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Natural Units

As I mentioned in the Chapter on UNITS AND DIMENSIONS, in any context where the speed of travel is virtually (or, in this case, exactly) a constant, people automatically begin to express distances in time units. [Q: "How far is is from New York to Boston?" A: "Oh, about three hours."] This is equivalent to defining the speed of travel to be a dimensionless constant of magnitude 1. Relativistic Physics is no different. Anyone who has to discuss relativistic phenomena at any length will usually slip into " NATURAL UNITS" where

$$c \; = \; 1$$

and distance and time are measured in the same units. You get to pick your favourite unit - seconds, meters, light years or (as we shall see later) inverse masses! The list is endless. Then  $\beta$  is just "the velocity" measured in natural units and the calculations become much simpler. But you have to convert all your other units accordingly, and this can be interesting. It does take a little getting used to, but the exercise is illuminating.