Marty meets his mom and dad before he was born in Back to the Future.

Picking up where I left off last weekend writing about time traveling forward and back.  Traveling back does offer the possibility of the problem of paradoxes.

The dictionary will tell you that a paradox is a statement or proposition that seems self-contradictory or absurd but in reality expresses a possible truth.

In time travel, a number of paradoxes emerge. If I was to travel back before my birth, I wouldn’t exist. Could I be my adult self the year I was only 5 years old? Cause comes before effect, right? If I travel back 100 years, the effect (that’s me) would come before the cause (my birth).

The most famous time travel paradox is known as the “grandfather paradox.” A variation on this is in the popular movie Back to the Future. Let’s say I can travel back in time before my birth and I am my adult self. While I’m there, I get into a bad situation and I end up killing a man. That man turns out to be my own grandfather. I killed my grandfather, so his son (my dad) will never be born, and either will I be born.

So then, who traveled back and killed grandpa? That is one inconsistent causal loop.

So to keep it consistent, we have to make it rather boring. I go back, but I get younger as I travel and at the point of my birth I vanish. If I travel back only one year to 2011, I am a year younger and everything that happened happens all over again. I can change nothing. In fact, I am the same person, with no knowledge of my future self. Following this theory, I might be traveling right now! And when I move forward (again) to 2012, at that same point in time, I will travel back to 2011 – again. I am stuck in a loop, albeit a consistent one. With all that repetition, it’s no wonder I keep having déjà vu moments. I HAVE done this before!

This consistent causal loop is paradox-free and it has been theorized by a number of people including physicist Paul Davies. He proposed a scenario like this: A math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with.

The machine Peter used to bridge parallel universes on Fringe

If you want to really do some divergent thinking, consider that the future or past you travel into might just be a parallel universe. In that case, you can change whatever you want, because it doesn’t affect your universe. If you watch the TV show Fringe, they have been playing with this idea for a few years.