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The Question

(Submitted May 31, 2000)

I have a question that I could not find in your archive: How does anti matter react to the gravity of normal matter? Are there experiments? (I could imagine anti neutrons in the earth field, but I don't know anything.)

The Answer

There is a popular level summary in the Usenet Physics FAQ:

The one argument that I like is by Schiff: Eotovos' famous experiments have established the equivalence principle, that the ratio of gravitational to inertial mass is identical in different materials. Mass of these "normal" materials include contributions from virtual positrons which are constantly created and destroyed (along with virtual electrons), and the virtual positron contribution is different in different materials (more in higher Z elements). Thus, the Eotovos experiments prove that the virtual positrons react to gravity the same way ordinary matter does.

Well, are there direct experiments using real antimatter? Not yet, at least nothing conclusive, but there may well be one (or more) in the near future. CERN's ATHENA (AnTHydrogEN Apparatus) group hope to perform precision tests of WEP (weak Equivalence Principle) using trapped antihydrogen atoms, and they probably are not the only group pursuing this goal. ATHENA's public information page:

have many links that you may find interesting; in particular, there are several papers in the "Proceedings of Intl. Workshop on antimatter gravity and antihydrogen spectroscopy" with intriguing titles/abstracts.

Best wishes,

Koji Mukai & Bram Boroson
for "Ask an Astrophysicist"

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