But if the beer has fermented, it has alcohol and alcohol is lighter than water. Specific gravity is the ratio of the weight of a given volume of liquid to the same volume of pure water, so you either have an extremely thin beer or a very high ratio of alcohol to dissolved solids like complex sugars and proteins. That's why I asked how it tasted: Some infections eat the complex sugars, reducing the gravity below what you'd normally expect. Extremely high proportions of sugary adjuncts can do this to a beer as well - if you used a lot of honey or cane sugar you'd get SG's lower than normally expected. Some Belgian yeast strains really dry out the beers, particularly in the presence of a lot of simple sugars. So I propose the ultimate test: Taste it and if it tastes good, drink it. It'll be very dry but that may be just the thing!