@james 68 Sorry, just got 'round to noticing this!

I think the issue is that the earth's gravity would attract a "normal", positive mass, but would repel this exotic "negative" mass. (F=GMm/r^2; G=gravitational constant, M=earth's mass, and r=distance from the center of the earth are all positive, both for a "normal" mass and this "exotic" one; but the latter has opposite sign, causing the sign of F to flip.)

So the earth's gravity causes a repelling force on the negative-mass object... which causes it to be attracted. It is, effectively, a double negative cancelling out.

You're right that it's a serious question, in the added sense that the answer is (to both of us, and I'm pretty sure to physicists in general) unclear and may remain that way until an actual experiment is done.

Come to think of it... when I suggested that a positive mass would attract a negative mass, which would in turn repel the positive mass, so that they would accelerate across the universe, I figured it wouldn't help for spacecraft propulsion unless you got (say) moon-sized masses of each. But if you take a spacecraft and add a matching amount of negative mass, so that their net mass is zero, and you push it... does the acceleration (force/mass) tend toward infinity as the mass approaches zero? Inquiring minds _definitely_ want to find out!