Otherwise known as the bootstrap or predestination paradox, a causal paradox results from time travel. In one of the generic forms, you come across a time machine (it does not matter if you invented it or happened upon it) and go back in time. While you are there, you meet someone you find attractive and together have a child. Later on, you find out that the child is a direct ancestor of you, and without him/her you would not exist. But if the child was always required for you to live, then you were also always required for the same reason. But how could you be around to have the child in the first place if the child was necessary for your existence?
To describe this paradox in more encompassing terms, this paradox involves you completing some event in the past that is required for a future event that you are dependent upon in order to go back in time. Frequently this is used in the above-mentioned case, but it could also be a historical event, such as you replacing a founding father of the United States and taking his place. Whatever the example may be, the paradox always arises because the event that created you happens before you were created.
Grandfather Time Travel Paradox
This is another infamous time travel paradox. On your adventure into the past, you meet a man, and for whatever reason (many exist, including an attempted theft to an accident), you end up killing him. You realize after he is dead that it is, in fact, your grandfather, specifically at an age where he had not yet met your grandmother. Because he is now dead, he never will meet her, so your mother/father will not exist, so you cannot exist either. But then, because you do not exist, you could not go back in time and kill him. So he never died, so you did exist, and you still went back in time. How can you exist and not exist at the same time?
In general, the grandfather paradox is simply you going back in time and preventing an event from occurring that is integral to your existence. Just like the predestination paradox, this deals with you being the source of a major event in your life but in this case an event of destruction.
The One About God
Imagine we have someone who can see how future events will play out. Ignoring the logistics of this idea, what if that person takes actions to prevent that future from happening? That would mean he changed his future so what he saw was wrong and therefore cannot really see into the future, making his actions impossible and so he can see the future fine, and we, therefore, have our paradox (Al 157-8).
The heart of this paradox arises from the deterministic viewpoint of the Universe, which is that the future can be predicted. Paradox seems to indicate we can’t, but all the physics up to this point say yes. A simple solution that resolves the paradox is something physics predicts can be true, but let’s face it: It will seem awfully convenient. What if we live in a multiverse where every possible future plays out? You can see a potential future and then make changes to ensure it doesn’t happen for you, but it will play out in its own branch of reality. But then this takes the whole air out of the demon’s ability to predict because if anything can happen, then what are you really anticipating? (159-161)
As sad as it is to admit it, this paradox addresses an impossible scenario. One simply cannot predict the total future because no one can know all the information about the Universe at any given moment. But what about in a quantum sense? Then we are cooking because we have probabilities of certain futures, and we don’t need all the info to analyze that scenario (161).
Most people say that because such paradoxes cannot happen and remain consistent with their timelines, we need not even consider them in the first place. But the math does not lie, and it says backward time travel is possible. So how can we explain these possible problems?
Note that to travel back in time as we know it, we need a closed time like curve, or a CTC. They are simply moments of spacetime that loop back onto themselves, generally caused by a large gravitational source. David Deutcsh in 1991 was able to show that when you go back in time to perform the grandfather paradox, you have a 50/50 shot of actually doing it or not. According to quantum mechanics, this is good enough to say that the not-chance happens. Twenty years after David developed this idea, Tim Ralph and his team were able to test out the theory using polarized particles. One particle would change states, and a second particle would be given the same traits as the other before its evolution. These two photons would then interact and change. Using this system, the team was able to show that the second particle would become the same as the first one after its evolution. Apply some math, and you have an equivalent CTC example (Billings).
Another possibility developed by Seth Lloyd in 2009 combines quantum teleportation and post-selection. His theory eliminates the possibility of alternative universes that David's CTC method predicts and instead keeps the time-traveler in his own universe. But more solutions are sure to crop up, so keep an eye out!
Al-Khalili, Jim. Paradox: The Nine Greatest Enigmas in Physics. Broadway Paperbacks, New York, 2012: 157-161. Print.
Billings, Lee. "New Time Travel Simulation May Help Resolve Grandfather Paradox." HuffingtonPost.com. HuffingtonPost.com, Sept. 03, 2014. Web. Oct. 25, 2014.
© 2017 Leonard Kelley
Leonard Kelley (author) on December 11, 2017:
Nadine, sometimes time travel reminds me of another show -Quantum Leap!
Nadine May from Cape Town, Western Cape, South Africa on December 11, 2017:
Wow! Wow, I found your article very fascinating, to say the least. It reminded me of the series travelers. (Hundreds of years from now, surviving humans discover how to send consciousness back through time, into people of the 21st century, while attempting to save humanity from a terrible future.)