Measurements of the positions of events must be made with respect to an origin and axes (i.e. a coordinate system), which we shall call a reference frame. A typical Cartesian coordinate system is shown in Fig. 1.1. The position of an event, taking place at the time t, can be specified by the coordinates x, y and z. Suitable units must be chosen for measuring length, mass and time. Until we reach Section 4.3 in Chapter 4, we shall find it convenient to define the metre independently of the speed of light, and we shall adopt the 1960 definitions of length, mass and time, which are fully consistent with Newtonian mechanics. They are as follows:

2the metre (m) is equal to 1 650 763.73 wavelengths of the orange-red line of the krypton-86 atom; A Cartesian coordinate system is used to plot the path of an accelerating particle. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780203983805/89768c00-4d8a-484e-b69a-f8a52bfbf6b8/content/fig1_1.tif"/>

the kilogram (kg) is defined as the mass of a cylinder of platinum-iridium kept at Sèvres, near Paris;

the second (s) is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.