The second is defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second.
With those definitions, we are left with no way to check the invariance of $c$ (the local speed of light in freely falling and non-rotating reference frames). Is there a definition of length which would be independent on anything involving light, and allow to check the invariance of $c$?
I am aware of classical definitions based on pendulum or earth's meridian. I red a definition involving the wavelength of a certain type of radiation, but this again depends on the speed of light. I am wondering it there exists a modern alternative definition of the meter, light independent and usable for precise experimenting.