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Hi, my friend was concerned about how constants are defined. He was thinking that everything is defined circularly such as

the speed of light defined by 3.8 x 10^8 m/s but the meter is defined by some distance light can travel in some amount of time, the Planck length is defined by how far light can travel in one unit of Planck time, how do you define Planck time, etc. Is the speed of light the source for all these measurements? I realize at some level measurement must be arbitrary but he was concerned that it was circular in nature. Thanks.

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One meter = 39", which even though a known measurement, is just another "arbitrary" unit of measurement, But constants, are set in stone. Pi is a constant, we just cant figure it out to the exact number (22/3).
Constants, by definition are unchanging. An Inch is an Inch, a Meter is a Meter, you could destroy every ruler in the world, and we could still measure it with a programmed laser.
Planck Time, takes into account different variables, gravity, etc.
So where the speed of light is constant (mostly), except when you consider the Planck effect.
If you look at it one way, it is circular, But since we know how far a meter is, that is a constant, and that's what makes the whole thing magically work together.

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