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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.