I am taking a class on relativity this summer. We are using the book Six Ideas That Shaped Physics. In chapter 2 the concept of SR Units is discussed. The light-second is chosen for our measurements of distance. The author sates on page 28.
"In SR unites, the light-second is considered to be equivalent to the second of time, and both units are simply referred to as seconds. This means that these units can be canceled if one appears in the numerator and the other in the denominator. For example, in SI units, velocity has units of meters per second; but in SR units it has units of seconds per second = unitless(!)"
The consequence of this is that the SR units for energy, momentum and mass all have units of Kg. Now that is interesting. However something doesn't feel right here and I am looking for a few of you to guide my thinking. I understand that a light-second can be used as a unit of distance. And I guess you can call it a second (for short) as the author says elsewhere if you want to. But calling it a second and having it be equivalent to a second are two different things. How can a light-second in the denominator cancel a second in the numerator? They are not the same thing!
Cliff Parker
Never express yourself more clearly than you can think. Niels Bohr