Mike Marcum didn’t have a lab, funding, or a PhD. Just a pile of salvaged electronics — and a theory. One night in Missouri, he fired up his homemade device. It formed a vortex. He tossed in a screw.

This “Back to the Future” superfan’s hand-built DeLorean time machine may look exactly like the one from the movie, but it still does need roads. And it recently drove down one that’s considered among ...