>Sorry to but in, Dave, but isn't a picometer per picosecond the same as a meter per second?

Mike

10 pm/psec = 0.01 nm/nsec = 0.00001 um/usec = 0.00000001 mm/msec = 0.00000000001 m/sec. So saying pm/ps is just a way of getting rid of some zeros.

As another example measuring the distance from Austin TX to Cambridge MA (1995.94 miles according to Mapquest) in km accurate to 15 significant digits would need to be expressed as 3212.xxxxxxxxxxx km where the last 2 Xs are nanometers so that's accurate to 10 nanometers. Which is clearly impossible, it's on the order of a million times more accurate than a distance that long can be measured.