XV. c

"It's better to light a candle than to curse the darkness" — Carl Sagan

We all love things that are constant, predictable, with no surprises and as a consequence the world of Science is full of it... constants. Here's a few for illustration purposes:

Speed of light in vacuum
Gravitational constant
Planck constant
Electric constant / Vacuum Permittivity
Magnetic constant / Vacuum Permeability
Cosmological constant
There are loads of them used for the most varied purposes, from rocket launches to utilities meter readings. We tend to forget how they came about, the process that lead to the calculated figures above. It all started with measuring, collecting data from experiments, finding that there is a relationship between the measurements and the common factor between them all. A constant is just a common factor between physical entities. Through time, measurements got more accurate, experiments more precise, predictions on new outcomes more reliable. The problem with common factors is that where a new prediction carries an unexpected result, the blame is apportioned to the physical entities that took part on it (including humans)... the constants remain blameless. 

I deliberately stated the constants above as they are mentioned ad nauseum in multiple science programs, books, boxsets, etc.; the gravitational constant G is constantly in place in our world, from orbit calculations, planets and stars' locations to a kilogram of apples. The Planck constant h is again everywhere, from radiation, quanta, Quantum Mechanics to the Heisenberg Uncertainty Principle, although I always have to smile about the intellectual contradiction of using an "uncertain" formula based on a "constant". I also have to smile about the Cosmological "constant" Λ (Lambda) which in the past 100 years has been positive, negative and 0 depending on the most current fashion of Astrophysics, a true statement of the "precision" of Science.

And then I come about that little letter c, the speed of light, the ultimate frontier. The figure given above relates to propagation in the vacuum, that "ethereal" word where there is nothing. And yet there is always something, as the speed of light is intrinsically linked to the properties of the medium where it is propagating (the vacuum and Earth's atmosphere have almost the same values for practical purposes). The speed of light is calculated by this formula:
That is the reason why I mentioned the values of ε0 (epsilon zero) and μ0 (meeoo zero) above (feel free to do some number crunching on a calculator to confirm that value). Both permittivity and permeability are constants, depending on the medium used. To this date the vacuum has the lowest values ever found for both concepts. Other media, like glass or water have different values with the correspondent change on the speed of light. 

All these constants and measurements are very paroquial, obtained in our vicinity and extrapolated through cosmic distances, cosmic times, cosmic intellectual processes, a sample of 1 as I mentioned before. That oldest observed distant galaxy that is 13.2 billion light years away and emitted photons 13.2 billion years ago, has exactly the same behaviour as we have in here, nothing changed since that time, the photon that reached our telescopes on that long voyage did it with a constant speed, crossing a constant permeability and permittivity all the time. That is impossible! Constant?... I have a problem with that and so should you.

PS: Regarding the vacuum permeability it appears that from 20 May 2019 (my birthday btw) and I quote "the vacuum permeability will no longer be a defined constant, but rather will need to be determined experimentally". This will imply a little problem for the humble c.