Without physical things to measure, there would be no math. On the other hand, physical things have to behave logically, which is mathematical.

Pi is the circumference-to-diameter ratio of a circle. That's the simplest definition. The value of pi can change: if a disk is spinning, then the outer edge has a relative velocity. Special relativity states that the circumference of the disk will contract but the radius won't. So the value of pi will decrease!

But that's the physical definition of pi.... there is a mathematical definition that might not be subject to relativity (but I'm not sure):

Pi can also be defined by the formula Pi = 4*arctan(1)

This definition of Pi is based on angular separation of points on the circumference of a circle. Whether the disk is spinning or not, Pi should always have the same value!

Now I've confused myself. Ha! How 'bout that!