The question of why the fifth force has hitherto not been observed is therefore tantamount to the question of why the scalar quantity Q ≡ uαuβ(∂gαβ/∂l) is small. [Clearly uµ
= 0 in general, dl/ds=0 would effectively reduce the metric from 5D to 4D, and the null wave l=l0e±is/L satisfies (12) with no constraint on Q.] Our answer to this is as follows: Q=0 if∂gαβ/ ∂l=0, meaning that there is no intrusion of the fifth dimension into spacetime, irrespective of the physical identification of x4 = l. This says that the 4D weak equivalence principle is a symmetry of the 5D metric.