  The proposed Antihydrogen Gravity experiment at Fermilab (P981) will directly
measure the gravitational attraction ("gbar") between antihydrogen and the
Earth, with an accuracy of 1% or better. The following key question has been
asked by the PAC:
  Is a possible 1% difference between gbar and g already ruled out by other
evidence?
  This memo presents the key points of existing evidence, to answer whether
such a difference is ruled out (a) on the basis of direct observational
evidence; and/or (b) on the basis of indirect evidence, combined with reasoning
based on strongly held theoretical assumptions. The bottom line is that there
are no direct observations or measurements of gravitational asymmetry which
address the antimatter sector. There is evidence which by indirect reasoning
can be taken to rule out such a difference, but the analysis needed to draw
that conclusion rests on models and assumptions which are in question for other
reasons and are thus worth testing. There is no compelling evidence or
theoretical reason to rule out such a difference at the 1% level.
