Larry Niven

Mike was a recent but ardent science-fiction fan. “I want to change it, Dr. Stuart,” he said earnestly. “I want to go back to four weeks ago and take away Tony’s Flexy.” He meant it, of course.

There were timelines branching and branching, a mega-universe of universes, millions more every minute. Billions? Trillions? Trimble didn’t understand the theory, though God knows he’d tried. The universe split every time someone made a decision. Split, so that every decision every made could go both ways. Every choice ever made by every man, woman and child on Earth was reversed in the universe next door.

Did nobody ever finish one of these, ah, time machines?

What if these German spies see that Astounding has suddenly stopped publishing anything about atomic bombs? What would they do? They’d think we were hiding something.