How many paradoxes are there in the field of time travel?

Answer by Sahaj Ramachandran:

There are three major paradoxes related to time travel. In science fiction, these have been extensively used, so much so that most time travel movies have become predictable.

Let’s start with the most famous one, and the easiest to understand: The Grandfather paradox.

The scenario is simple. Somehow, in the future, you are able to travel through time. And just because you feel bored right now, you decide to travel back in time, even before you were born, just to see what would happen if you killed your grandfather.

The Paradox here is because you have altered a significant event that is related your personal timeline. Your grandfather’s death would inevitably affect your birth, and thus, you would never be born. And if you were never born, you couldn’t have travelled back in time and killed your grandfather.

Note: This would work even if you kill your younger self, or even your parents. These kind of situations are referred to as the grandfather paradox.

Solutions to the paradox:

  • Multiverses: Put simply, when you travel back in time, you travel to the past of a different universe, and in that particular universe, you die, whereas in your universe, you will still be alive. You just have to make it back to the exact universe you came from, somehow.
  • Retro-causality: Events (effects) are allowed to take place before their cause, in a hypothetical world.
  • The past cannot be easily changed: Even if you were to travel back in time, naturally, you would not be able to do anything that might potentially harm your timeline. For example, when you try to kill your grandfather, the gun might malfunction.

Movies that have this paradox: The Grandfather Paradox (2006), Back to the Future.

​Let’s move on to the Predestination paradox. Again, since you are bored, you try time travel and this time, end up in a different time. For a change, you don’t feel like killing someone. Instead you befriend your dad. In a series of events that follow, you realize your friendship with your dad is the only reason why he met your mother.

In this case, you are responsible for your own birth. It is cool, but it has a very big problem: You will need to come back every time from the future to do the same exact things that would lead to the marriage of your parents. Otherwise, you would cease to exist.

Solutions to this paradox:
In order to exist, there is only one way to go: Do the same exact set of events, infinitely. There is no other choice. If you are to time travel, make sure you don’t create events that are directly linked to your timeline.

You can, however, live out a different life only if the events aren’t linked to your birth. For this, you can deliberately alter the events and take a break from doing the same things over and over again. This would alter your timeline and give you a different life to live. [Note that in this case, there is another problem. Another timeline means that there is a possibility of you not travelling back in time ever, and the previous events are cancelled -> the alternate timeline doesn’t exist. Thus, this paradox is inescapable].

Movies that have this paradox: Predestination (duh!), Looper, Men in Black III.

​The last major one is actually similar to the Predestination paradox: The Bootstrap Paradox.

Imagine that you are a music composer. But your creativity level is near to zero. Doesn’t matter. You have a goddamn time-machine. Using the time machine, you go to the future, and steal your own song, which is apparently a big hit. You come back in time and release this song for the world to hear. The next day you realize that the song is going viral. Months later, while you are performing the song on stage, a younger self comes from the past and steals it. Later, you realize it was you who stole it.

Who composed the original song? Your younger or older self? The song has no definite origin. And in this case, it is the bootstrap paradox. It can be any object, like even a watch, or information.

Solutions to this paradox:
In our scenario, the song could’ve been composed by somebody else and was probably heard by one of yourselves. And then, the subsequent hearing is done by your two selves. This is where an “entry-point” is defined in the loop that follows.

Movies that have this paradox: Somewhere in time (the pocket watch), Predestination (the gun), Time Lapse.

​Other paradoxes include the butterfly effect, where a small seemingly insignificant event produces a chain reaction of events that eventually cause a bigger event. As an example, a small change you make in the past can lead to a bigger event that could possibly stop you from travelling back in time in the first place.

The Hitler’s murder paradox is where a change in major events can prevent future events from happening, or even cause worse events to happen. For example, you can save the Titanic from sinking and save a lot of lives. But that many people living, history can go a different way and create events that are undesirable. Perhaps your dad married your mom because his previous engagement died on the Titanic. So if you saved the Titanic, you would never have been born to save it in the first place.

Did you know that Professor Utonium created the powerpuff girls because he saw them when he was a kid? The girls went back in time in one episode that encouraged the professor to create them. Thus, a predestination paradox.

I would’ve never known Adam if he didn’t create Quora. After seeing Quora, I go back in time to prevent him from creating it. And since he never created Quora, I never knew Adam and never really thought of preventing it in the first place. This would be a variation of the grandfather’s paradox.

BTW, I just stole this answer from the future. This answer would be showing up in my friend’s feed a week from now, and I deliberately steal it and post it now. What’s the source of this answer? It doesn’t have one. It’s caught in a bootstrap paradox 🙁

Further reading: List of paradoxes on Time

How many paradoxes are there in the field of time travel?