The Instant of Now by Irving E. Cox

Luckily old Dr. Kramer had asked no awkward questions when Eddie excused himself from the balance of the lecture. If the kindly bumbling professor had been inquisitive, Eddie had no idea how he would have answered. Glenna was his fiancée, Hurd his best friend—and their disaster meant disaster for the underground movement that had become the guiding purpose of his entire life.