### 8.2 Restriction on the velocity

Note that Equation (37) is, disregarding the difference in notation, identical to the first equation in
(35). However, in the present setting, there is a restriction on , which did not appear in the polarized
case. The essential part of this restriction is the inequality ; the sign of is
not of central importance. The restriction is due to the fact that there is a potential
inconsistency with Equation (16) if ; assuming the derivatives of and with respect to
time and space to tend to zero, all the terms in Equation (16) tend to zero except, possibly,
, which is roughly . In fact, if and , the
expansions are clearly inconsistent. Furthermore, considering Equation (16), the term
would in that case seem to have the effect of causing to decrease. For a further discussion,
see [10].