FUTURES NATURE|Vol 464|29 April 2010 Grandfather paradox A question of time. Ian Stewart reworked the theory … and discovered a would resolve the paradox by excising the sign error. His machine could travel only time machine, and snap back to a consist- I didn’t turn round. into the future. By so doing, he had created ent history in which Hubert married Rosie, I knew what was coming. It happened a paradox. If he never got back … who had with all of its consequences. once a year, on his birthday. His choice: married Rosie? “You must help me!” he pleaded. His moral blackmail, perhaps? I sensed the He started skipping ahead a few weeks body trembled, his eyes were wild. His life misty shimmer forming in the corner of at a time, in a frantic search for inspira- now consisted of closely spaced episodes the room behind me, the impossible twists tion. He haunted public libraries, boning in which he begged me for death. It was in directions that didn’t exist, the machin- up on physics and philosophy. As the years a horrible way to live, and we were both ery and its increasingly haggard passenger flicked by, he came to realize that he could becoming desperate. solidifying from thin air. never go back. Destroying the time machine wouldn’t On the sideboard was a faded mono- He watched his son’s christening, then help. Neither would suicide. Chronoclas- chrome picture: a confident young man his marriage. He was waiting outside the tic calculus allowed logic to be suspended and his beautiful new bride. Without turn- hospital when I was born. He developed inside the time machine’s causal loop, until ing, I spoke to the original. the loop closed. The agent of “The answer’s still no, his destruction had to be out- Hubert.” side the loop, and it had to JACEY “I’m not asking a lot,” he be a logical consequence of pleaded. his hypothetical return to his “Only murder,” I said, past. That meant my dad, me fingering the gun in my or my kids … My kids! pocket. This had to stop. We went through the Grandad climbed reluc- usual exchange. “If you tantly into his machine, don’t kill me, you didn’t get hesitated, and pulled the b or n .” start lever. As the machine “I was born.” began to fade, I took the gun “That’s because my causal from my pocket and shot loop is incomplete,” Hubert him. I couldn’t risk aiming said angrily. “You’ve seen at the controls: I would have the analysis. You know only one chance, and he was what won’t happen if you let a bigger target. me live.” I had, and I did, and I’d finally realized that it made no more sense now his calculus was as defec- than it had when The Beatles tive as his machine. Yes, my were recording Sergeant timeline contained a grand- Pepper. father who lived a happy life According to family tradi- with his beloved Rosie — but tion, Grandad had wanted to be it also contained a grandfather an inventor, failed, and ended up run- a ‘chronoclastic calculus’ of space-time in who materialized in a time machine. ning a pub. Actually, one invention had an attempt to rationalize his fragmented Hubert’s time-travelling causal loop was worked. I knew it was true. When a time life. Sitting in the back of the chapel at my logically entangled with mine; if the Uni- machine materializes before your eyes, you father’s funeral, he suddenly understood verse excised him, it would also excise believe. what had to be done. me and my kids. So I trapped Grandad’s Physicists and philosophers always say Time travel violates several conservation corpse in a frozen instant where no time that time travel into the future is straight- laws, but the Universe can borrow energy, passes and logic is suspended. forward. It’s travelling into the past that momentum or matter — provided it repays That faded photograph tells me I am no creates the paradoxes. Grandad discovered the debt when the time machine returns murderer. It lies. Oh, how it lies! that it’s not that simple. to its starting point. Hubert’s dual exist- My grandfather wanted me to kill him, The time machine had been Hubert’s ence broke no laws. So far. But it would if and when no other choice remained … I only success, an ingenious application of he could create a paradox so blatant that did. Hamilton’s quaternions. He flight-tested it it could not be resolved by repaying what And that’s the only reason why either of with a short hop into his own future, find- had been borrowed. This was why he kept us was ever alive. ■ ing that he and his new bride Rosie were begging me to kill him. Ian Stewart is enjoying retirement, which deliriously happy and a baby was on the According to Grandad’s calculus, the resembles his previous life so closely that way. Reassured and proud, he pushed the basis of the Universe is not energy or it is hard to spot the difference. He is now lever to return to the instant of his depar- information, but logic. If I killed him in my emeritus professor and digital media ture … timeline he would never have invented his fellow at the University of Warwick, UK. Nothing happened. machine — so I wouldn’t be able to kill him. Join the discussion of Futures in Nature at go.nature.com/QMAm2a FUTURES The machinery checked out, so he With its logical basis wrecked, the Universe 1398 © 2010 Macmillan Publishers Limited. All rights reserved.