Time and space acquire their meaning from measurements,
i.e. identifications of a relationship by means of a unit that
serves as a standard of measurement. The measurement process we shall
focus on is based on the emission, reflection, and reception of radar
pulses generated by a standard *geometrical clock*.

Such a clock consists of a pair of Fourier compatible radar units, say, A and B. Their reflective surfaces form the two ends of a one-dimensional cavity with its evenly spaced spectrum of allowed standing wave modes in between. The operation of the geometrical clock hinges on having an electromagnetic pulse travel back and forth between the reflective ends of the cavity. The back and forth travel rate is determined by the separation between the cavity ends. This rate need not, of course, be constant in relation to the atomic clocks carried at either end. A geometrical clock with mutually receding ends would exemplify such a circumstance.

The definition of a geometrical clock is therefore this: it is a one-dimensional cavity

- whose bounding ends are Fourier compatible and
- which accommodates an electromagnetic pulse bouncing back and forth between the left and right end of the cavity.

along the two straight world lines of radar units A and B in spacetime sector . Here

is the Doppler frequency shift factor and is the fixed comoving separation between A and B. The constant is the Minkowski time when the geometrical clock strikes zero.

For a clock with ends subjected to accelerations and
, the ticking events are located at

along the two hyperbolic world lines of A and B in boost-invariant sector .
Here

is the pseudo-gravitational frequency shift factor between them, and is the boost time between a ``tick'' and a ``tock''.

The spacetime history of such clocks and their bouncing pulses are exhibited in Figures 2 and 3.