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Here is the most precise Michelson-Morley experiment ever made: Laboratory Test of the Isotropy of Light Propagation at the 10^−17 Level

When they say:

We find no evidence for an isotropy violation at a 1 σ uncertainty level of 0.6 parts in 10^17 (RMS) and 2 parts in 10^17 for seven of eight coefficients of the SME.

What do they mean in terms of precision of the speed of light difference?

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  • $\begingroup$ The Michelson-Morley experiment does not measure the wavelength of light, so it is not clear what you are asking. $\endgroup$ – Buzz Oct 28 '18 at 3:04
  • $\begingroup$ Well they were expecting a wavelength shift between 2 light rays, no? Which will result in some fringe. $\endgroup$ – Phil Bouchard Oct 28 '18 at 3:05
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    $\begingroup$ No, the classic way of interpreting Michelson-Morely results is as a difference in the speeds of the waves in the two arms of the interferometers, not the wavelength. $\endgroup$ – Buzz Oct 28 '18 at 3:10
  • $\begingroup$ Well if the wavelength changes then the speed will change as well. Here is the relation: ${v_2}=-\frac{\left( {{\lambda }_2}-{{\lambda }_1}\right) {c_1}}{{{\lambda }_1}}$ $\endgroup$ – Phil Bouchard Oct 28 '18 at 3:15
  • $\begingroup$ Anyway I just changed the wavelength for speed of light. $\endgroup$ – Phil Bouchard Oct 28 '18 at 3:36
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They are just saying that their measurement indicates that the speed of light is either perfectly isotropic or that any anisotropy is less than $0.6 \; 10^{-17}\;c$. This works out to about $1.8 \; nm/s$

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  • $\begingroup$ Why are they saying in the following forum that it's around $1 \times 10^{-12} c$? physicsforums.com/threads/… Reference physicsforums.com/threads/… $\endgroup$ – Phil Bouchard Oct 28 '18 at 3:52
  • $\begingroup$ Shouldn’t you ask them that directly there? $\endgroup$ – Dale Oct 28 '18 at 3:54
  • $\begingroup$ The conversation is 3 years old so they won't answer. $\endgroup$ – Phil Bouchard Oct 28 '18 at 3:55
  • $\begingroup$ Regardless of their willingness to answer, I am neither a mind reader nor a psychologist and cannot tell you why someone else did something. I can barely figure out my own motivations $\endgroup$ – Dale Oct 28 '18 at 4:02
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    $\begingroup$ @PhilBouchard That commenter is just wrong about the $10^{-12}$. The experiment shows the fractional differences in the speed of light in different directions are at the $10^{-17}$ level or smaller. That is what eq. (3) in the PRL paper indicates. $\endgroup$ – Buzz Oct 28 '18 at 4:47

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