Re: In a galaxy composed of anti-matter, would gravity be reversed?

Hello, Albert -

        No, as far as we know, antimatter is affected by gravity in the
same way as matter. This is inherent in the equations that describe
particles and antiparticles (there is only one value for the mass, and it's
a positive number), and also in the equations of general relativity. There
was an experiment done to test this, but it was inconclusive (these
experiments are *very* hard, as the only antimatter we have created in the
laboratory is single particles, and the gravitational effects are tiny).
There is no reason to think that the equations are wrong, and a lot of
reasons to thing that they are right (since they work so well at predicting
so many phenomena), but it would be nice to test this particular prediction
directly.

        Here are some Web pages about antimatter:

http://phyhp.phy.ncku.edu.tw/chinese/uw_physics/grav_antimatter.html

Does Antimatter Fall Up or Down?, from the Physics FAQ.

http://pdg.lbl.gov/cpep/antimatter.html

The antimatter page from the Particle Data Group's particle physics
tutorial. If you are interested in particle physics at all, this whole
tutorial is excellent!

http://image.gsfc.nasa.gov/poetry/ask/a11025.html

Does Antimatter Produce Antigravity?, from Ask the Space Scientist.

        It is in any case not likely that antimatter galaxies exist;
galaxies are not all that far apart from each other, and collide frequently
(over their life cycle). If a matter and antimatter galaxy were to collide,
we would see annihilation radiation, which has not been observed. For more
information, see the Cosmology FAQ:

http://donald.phast.umass.edu/~linder/faq.html