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.

Time travel is one of the most captivating tropes in science fiction, inspiring countless stories of adventures through the ages. After all, what’s more tantalizing than the possibility of rewriting ...