (1990),
Braxmeier et al. (2002),
Wolf et al. (2004).
Tobar et al. (2009) which gave an upper limit of the velocity dependence of the speed of light of
.
Besides those terrestrial measurements, a Kennedy–Thorndike experiment was carried out by Müller & Soffel (1995) using Lunar Laser Ranging
, i.e., signals between Earth and Moon have been evaluated. This experiment gave a negative result as well.
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...
.
The modification is to make one arm of the classical Michelson–Morley (MM) apparatus shorter than the other one. While the Michelson-Morley experiment showed, that the speed of light is independent of the orientation of the apparatus, the Kennedy–Thorndike experiment showed that it is also independent of the velocity of the apparatus in different inertial frames. It also served as a test to indirectly verify time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
: while the negative result of the Michelson-Morley experiment can be explained by length contraction
Length contraction
In physics, length contraction – according to Hendrik Lorentz – is the physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer...
alone, the negative result of the Kennedy–Thorndike experiment also requires time dilation besides length contraction to explain why no phase shifts will be detected while the earth moves around the sun. The first direct confirmation of time dilation was achieved by the Ives–Stilwell experiment. See also Tests of special relativity.
The experiment
The original Michelson–Morley experiment was useful for testing the Lorentz–FitzGerald contraction hypothesisLength contraction
In physics, length contraction – according to Hendrik Lorentz – is the physical phenomenon of a decrease in length detected by an observer of objects that travel at any non-zero velocity relative to that observer...
only. Kennedy had already made several increasingly sophisticated versions of the MM experiment through the 1920s when he struck upon a way to test time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...
as well. In their own words:
By making one arm of the experiment much shorter than the other, a change in speed of the earth would cause changes in the travel times of the light rays, from which a fringe shift would result except if the frequency of the light source would change to the same degree. In order to determine if such a fringe shift
Fringe shift
A fringe shift is most often referred to in interferometry experiments such as the Michelson-Morley. It is the behavior of a pattern of "fringes" when the phase relationship between the component sources change....
took place, the interferometer was made extremely stable and the interference patterns were photographed for later comparison. The tests were done over a period of many months. As no significant fringe shift was found, the experimenters concluded that time dilation occurs as predicted by Special relativity.
Recent experiments
Such experiments have been repeated with increased precision until today, using laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
, maser
Maser
A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...
, cryogenic optical resonators, etc.. Examples that considerably reduce the possibility of anisotropy, are Hils and Hall (1990),
Braxmeier et al. (2002),
Wolf et al. (2004).
Tobar et al. (2009) which gave an upper limit of the velocity dependence of the speed of light of
.Besides those terrestrial measurements, a Kennedy–Thorndike experiment was carried out by Müller & Soffel (1995) using Lunar Laser Ranging
Lunar laser ranging experiment
The ongoing Lunar Laser Ranging Experiment measures the distance between the Earth and the Moon using laser ranging. Lasers on Earth are aimed at retroreflectors planted on the moon during the Apollo program, and the time for the reflected light to return is determined...
, i.e., signals between Earth and Moon have been evaluated. This experiment gave a negative result as well.