By Edward M Lerner.
"The lecture hall was pleasantly warm. Behind Prof. Thaddeus Fitch, busily writing on the chalkboard, pencils scratched earnestly in spiral notebooks, fluorescent lights hummed, feet shuffled. A Beach Boys tune wafted in through open windows from the quad. “And so,” he continued, “travel backwards in time would violate causality, and hence appears to be impossible.” He turned to face the class. “The problem is most commonly illustrated with the ‘Grandfather Paradox.’"