Direct Observation Limits on Antimatter Gravitation

TL;DR

This paper discusses the potential for direct measurement of antimatter's gravitational behavior, highlighting the lack of direct evidence and questioning indirect assumptions that suggest no difference at the 1% level.

Contribution

It clarifies the current state of evidence regarding antimatter gravity and emphasizes the need for direct experimental tests to confirm or refute potential differences.

Findings

No direct measurements of antimatter gravity exist.

Indirect evidence does not conclusively rule out a 1% difference.

Current models and assumptions are insufficient to dismiss a gravitational asymmetry.

Abstract

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.