Hi Marcoen,
As I noted in the essay, back in 1967 Fairbank and Witteborn managed to isolate electrons from external forces without undue difficulty. The problem they faced was the then puzzling absence of much larger forces which had been detected in other experiments. (Later they found that those larger forces are temperature dependent, which explained their absence in the low temperature environment of the original experiment). It was ultimately the failure to obtain funding for a decent positron source that prevented conducting the experiment with positrons. (I actually have an e-mail from Dr. Witteborn that confirms this). Same goes for the planned experiment utilizing antiprotons from LEAR at CERN in the late 1980's; it was not any technological barrier - only funding that failed.
There is a lot of misinformation out there. In fact many people, and even a good number of physicists, believe experiments to determine the gravitational acceleration of antimatter have already been done. Perhaps it's the fact that these experiments were planned and discussed extensively but never carried out as intended. Also there are articles like "The Light Stuff" in the 11/20/04 issue of New Scientist where physicist Paul Wesson wrote: "In a recent series of experiments, most notably at the Stanford Linear Accelerator in California, researchers looked to see if positrons, the antimatter partners of electrons, fell upwards in the Earth's gravitational field. But like balls, people, and all the other that we know about, they fall towards the center of the Earth." No such experiment was ever performed. Articles like this, in magazines and on the internet feed on themselves as people take information from them and write additional articles without investigating the validity of the original source.
My proposal is that there are three types of gravitational mass linked to baryon number which is in turn linked to color charge. Quarks (baryon number +1/3 and color charge; red, green, or blue) have a gravitational mass type 'c' (for color). Antiquarks (baryon number -1/3 and color charge; anti-red, anti-green, or anti-blue) have gravitational mass type 'c-bar' (for anti-color). Everything else (zero baryon number) has gravitational mass type 'w' (for color neutral or white). Our 'world' (leaving out antimatter, dark matter and dark energy), the world of protons, neutrons, electrons, photons, neutrinos, etc. is a world of type c and w gravitational mass with the gravitational interactions:
c c = attractive, c w = attractive, and w w = attractive
I wanted complete symmetry in the 'antiworld', the world of antiprotons, antineutrons, positrons, photons, and antineutrinos such that one could exchange the matter world for the antimatter world with no observable change taking place. Thus the 'antiworld' is a world of type c-bar and w gravitational mass with the gravitational interactions:
c-bar c-bar = attractive, c-bar w = attractive, and w w = attractive
That left c c-bar = repulsive to insure the long range separation of bulk matter and antimatter in a universe where baryon number is strictly conserved. Thus there is a beautiful symmetry in the universe. (Somewhere in a very distant antigalaxy on a small blue planet orbiting a G2 antistar, someone is wondering what happened to all the matter in the universe as physicists search in vain for antiproton decay).
Yes, by (3) under 'New Rules' in my essay, the gravitational interaction between protons and positrons is attractive. As I noted in the essay, under the hypothesis I proposed, positrons would fall just as electrons do in the Earth's gravitational field. So even if the Fairbank/Witteborn experiment had been performed using positrons they would behave as expected under standard gravitational theory. Yes, by (6) under 'New Rules' in my essay, the gravitational interaction between protons and antiprotons is likely to be mostly repulsive. (See my post just above yours, where I address the effect that the presence of gluons within the proton and antiproton has on the gravitational interaction between them). Yes, antiprotons and positrons have different properties. Antiprotons are composite particles made up of three antiquarks, each with a bayon number of -1/3 and an anticolor charge. Positrons, as far as we know, are fundamental particles with a baryon number of zero, and are color neutral (white).
As far as I know the notion of three types of gravitational mass linked to baryon number (and thus to quark color charge) and the resulting six distinct gravitational interactions is unique. Until now I have not seen it discussed or in print anywhere. FQXi wanted something original and I think I have delivered. However, that doesn't mean it is right. Only experiment can determine that.
Best Regards,
Steve