Time Travel in Einstein's Universe

Time travel in Newton's universe was inconceivable, but in Einstein's universe it has become a possibility. Now, a Princeton astrophysicist gives readers a guided tour of the potential of traveling through time. Line drawings.

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Back to the Future and the Grandmother Paradox

But what if, as an older man, the time traveler refuses to say hello and instead simply kills his younger self? Time travel to the past suggests such a paradox. When I do television interviews about time travel, the first question I am always asked is this: "what if you went back in time and killed your grandmother before she gave birth to your mother?" The problem is obvious: if you kill your grandmother, then your mother would have never been born, and you would never have been born; if you were never born, you could never go back in time, and so you could not kill your grandmother. This conundrum, known as the Grandmother Paradox, is often thought sufficiently potent to rule out time travel to the past.

A famous example from science-fiction stories that have explored this idea is the 1985 movie Back to the Future. The hero, played by Michael J. Fox, goes back in time to 1955 and accidentally interferes with the courtship of his parents. This creates a problem: if his parents don't fall in love, he will never be born, so his own existence is imperiled. He realizes he must act to ensure that his parents fall in love. Things don't go well at first -his mother begins to fall in love with him, the mysterious stranger, instead of his father. (Freud, take note.) To bring his parents together, he hatches an elaborate plan. He realizes it is failing when the images of himself and his brother and sister vanish from the family picture he carries in his wallet-a bad sign. Later he sees his own hand fading away. He can look right through it. His disappearing. He begins to feel faint. Because he has interrupted his parents' romance, he is slipping out of existence. Later, when his plan finally succeeds and his parents are united, he suddenly feels better and his hand returns to normal. He looks in his wallet; the pictures of himself and his brother and sister have reappeared.

A hand can fade in a fictional story, but in the physical realm, atoms just don't dematerialize that way. Besides, according to the parameters of the story, the boy is dematerializing because, as a time traveler, he prevented his parents from falling in love, thereby circumventing his own birth. But if he was never born, his entire world line, from the point of his birth to his adventures as a time traveler, should vanish, leaving no one to interfere with his parents-so his birth would have happened after all. Clearly, this fictional story has not resolved the Grandmother Paradox. Physically possible solutions to such time-travel paradoxes exist, but physicists are divided on which of the two approaches is correct.

TIMESCAPE AND THE MANY-WORLDS THEORY

First, the radical alternative. It involves quantum mechanics, that field of physics developed in the early twentieth century to explain the behavior of atoms and molecules. ---

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