# Precision of the latest Michelson-Morley experiment in terms of speed of light difference?

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?

• The Michelson-Morley experiment does not measure the wavelength of light, so it is not clear what you are asking. – Buzz Oct 28 '18 at 3:04
• Well they were expecting a wavelength shift between 2 light rays, no? Which will result in some fringe. – Phil Bouchard Oct 28 '18 at 3:05
• 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. – Buzz Oct 28 '18 at 3:10
• Well if the wavelength changes then the speed will change as well. Here is the relation: ${v_2}=-\frac{\left( {{\lambda }_2}-{{\lambda }_1}\right) {c_1}}{{{\lambda }_1}}$ – Phil Bouchard Oct 28 '18 at 3:15
• Anyway I just changed the wavelength for speed of light. – Phil Bouchard Oct 28 '18 at 3:36

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 $$1.8 \; nm/s$$
• Why are they saying in the following forum that it's around $1 \times 10^{-12} c$? physicsforums.com/threads/… Reference physicsforums.com/threads/… – Phil Bouchard Oct 28 '18 at 3:52
• @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. – Buzz Oct 28 '18 at 4:47