Stephen Cass, an editor at Discover magazine, was determined to investigate some of the invisible things that are part of his daily life. As he walked toward his office in New York City, he mused: “If I could see radio waves, the top of the Empire State Building [with its host of radio and TV antennas] would be lit like a kaleidoscopic flare, illuminating the entire city.” He realized he was surrounded by an invisible electromagnetic bedlam of radio and TV signals, Wi-Fi, and more.

Elisha’s servant learned about another kind of unseen reality—the invisible spiritual world—one morning. He awoke to find…