  We point out that the size of the photon single spin asymmetry in
high--energy proton proton collisions with one transversely polarized proton
can be related to $d^{(2)}$, the twist three contribution to the second moment
of $g_2$. Both quantities should be measured in the near future. The first was
analysed by Qiu and Sterman, the second was estimated by Balitsky, Braun, and
Kolesnichenko. Both experiments measure effectively the strength of the
collective gluon field in the nucleon oriented relative to the nucleon spin.
The sum rule results suggest that the single spin asymmetry is rather small for
the proton, but could be substantial for the neutron.
