> But the question "how can you define the meter in terms of the speed of light if the speed of light depends on the definition of the meter?" is totally incohe

> But the question "how can you define the meter in terms of the speed of light if the speed of light depends on the definition of the meter?" is totally incoherent; I agree, I messed that up. What I meant to say was: "how can the speed of light have an unchanging numerical value if the definition of the meter is not an unchanging definition?" Prior to 2019 the meter was not defined in terms of speed of light over time. My point is that, unlike dimensionless constants like pi, constants with units can have arbitrary numerical values depending on the definitions of the units. And if you make a law that changes the definition of a unit, you therefore change the numerical representation of the corresponding constants. In other words, if you defined a meter to be "the distance light travels in 1 second", then the constant c would now be 1 m/s. > I can't tell why you think it's "problematic" for a physical constant to have units Because the definitions of units can change[1]. The definition of the meter changed earlier this year, and thus the speed of light constant, c also changed. Before May 9, 2019, c had an infinite number of digits using SI units. Now it only has 9 digits. Planck's constant, along with a bunch of others changed as well during the units redefinition[1]. [1] https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_ba... Again, I'm not arguing that the speed of light can change, but that those constants are only unchangeable in their symbolic forms. Laws can and do change their numeric representations by proxy of defining units.