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Time Travel February 17, 2009

Posted by phillymb in Metaphysics, Philosophy of physics.

Going along with Greg’s post on the topography of time, specifically the portion on Time Dilation, I will explore the mathematical possibility and philosophical implications of time travel in physics.

Who wouldn’t want to travel back to the past to see what life was like or peek into the future?  What sort of implications would this have?  The idea of time travel has entertained us for years from black and white TV shows like the Twilight Zone until now with shows like Lost.  However, from a physical perspective how would time travel be possible?

There are two ways to travel into another person’s future:

  1. Traveling at speeds >10% of the speed of light (https://planetparadigm.wordpress.com/2009/02/12/topography-of-time/)
  2. Taking advantage of an intense gravitational field

Wormholes are an example of an intense gravitational field:

Hypothetical tunnel connecting two different points in space-time in such a way they could allow travel from one part of the universe to another part of that same universe very quickly or would allow travel from one universe to another
•    Would be superluminal
•    Created by the connection of a black hole and a white hole
•    Highly unstable: light would not be able to pass through it without collapsing unless an exotic matter (negative energy density and a negative pressure or tension) held the tunnel open

Theoretical Wormhole in Curved Space-time

Theoretical Wormhole in Curved Space-time http://www.daviddarling.info/images/wormhole_graphic.jpg

This leads into the many what-ifs of time travel.

It would be convenient if we could just simply ask a time traveler to make sense of all this for us…but for now we’ll look at the problems with time travel.

The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and, for example, tries to kill his/her grandfather at a time before the grandfather had fathered any children.  If the time traveler never came into existence, then how would he/she be able to return  to kill thegrandfather?  Stephen Hawking’s “Chronology Protection Conjecture” claims that the laws of physics conspire to prevent such a paradox from happening because physics would not let such large inconsistencies occur. For example, a bird would fly in the way of the homicidal time traveler or the gun would lock up.

When I mentioned this in class someone asked me if this would be considered fatalism, which states that whatever will happen in the future is unavoidable. However, I interpret the situation not as a determined outcome, but as an avoidance of a paradox. This  is also called the theory of compossibility.

I do not think that we will ever be traversing wormholes in the future (or past).  Although the math is elegant, I do not think we will be able to obtain the exotic matter needed to hold open a wormhole for a photon to pass through, let alone a human.

Thanks for reading,

Mike Brindak


Ned Markosian, “Time Travel,” in The Stanford Encyclopedia

Joel Hunter, “Time Travel,” in The Internet Encyclopedia of Philosophy



1. Evelyn Brister - March 8, 2009

Mike, I just watched a time travel film that you, Mark and Greg might like. It’s called Primer:

At first I thought the plot completely fell apart and the time travel element didn’t make any sense, but the more I thought about it, the more the pieces started to fit. I do think it could have used more in the way of explanatory dialogue!

From the review by A.O. Scott (http://movies.nytimes.com/2004/10/08/movies/08PRIM.html):
“Shane Carruth’s “Primer,” a debut feature shot on 16-millimeter for a budget of around $7,000, is an ingenious movie about the perils of ingenuity. Two would-be inventors, Abe and Aaron, working after hours in their suburban garage, stumble onto an invention whose application is not obvious at first but whose ethical and metaphysical implications quickly become enormous. Abe (David Sullivan) describes it to Aaron as “the most important thing that any living organism has ever witnessed,” which may be a slight exaggeration. To call the gizmo a time machine, which it more or less is, would be to create a slightly misleading impression, evoking splashy Hollywood confections like the “Terminator” and “Back to the Future” franchises, which “Primer” does not resemble in the least.”

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