ABSTRACT
With the discovery of the quantum theory (describing the very small) and the special theory of relativity (describing the very fast), it was natural to seek a theory that described very fast subatomic particles. Such a theory had to combine STR and the quantum theory, resulting in an equation that would replace the Schro¨dinger equation. Early attempts yielded results, which, although deemed accurate later on, were considered nonsensical at the time. For example, one formulation appeared to yield negative probability.1 The negative sign of the probability can be traced to Equation (28.7), which contains the square of the energy, making it possible for the energy to be negative (keeping its square positive). This negative energy led to what was interpreted as negative probability.
