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The grandfather paradox

Updated on February 10, 2015

Any lover of the “Star Trek” television series and films has heard of the problems inherent in time travel. Captain Kirk is concerned that a famous person is about to be run over by a carriage. Spock explains that the person is known to survive the accident and go on to write his famous treatise, so leave well enough alone. A Vulcan recalls the story of her grandmother crash-landing on Earth, selling the design for Velcro to a manufacturer to raise money for an Earth boy to go to college, then being rescued. The stories abound in science fiction, perhaps culminating in Robert Heinlein’s short story "All You Zombies." In it, a woman becomes her own mother, her own lover and her own abductor. In the final episode of “Star Trek: New Generation” the crew has to experience a rash of different time lines to find the right choice to prevent the end of Earth.

The basis of most conversations and creations related to time travel is the grandfather paradox, first appearing in 1943 in Rene Barjaval’s book Future Times Three. He proposes that if you build a time travel machine and travel back to your grandfather’s time and kill your grandfather before he marries, you commit suicide – you will not exist. But if you don’t exist, how can you build a time machine to begin with?

Does the baby exist? Is this man cradling his own grandfather?
Does the baby exist? Is this man cradling his own grandfather?

Since time travel is not yet achieved (we think), we have only theory to fall back on. One theory is that, because of the grandfather paradox, time travel is impossible. Or we could travel in time, but not change anything. If we tried to change anything, accidents or a change of events would cause the event we are trying to prevent to occur anyway. This assumes that small changes would occur in history, but the Big Picture would not change, as time resolves itself.

For instance, a time traveler goes back to 1944, infiltrates Hitler’s generals and convinces them to assassinate Hitler by blowing up his bunker. History prevails – Hitler is not in the bunker when the place is blown to smithereens.

You kill your grandfather, but he was not a good boy – your grandmother is already pregnant with your father.

While writers with great imaginations play with all the possibilities, physicists have actually gotten into the fray.Einstein’s theory of relativity suggests that there are closed timelike curves (CTCs), paths in spacetime making it possible to travel back in time. Presenting time as a fourth dimension, it has already been demonstrated with atomic clocks that time travels at a much slower rate in space as it does on earth.

A global team of physicists led by Seth Lloyd from MIT are actually performing experiments to prove the existence of these CTCs; they have arrived at a theory that time travel is possible as long as a paradox does not occur, much like teleportation through time instead of space. One could only travel back to one’s own time line “universe”. David Deutsch. On the other hand, theorized that time travel is only possible when traveling to a different past than the one the traveler remembers.

Quantum physicists suggest that at every point in time where we can make a choice, several paths exist, in parallel universes. Thus if we traveled back and changed a decision we would simply find ourselves in one of those parallel worlds. They even suggest that we can manipulate our futures by opening up these alternate choices.

According to Steven Gibbs, the paradox doesn’t exist. If you try to change the time line, all you do is split time and travel a new line, as in splitting a river. But like the river, the more you split it, the shallower the water, until there is no more and you stop. And it doesn’t explain how you could exist even in a different path if you killed your grandfather.

Some theorize that one could change events in the past unless those events are within the grandfather paradox. In any case, since the future would be changed, one’s memory would be changed as well, so the time traveler would not be aware that he caused any rift in history. Then, did he actually cause a change? Was Spock simply spouting his new-found memory of history?

This brings up the idea of free will. We believe that we choose the lives we lead. But are these lives actually being manipulated by another human being from the future? And if we can travel to the past, can we also travel to the future? Would knowledge of the future change our method of coping with the present? Or would knowledge of the future guide us as to what we need to do in the present?

A host of new questions arises if we assume that man could travel through time. Perhaps, as many scientists believe, it simply isn’t possible, or all we could do is observe what we already know. That would at least allow some of us to sleep at night. But it isn’t half the fun as science fiction authors can offer us in their brilliant hypotheses.

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