The "time travel paradox", also known as the "grandfather paradox", involves a person traveling back in time and killing their grandfather before he meets their grandmother, preventing their own birth. Therefore, the traveler never existed and could not have made the trip. This implies that both the father and the traveler can exist, so the traveler could go back and kill the grandfather, and so on, endlessly. This paradox is often used to deny the possibility of time travel, but many believe it only restricts what can be done on a trip to the past.
Time travel and space travel are important elements in many science fiction novels and films. But while space travel has become routine for many years, time travel has not yet left the realm of fantasy. Our knowledge of physical laws contains no impediment to time travel.
We won't prove it here, but a trip to the past does not contradict fundamental physical laws such as the conservation of energy or the second law of thermodynamics. That is why the fact that we don't have a device to do it doesn't prevent scientists, philosophers, and enthusiasts from trying to understand the consequences these trips could have on the consistency of our universe.
One of the most interesting consequences of a trip to the past is the so-called "grandfather paradox". Imagine you have a time machine - surely a brand ACME time machine - that can take you to the past. You get in, press some buttons, and appear in the era when your grandfather was still a young boy of 12 or 13, who - of course - had not yet married your grandmother, let alone conceived your father.
You find him and end up killing him. This means your father and, therefore, you could never have been conceived, so your trip to the past (including the murder) never happened. But if your grandfather wasn't killed, then you can exist and make the trip to kill him. This "circular reasoning" can continue as long as you want, over and over. This paradox is believed to have been first expressed by French science fiction writer René Barjavel in his novel "The Imprudent Traveler" (Le voyageur imprudent) from 1943.
The paradox arises because, as far as we know, the current state of the universe is determined by its previous states. This implies that by changing one of these past states, the change propagates forward in time, affecting the current state. One way to avoid paradoxes would be for the traveler to be content with only being an observer of the past, without trying to change it.
This imposes restrictions on what the time traveler can do, but it does not exclude the possibility of the trip. However, some believe that the hypothetical traveler couldn't even observe the past, since one of the principles governing quantum physics - the uncertainty principle, formulated by Heisenberg - implies that the mere observation of a particle modifies its state. By observing the past, the traveler would set off a series of "cascade" effects that would propagate at the speed of light and end up altering the past and everything that follows.
Solutions to the paradox
One possible solution to the grandfather paradox is the existence of parallel universes. When you get on the ACME machine and travel back in time to kill your grandfather, you move to a parallel universe in which you will never be conceived. However, you would still exist in your original universe. Science fiction has used this resource a lot. Alfred Bester, for example, did it in his novel "The Men Who Murdered Mohammed" and Michael Crichton has used a similar argument in "Timeline". However, this is a tricky solution, since instead of traveling in time you are "jumping" from one universe to another.
Another possibility is that the universe lacks an absolute timeline that must remain unchanged from the beginning of time. Instead, each particle (and therefore everything contained in the universe, including humans) would have its own timeline. If all the particles that make up the traveler go back in time, that person could kill their own grandfather and a new timeline would develop.
This theory is similar to the theory of parallel universes, although everything occurs in a single universe. Lately, many scientists have begun to believe that this is really how the universe works, especially those who claim that all possible quantum states exist simultaneously and that, when examining them (and collapsing the associated wave function), a universe is chosen from the possible ones. In some way, Schrödinger's cat is alive in one universe and dead in another.
It is also possible that traveling to the past creates a new future, so the traveler would not modify the past, but his future. This may sound familiar, since it is the main plot of the movie "Back to the Future II". When future Biff Tannen meets Marty McFly in 2015, he gets a summary of sports results from the last fifty years. Tannen learns of the existence of a time machine, steals McFly's sports almanac, and travels to the past to give it to his younger self. When McFly returns to his time, he discovers that everything has changed and a millionaire Biff Tannen has married his mother. Homer Simpson has also suffered it when, in the segment "Time and Punishment" from the episode "Treehouse of Horror V", he creates (accidentally, of course) a time machine that affects the present with each trip.
Other animated series have frequently resorted to the grandfather paradox. One of the most memorable is the Futurama episode in which Phillip J. Fry travels to the past. There he meets his grandfather and his then-girlfriend and, despite doing everything possible to prevent anything bad from happening, his grandfather dies in an accident. Fry sees that he still exists, so he assumes that his grandfather's girlfriend was not really his grandmother and sleeps with her. Finally, it is discovered that she was indeed his grandmother, and the only reason he is still alive is because he has become his own grandfather.
Many scientists are not convinced by the parallel universes hypothesis. Stephen Hawking doubted that time travel would ever be possible. He based his reasoning on the fact that if it were ever possible, travelers from the future would surely be among us, but who has ever seen a tourist from the future? Hawking has also contributed to the question with the "chronological protection" conjecture. According to this hypothesis, even if time travel were possible, any change in the past would have no effect on the present, because we are bound to a single universe, with a single history. According to the scientist, accepting this hypothesis would avoid falling into the grandfather paradox.
It is difficult to understand how time travel could be physically possible and yet lead to unsolvable paradoxes. Physicists are not sure how this is resolved, but they all trust not to have to go through the traumatic experience of Phillip J. Fry and end up sleeping with their grandmothers. And you, what do you think?
