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.