Chandra X-ray Observatory close-up of the core of the M87 galaxy. Image Credit: NASA/CXC/Villanova University/J. Neilsen

We’ve got the DeLorean warmed up, Marty and Doc in the back and we are heading into the mind-boggling world of Time travel paradoxes. Time travel itself is a paradox the whole journey back in time contradicts itself and as we’ve seen in movies things rarely work out to the traveller’s expectations of requirements. Closing loops are important if you are planning on time travelling but sometimes it’s unavoidable to accidentally wipe yourself or an event out of total existence.

The Grandfather Paradox

So, imagine your grandfather as a terrible person who has done a terrible thing and has left a terrible legacy affecting many people. You go back in time to kill him, knowing that you and your family will no longer exist. From here the Grandfather Paradox splits into two outcomes. You find your grandfather and point a gun at his head and then “click”. It will not fire. You test the gun at the wall, “BANG” fires perfect, you point it back at his head and “click, click” nothing. You try countless times to do away with him by many different means but each time he lives and only sustains an injury. He is protected by an already written fate. This is known as the “Time Line Protection Hypothesis” No matter what you do to try to change the future, it is already written, and all efforts make no change to the future outcome.

The other alternative is the Multiple Universe Hypothesis where you shoot your Grandfather dead, you return to the present to see that you’ve never existed here, yet you are still alive. You have created a parallel universe.

The Bootstrap Paradox
A man time travelling. Image Credit: Astronomy Trek

This is when a piece of information, human, or an item is sent back in time and results in an infinite loop with no origin. Described very well in Doctor Who, A fan of Beethoven travels back in time to meet him and to get his manuscripts signed by the composer. Upon arriving in the past, he discovers that nobody has heard of Beethoven, no one can recall his music and that he does not exist. Worried about a world without the music of Beethoven, our time traveller copies all of Beethoven’s music down and signs it as such. Therefore, becoming Beethoven. The music has no origin, who wrote the original music? It exists without ever being created.

The Predestination Paradox

The best way to describe it is the scenario from, where your lover dies in a hit and run, so you travel back in time in hopes to save them from their ill fate, however you discover that in your efforts to protect them you are the one that kills them. Your attempt to change the past had resulted in the predestination and this paradox suggests that things will always turn out the way it is destined to be. Check out the movie 12 Monkeys for a great example of this.

This is a time loop where the event that is played out is determined by the time traveller, who travelled in time to alter the event. Thus, finding that it has always been a time loop.

The Let’s kill Hitler Paradox

Like the Grandfather Paradox, the let’s kill Hitler Paradox is on a much grander scale, yet quite simple. If you go back in time to Kill Hitler and succeed, then he would have never created the evil he did and therefore there would be no reason to travel back in time to kill him. He would have never existed, no one would know his name.