content: Currently it's totally impractical, so it's more important to understand the physics, and if we figure that out, then maybe something useful can come of it.  Right now it's about as useful as using  sound waves to move furniture.  When we understand it, it may end up not serving as propulsion at all, but something else.  When we discovered fire, it would be hard to comprehend how it would be used in the internal combustion engines, scramjet engine, etc.