It's easy to over estimate how much supposedly clever people really know about things. I used to have a friend who's a race engine builder, as am I, and to do it right requires a lot of maths, physics and even chemistry is useful in understanding by products of combustion. One night down the pub the topic turned for some reason to that of vacuum. He said there's no such thing as a perfect vacuum which is indeed true in most cases but I responded that the vacuum in deep space is near as dammit perfect barring maybe a few stray atoms per cubic metre.

He looked at me askance and said that space can't possibly be a vacuum. I asked why he thought that.

"Well if space was a vacuum then what would rockets have to push against."

I nearly fell off my bar stool.