#### BELIEVE ME NOT! **-** **-** A SKEPTICs GUIDE

** Next:** A Rotational Analogy
** Up:** The Special Theory of Relativity
** Previous:** Relativistic Travel

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

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
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.

*Jess H. Brewer *

1999-03-19