representation for the canonical variables used in conventional theory, but its utilization of "state" as defined by its expectation value form, rather than as a vector in a Hilbert space, means that, a priori, the energy of a state is a somewhat vague mathematical object. The problem is thus that of dealing, in a fashion which is effective from a quite broad point of view, with the vacuum for free fields, the ultimate aim being to arrive at a simple characteri- zation which may be adapted with a reasonable degree of confidence to the case of interacting fields.
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I. E. Segal (1962) studied this question.