Where would a negative reading be on a hydrometer that is measuring specific gravity? If the gravity of the must starts at say 1.090 (+/- a gallon of water with a little more than 2 lbs of sugar dissolved in it) and the yeast works on the dissolved sugar and so the gravity drops until the wine has about the same density as water (1.000 - still SOME sugar in it, but a fair amount of alcohol) and it continues to drop until there is no trace of fermentable sugar left and the density of that liquor is now less than water because alcohol is not as dense as water and so the gravity reading might be 0.996. Point nine nine six is not zero and zero will be less dense than 100 percent pure alcohol ( 1= water, .996 is a typically dry wine). A gravity reading (specific gravity as a unit) of MINUS 1.030 is... is ... well it's beyond my imagination... An hydrometer measuring density is simply not designed to measure that lack of density. I don't know if the vacuum on the moon would have that kind of "density".