I am not aware of any approaches that even remotely could deliver a perpetual motion engine or a reactionless drive. You need something to start from but there is nothing in sight.

We have lots of things that didn't seem possible. The first barrier isn't finding the previous art. You are not even supposed to talk about it.

How strange was nuclear before innital discovery?

I'm not aware of [other] approaches that will allow us to travel to the stars.