However, when a physicist says "energy is conserved" (to use the same example as my last posting, but in a different context) we only "know" this to be a fact within the accuracy of our measurements.

But momentum is conserved, because those frictional forces are all internal.

Again, we have to assume that momentum is conserved.

The velocity of the center of mass has not changed as seen from your frame of reference, because the velocity of center of mass is always the same, remember, because momentum is conserved.

The wavelengths of that light are our key to assessing conservation.