So, it looks like I'm not likely to get an answer to the https://www.physicsforums.com/showthread.php?t=345179", but I figured whilst I was here I'd ask something that's had me curious for years. Buoyancy causes things to float/fly when the average density of a body is less than that of the density is travelling through, yes? A zeppelin flies (well okay, not so many of them anymore) because the helium or hydrogen inside it is less dense than the air outside. The least dense thing I can think of is a vacuum. So, if a container could be made that was sufficiently strong so that it wouldn't collapse under the air pressure, and sufficiently large (and light) that the volume of the vacuum it contains is enough to reduce the average densite of the whole body, would it fly? Is there a physical reason why this wouldn't work, or is the reason we don't have vacuum powered airships due to difficulties with engineering - inadequate materials or prohibitive cost?