One-way speed of light

[1][2] Experiments that attempt to directly probe the one-way speed of light independent of synchronization have been proposed, but none have succeeded in doing so.

Thus the measured value of the average one-way speed is dependent on the method used to synchronize the start and finish clocks.

The Lorentz transformation is defined such that the one-way speed of light will be measured to be independent of the inertial frame chosen.

[8] Some authors such as Mansouri and Sexl (1977)[9][10] as well as Will (1992)[11] argued that this problem doesn't affect measurements of the isotropy of the one-way speed of light, for instance, due to direction-dependent changes relative to a "preferred" (aether) frame Σ.

They concluded: "...one cannot hope even to test the isotropy of the speed of light without, in the course of the same experiment, deriving a one-way numerical value at least in principle, which then would contradict the conventionality of synchrony.

In isotropy experiments, simultaneity conventions are often not explicitly stated but are implicitly present in the way coordinates are defined or in the laws of physics employed.

), meaning time dilation can no longer be ignored at small velocities, and slow clock-transport will fail to detect this anisotropy.

Changing the convention of synchronization for A and B makes the value for time dilation (like the one-way speed of light) directional dependent.

[20] Thus the so-called twin paradox occurs in all transformations preserving the constancy of the two-way speed of light.

It was argued against the conventionality of the one-way speed of light that this concept is closely related to dynamics, the laws of motion and inertial reference frames.

[21] Similarly, Ohanian argued that inertial reference frames are defined so that Newton's laws of motion hold in first approximation.

Therefore, since the laws of motion predict isotropic one-way speeds of moving bodies with equal acceleration, and because of the experiments demonstrating the equivalence between Einstein synchronization and slow clock-transport synchronization, it appears to be required and directly measured that the one-way speed of light is isotropic in inertial frames.

Otherwise, both the concept of inertial reference frames and the laws of motion must be replaced by much more complex ones involving anisotropic coordinates.

[4] Salmon argued that momentum conservation in its standard form assumes isotropic one-way speed of moving bodies from the outset.

[21] And in response to Ohanian, both Macdonald and Martinez argued that even though the laws of physics become more complicated with non-standard synchrony, they still are a consistent way to describe the phenomena.

[26] In the October 2009 issue of the American Journal of Physics, Greaves, Rodriguez, and Ruiz-Camacho proposed a new method of measurement of the one-way speed of light.

[27] In the June 2013 issue of the American Journal of Physics, Hankins, Rackson, and Kim repeated the Greaves et al. experiment intending to obtain with greater accuracy the one-way speed of light.

J. Finkelstein showed that the Greaves et al. experiment actually measures the round-trip (two-way) speed of light.

This experiment, carried out in 1990 by the NASA Jet Propulsion Laboratory, measured the time of flight of light signals through a fiber optic link between two hydrogen maser clocks.

[41] Similarly, differences in the one-way propagation between left- and right-handed photons, leading to vacuum birefringence, were excluded by observation of the simultaneous arrival of distant star light.

However, the partially conventional character of those quantities was demonstrated by Kostelecky et al, pointing out that such variations in the speed of light can be removed by suitable coordinate transformations and field redefinitions.

[43] There are one-way coefficients of the SME that cannot be redefined into other sectors, since different light rays from the same distance location are directly compared with each other; see the previous section.

In 1904 and 1905, Hendrik Lorentz and Henri Poincaré proposed a theory which explained the negative result of the Michelson-Morley experiment as being due to the effect of motion through the aether on the lengths of physical objects and the speed at which clocks ran.

In 1905 these transformations became the basic equations of Einstein's special theory of relativity which proposed the same results without reference to an aether.

However, the difference between the one-way and two-way speeds of light can never be observed due to the action of the aether on the clocks and lengths.

Therefore, the Poincaré-Einstein convention is also employed in this model, making the one-way speed of light isotropic in all frames of reference.

A synchronization scheme proposed by Reichenbach and Grünbaum, which they called ε-synchronization, was further developed by authors such as Edwards (1963),[47] Winnie (1970),[18] Anderson and Stedman (1977), who reformulated the Lorentz transformation without changing its physical predictions.

As shown by Edwards, Winnie, and Mansouri-Sexl, by suitable rearrangement of the synchrony parameters even some sort of "absolute simultaneity" can be achieved, in order to simulate the basic assumption of Lorentz ether theory.

[48] A number of theories have been developed to allow assessment of the degree to which experimental results differ from the predictions of relativity.

However, it was pointed out that such variations in the speed of light can be removed by suitable redefinitions of the coordinates and fields employed.