You, Grandpa, and Time Travel Paradoxes
Tuesday, November 11, 2008 - Ran Levi
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understand quantum theory?
Currently, time travel research is performed only by theoretical physicists. We therefore cannot honestly expect a real functional time machine any time soon – after all, these are theoretical physicists. Even if they had the ability to invent a time machine, they would have probably crashed it while trying to get it out of its parking space.
That being the case, in this column I will talk about paradoxes of time travel. In other words, had a time machine been possible and available here and now, what problems and strange paradoxes would arise.
What is a paradox? A paradox is a contradiction. In a paradox, we start with a basic postulation and by following a series of logical steps we end up with a conclusion that contradicts our basic postulation.
(Credit: Weber State University)
This paradox is solved with the use of a mathematical tool called "Limit,” which basically puts a finite value on how far the tortoise can go in a given time. This solution merely gives a mathematical description of some facts of life that we already know are true. But time travel paradoxes are different: we do not have any experience with time travel; hence we can not easily solve its paradoxes.
The idea of time travel is especially enticing for science fiction writers. Time travel allows the writer to begin the story from a familiar starting point that the reader can identify with – for example, a bored high-tech worker that spends his days in front of a computer monitor – and then transfer the reader to another, much more exciting reality, thousands of years into the future or millions of years backwards to the dinosaur era. Time travel also allows the writer to speculate about possible future technological advances and other topics that science fiction likes to deal with.
But good ideas do not come free of charge. Time travel trails many paradoxes and logical problems that the writer or screenwriter must deal with and solve, or hope that the special effects department does an exceptionally good job and then no one would notice the problem.
One famous time travel paradox is the “grandfather paradox.” The name “grandfather paradox” was given to this problem because the general idea is as follows: the time traveler goes back to the past and shoots his young grandfather. Since the grandfather is dead, the father of the time traveler was never born, meaning that the time traveler himself doesn’t exist. But the traveler exists, otherwise who shot the grandfather? That is the paradox. Interestingly, almost all the examples of time travel paradoxes include killing a family member. Sometimes you travel to the past to stab your grandmother, sometimes you run over mom; some people will even take the trouble to kill their young selves. When reading about time travel paradoxes, it seems like no one goes back to the past to enjoy a good meal in a restaurant that already closed – everybody tries to settle a debt with their grandfather for not giving them enough Christmas money.
The solution of the screenwriters of “Back to the Future” was to transform the deterioration of the family into a gradual process: as time passes and Marty cannot solve the problem, he and his brother and sister begin to slowly disappear from the family picture, as though they never existed. During this time, Marty tries to fix the mistake and make his parents fall in love with each other.
This sounds like a good solution, but only if you disregard the chaos theory. According to the chaos theory, in the long term, small and seemingly insignificant actions might have dramatic effects on the world. If the screenwriters had taken the chaos theory into consideration and integrate it into the script, they would have been in big trouble since every action of Marty in the past could dramatically change the future. If, for example, Marty had scared a small butterfly sitting on a flower, the flapping of the butterfly’s wings could eventually cause a hurricane that would have completely destroyed the town. What we get is a “grandfather paradox” multiplied several fold.
The screenwriters of “Back to the Future” decided to gracefully ignore the chaos theory, but scientists did try to deal with this paradox. One possible solution is the “many-worlds interpretation.” According to this theory, when a time traveler goes back to the past and changes something – be it a dramatic or a tiny change – the universe splits at that moment into two parallel universes.
One universe is the original one, where nothing has changed and everything remained the same. If we continue the previous example, this would be the universe where Marty couldn’t save his father from the car accident. The critical point for understanding this solution is that the time traveler always remains in the second universe, the universe where the change happens. In this universe, he really was never born, but there is no paradox since Marty comes from a parallel universe, where the accident did take place, the parents fell in love, and everything continued as normal.
Some people think that this solution, splitting into two parallel universes, voids the meaning of time travel because the traveler cannot really affect “his” universe, the universe from which he came. If, for example, the time traveler went back in time in order to kill Hitler and prevent the Second World War, he cannot truly prevent the war. There will be two parallel universes, one in which the war, holocaust, and suffering remain exactly as they were – and a parallel universe where none of it happens.
Another paradox that might arise from time travel is the “causality paradox.” Common sense tells us that everything has a reason. If every night I open the refrigerator and devour an entire box of ice cream and later at work people tell me I’ve gained weight, obviously there is a reason. I have big bones. Genetics.
In the sequel, we find that Skynet was built by engineers through analysis and reverse-engineering of the remains of the robot (Schwarzenegger) from the first movie. If Skynet sent the robot from the future and through the knowledge that was taken from this robot the engineers could design Skynet, then where did this knowledge come to the world? According to logic, somebody had to invent the engineering knowledge that brought to create Skynet and the robot, but the paradox forces us to believe that this knowledge was there all along, and nobody invented it.
There is another paradox that is derived from the causality paradox. Let’s suppose I went back to the past, searched for young Ran Levi, gave him an ice cream spoon and told him “Here, at least eat the ice cream like a decent human being and not with your hands.” Ran keeps the ice cream spoon all these years, then goes back to the past to give the spoon to young Ran. Beyond the question of “Where did the spoon originally come from?” we must ask ourselves, “What will happen to the spoon in the future?” In each of these cycles, the spoon becomes older and damaged. Another cycle goes by and another one, and the spoon rusts, becomes crooked – and eventually I have to go back to eating the ice cream with my hands. If this is the case then this loop cannot last forever, even though it seems there was never a starting point and it always existed.
cosmic censorship hypothesis
(Credit: Brookhaven Lab/
According to this hypothesis, if time travel is even possible, then it can only be performed in very specific places in the universe, called “singular points” – like at the heart of black holes. A singular point is a place where the laws of physics are not valid and, perhaps, there is no meaning to causality and what happened first. The only problem is that matter that is “swallowed” by a black hole, for example, can never get out. Therefore if a scientist succeeded in going back to the past inside a black hole, he can never get out to tell the tale. This is cosmic censorship and currently it is considered to be only a hypothesis (although some have tried to find actual proof).
Let’s return to “Back to the Future” and Marty McFly. In one of the scenes from the first movie Marty goes up on stage and plays the song “Johnny B. Goode” by Chuck Berry. In the second movie we see Marty, the one who went back to the past at the end of the first movie, climbing the scaffolds above the stage looking at Marty from the first movie playing the song. Therefore, at the same time, in the same universe, there are two identical copies of Marty. This is strange; it might even raise the production cost of the movie, but it is still not the paradox.
The real problem begins when we realize that in principal, we can create an infinite number of Marty McFlys by this method. For example, if every time future Marty goes back to the past a few seconds earlier than the previous Marty, theoretically, the universe could be filled with identical clones of Marty McFly until there is no space for anything else.
By the way, the screenwriters managed to squeeze into this single scene, in which Marty plays on stage, two other paradoxes simultaneously. At the same time Marty plays “Johnny B. Goode,” one of the band members phones his uncle, Chuck Berry, and tells him about the cool song – a song that Chuck Berry ends up singing himself. We have here the causality paradox since it seems that nobody wrote the song “Johnny B. Goode” and it always looped between Marty and Chuck. Later in the scene Marty’s future mom asks Marty for his name, and when he answers she falls in love with the name and decides to call her future son by that name. The paradox here is clear, I think.
a causality paradox
(Credit: Atlas Entertainment)
It is a very common approach in the science fiction world for solving paradoxes, that there is some mysterious force in the universe that prevents time travelers from changing the past. For example, Cole meets in a psychiatric hospital a man named Jeffrey – Brad Pitt in an amazing role – whom he suspects to be the spreader of the original virus. Later in the movie Jeffrey claims that the idea to eradicate humanity never even occurred to him until Cole brought it up in one of their conversations- a classic causality paradox.
(Credit: University of Copenhagen)
The discussion about time travel paradoxes is long and complex and in this column I only covered a small portion of it. For example, some scientists claim that if time travel is possible, it will only be possible to go back in time up to the point when the time machine was invented and that’s why we don’t see tourists from the future around us. Some science fiction writers predict that if a time travel paradox occurs, the universe will decide it’s had enough and commit suicide. We probably won’t get an answer to our questions... at least not in the near future. If time travel were possible, what would you change in your own past?...
Special thanks to Pazit Polak for translating the column from the original Hebrew version.