In this chapter we are going to discuss some aspects touching upon the discretization of time in the context of Einstein's theories of special relativity (SR) and general relativity (GR).

Snyder's quantized spacetime

At first sight SR seems the wrong theory for temporal discretization. In Newtonian mechanics, time is absolute, which means that simultaneity is absolute: it is the same for all inertial observers. Therefore, if absolute time is discretized for one inertial observer, it is discretized in exactly the same way for all other inertial observers. In that respect, Newtonian discrete time mechanics does not require a preferred inertial frame. Simultaneity is not absolute in SR, however, which suggests that discretizing time will break Lorentz covariance: the use of a preferred inertial frame in which to temporally discretize seems inevitable.

This conclusion is based on classical thinking. The example of quantized angular momentum demonstrates that it is possible to reconcile having a continuous parameter space with a discrete spectrum of observable values. Suppose that an observer is conducting a Stern–Gerlach (SG) experiment, firing a beam of electrons through an apparatus containing a strong inhomogeneous magnetic field (Gerlach and Stern, 1922a). In such an experiment, the observer will first fix the orientation of the apparatus relative to the source. Then it will be found that an electron passing through the apparatus will be detected finally in only one of two possible angular momentum states.