Time Travel Paradoxes

A direct link to the above video is at http://www.youtube.com/watch?v=-x8qCX78MVk


A direct link to the above video is at http://www.youtube.com/watch?v=6ICmPQFNGog

The above video accompanies my previous blog entry Just Six Things, which was published earlier this year. It connects to the video we looked at last time in Bees and the LHC, in which we considered the possibility that bees can somehow perceive the sixth dimension (as has been proposed by a scientist analyzing the "waggle dance" they perform to communicate to each other), and the controversial leap I made that perhaps the declining bee population of the world is connected to their sixth-dimensional perception.

In entries such as Just Six Things, I've been relating the quantum mechanics concept of there being a wave function of all possible states for our universe to my way of describing the sixth dimension, the entirety of which then becomes a point in the seventh dimension, which gives our universe a certain location within the multiverse landscape. Thinking that there is a set of all possible states that already exists for our universe leads some back to the idea that free will is an illusion, since within that wave function we can see that everything that can happen already has happened from the perspective of timelessness.

Everett's "Theory of the Universal Wavefunction" (also referred to as "Many Worlds") is the way out of this quandary, which brings us to today's topic.

A YouTube user named Lee Ann recently asked me a question about time travel paradoxes, and here's how I responded to her:

Hi Lee Ann, that's a great question.

I subscribe to Everett's Many Worlds Interpretation of quantum mechanics, because it allows a logical consistency to such discussions, and avoids the Grandfather Paradox issues. I presume you're familiar with the Grandfather Paradox where someone goes back in time, kills their grandfather, which means they were never born, so the grandfather is not killed, so the person is born and goes back and kills the grandfather, and so on?

Everett's interpretation makes more sense to me. So to answer your question specifically, assuming reliable time travel technology now exists it's December 2010 and you and your son climb into your time machine. There will be two main branches of you created now: the one where you go back in time (B) and the version that said "I choose not to go back in time" (A). Version A continues on with their life, no time travel occurred so things happen accordingly with their life from there on.

Version B goes back to 1992 and does something - carves their name in a tree, blows up their house, whatever you care to imagine. They could meet with and talk to themselves --the 18 years younger versions of themselves-- but in doing so they would be creating a new timeline, a version C of themselves who in December 2010 remembers that time 18 years ago when the time travelers appeared.

Does version A ever see the carving in the tree? No. Does version B become the 18 years younger version of themselves when they travel back in time? No. And as time travelers are they able to cause things to happen like causing their parents to never fall in love so they're not born (a la Back to the Future)? No.

In fact, here's a couple of paragraphs from my book on the Back to the Future question:

It should be clearer now that "Back to A Future" might have been a better title for this movie trilogy, since anyone who goes into their own past will never be able to get back to the present they had started from--because in that present time they started from they had never traveled to their own past. Their action of traveling to the past automatically puts them on a new timeline, the one where they had been in their own past, which might end up arriving at a present extremely similar to their original timeline, but is more likely to be completely different. Paradoxes such as this are built into any discussion of time travel--but agreeing that all possible pasts and futures really do exist is still easier to deal with than the alternative, which would be to imagine that there is only one real timeline and every aspect of the past and future is already carved into stone. If each of us is correct in our belief that we have free will, then the future must be filled with options, not just a single track.

There could be a whole other set of "Back to A Future" movies created where the inherent paradoxes are explored--for instance there is the plot where Marty travels to the new and improved future (the ending of the original movie), and discovers a family and friends who have lived a happy life with some other "timeline twin" of Marty, who we will call "New Marty". "Old Marty" realizes that "New Marty" must exist, because otherwise his new and improved family wouldn't have recognized him, and would have chased him out of the house as a stranger. So "Old Marty" has a gruesome task ahead of him: he must find and kill "New Marty" and dispose of the body before anyone finds out they both exist. The rest of the movie would entertain us with the hilarious slip-ups as "Old Marty" then tries to interact with his new and improved family despite never having shared any of their past years together.

Next time we're going to discuss one of my favorite blog entries of 2010, which finally has a new video up on YouTube: the entry will be called Gravity and Love.

Till then, enjoy the journey!

Rob Bryanton