We identify physical properties with constant values and no apparent deviation through observations and measurements.
The speed of light in a vacuum is a crucial example. It is extremely important for observation as it allows us to see the world around us with precision, not only in our immediate environment but also over large distances. The velocity of light is such an absolute physical constant.
There is also scientific consensus that the gravitational constant G, the Planck constant h, the electric constant ε0, and the elementary charge e are absolute physical constants. Regular science assumes that these physical constants are constant in space and time. The invariance of the speed of light has been proven over time, dating back to the Big Bang. For the other constants, there is a high level of confidence that they are also constant over extremely long periods of time, but these conclusions are based on secondary observations.
In particle physics, it is complicated to measure some of these physical constants, due to the small scale of these particles. The measuring method itself has a measuring tolerance, but in general, there is consensus on most of the important physical constants as highlighted in the Standard Model.
There is no reasonable doubt among scientists about matters like the “invariant mass” of an electron and a proton.