# Opening

Taylor and Wheeler begin their excellent book, Spacetime Physics with a parable of considerable significance:

Once upon a time there was a Daytime surveyor who measured off the king's lands. He took his directions of north and east from a magnetic compass needle. Eastward directions from the center of the town square he measured in meters. . . . Northward directions were sacred and were measured in a different unit, in miles. . . . His records were complete and accurate and were often consulted by the Daytimers.

Nighttimers used the services of another surveyor. His north and east direction were based on the North Star. He too measured distances from the center of the town square in meters. . . and sacred distances north in miles. . . His records were complete and accurate. Every corner of a plot appeared in his book with its two coordinates, . . .

Our task, in Relativity, is to reconcile the observations of two observers -- not unlike the surveyors in this fable. But in our case the two observers are moving relative to each other. In this chapter we shall also restrict our attention to the special case of two observers (and their reference frames) moving relative to each other along a straight line with constant velocity. This is the Special Theory of Relativity. It was developed by Albert Einstein in 1905.

What is the point, then, of this chapter? If we are able to measure with enough accuracy or if the speeds involved are great enough, we find that the clear, straightforward, obvious relationships we might expect simply do not give the right answers! But they do work quite well for ordinary, everyday speeds (like a car at 100 km/hr or even a spacecraft at 30,000 km/hr).

Until now -- especially in mechanics but even in optics and electromagnetism -- you have already had some experience with the ideas discussed. You have seen a baseball move along a parabolic trajectory so when we derived that you were not surprised. You have a common-sense, "gut feeling", or intuitive picture of how a car behaves as it rounds a corner at 100 km/hr or as an airplane banks at 500 km/hr. But none of us have similar personal, first-hand experiences with objects traveling close to the speed of light. The results of Special Relativity will be new to us -- and quite different from our ordinary, intuitive expectations. This is disconcerting to some people and exhilarating to some. Let it be exhilarating. Come join Alice on another trip down a rabbit hole where you will find interesting and intriguing things you would never have expected!