Sunday, July 5, 2015

Terminator Genisys: Part One: Is Time Travel Possible?

Terminator Genisys is not about time travel.  Like many other time travel stories, from Back to the Future to Looper, it’s really about travel between different universes or different worlds.  My claim requires some examination (appropriate enough in a blog called Examined Worlds).  The examination will require two parts.  In the first, I’ll concentrate on whether time travel is possible, which will provide the background for the second part, where I’ll explain why Genisys isn’t really about time travel.

Terminator Genisys

The movie has gotten mostly negative reviews (currently 27% fresh on Rotten Tomatoes), but I really liked it.  I loved Schwarzenegger’s movies as a kid, so it’s nice to see Arnold out of politics and back in Hollywood.  He even reminded me that he can be funny. Genisys isn’t on par with the first two Terminator movies, which ought to be enshrined as science fiction classics, but I liked Genisys a lot better than the third and fourth movies.  

Philosophically, Genisys brings up all the major issues of the previous films.  What would artificial intelligence be like?  Would it want to kill us?  Could it be reasoned with?  Could AIs be considered persons in a moral or legal sense?  Could we form personal relationships with AIs?  Might AIs do a better job running things than we do?  (I covered some of these issues in my reviews of Robot and Frank and Ex Machina and in my many discussions of Iain M. Banks).  There’s also the classic problem of freedom and determinism: Is the future in some sense set or can our choices and actions change it?  Are individuals so important as the Great Man theory of history posits, or could other people have fulfilled the roles of Sarah Connor, Kyle Reese, and John Connor in the human resistance?

Is time travel possible?

But the biggest science fiction trope of the Terminator series is supposed to be time travel (see this excellent piece for a thorough attempt to understand all the timelines in the series).  Is time travel really possible, at least outside of the normal rate of one second per second into the future at which we normally travel?

First, let’s distinguish between physical possibility and logical possibility.  Time travel to the future is physically possible, which means that scientific laws as we currently understand them would allow for it.  According to Einstein’s special and general relativity, you could experience time dilation by traveling very fast or by being near extremely massive objects (this is what Interstellar depicted so well when a few hours for some of the characters traveling near a black hole was over 20 years back on Earth).  However, time travel to the past is physically impossible given current scientific understandings: Einstein says, “No way!” (with a possible, but very unusual caveat involving wormholes, which is explored in this piece).
"No way!"
The Grandfather/Connor Paradox

Time travel to both the future and the past is logically possible, which means that it doesn’t engender any logical contradictions to imagine it.  (The classic example of something that isn't logically possible is a square circle). Some people think the Grandfather Paradox or causal loops demonstrate the contradictory nature of backwards time travel, which makes it logically impossible.  Could you go back in time and kill your own grandfather before he met your grandmother?  Or in Terminator terms, could the machines go back in time and kill Sarah Connor before she gives birth to John Connor?  But how could either of these happen, since you exist because of your grandparents and John Connor is already leading a resistance and hence must exist?

Causal loops: Primates encouraged

Or consider causal loops.  By instigating the resistance to send Kyle Reese back in time to stop them from killing Sarah Connor, the machines actually initiated a chain of events that made John Connor exist in the first place!  Without their actions, the resistance wouldn’t send Reese back in time, and without that (and a couple hours for romance with Sarah while hiding from a killer cyborg) Reese would not have fathered John, and there wouldn't be a John Connor to worry about!  

Cornelius and Zira arrive in 1971
My favorite causal loop is in the third and fourth of the original Planet of the Apes movies (Escape and Conquest); the future intelligent apes Cornelius and Zira go back in time to a fabulous 1971 Earth where their baby, Caesar, grows up and leads the ape revolution that paves the way for Cornelius and Zira to exist hundreds or thousands of years later, who then go back in time with their baby, who starts the revolution… (a nice treatment of this can be found in Ralph Shain’s chapter from Planet of the Apes and Philosophy: Great Apes Think Alike).

My second favorite causal loop is the Futurama episode "Roswell that Ends Well," which also includes a riff on the Grandfather Paradox: Fry kills the man he believes to be his grandfather in 1947, but then he meets his grandmother and becomes his own grandfather.

No paradox, but maybe a pair o' docs

Some people think these are intolerable paradoxes, which means that travel to the past is contradictory and hence a logical impossibility.  I don’t see a problem.  David Lewis’s famous article, “The Paradoxes of Time Travel,” makes the case that the Grandfather Paradox could be solved by simply stipulating that no matter what you do, you will never be able to kill your grandfather: your foot will slip while aiming the gun, you will lose your nerve at the last minute, or maybe two doctors will talk you out of it, who would be the only pair o' docs involved! (My lame joke, not Lewis's - sorry, I couldn't resist).  Remember that sending Arnold back to 1984 was a Hail Mary effort for the machines when they were about to be defeated (a point nicely dramatized in Genisys), so even the machines were probably unclear whether this would work.  And, at least in the first movie, it didn’t work.  Not only did they not kill Sarah Connor, but they inadvertently created a causal loop that brought about John’s existence!  Those bumbling murderous machines!

Causal loops are loopy, but not impossible

As for causal loops, Lewis claims, and I’m inclined to agree, that this is only paradoxical if you assume that causal explanations must be linear (A explains B, which explains C, etc.).  But why couldn’t it be circular?  Surely causal loops would be odd, but why say they’re impossible?  As Lewis points out, plenty of people think God or the Big Bang can exist with no causal explanation, so they’d have no basis to rule out causal loops on account of their lack of explanation. (For more on Lewis and others, see this article).

Si, se puede 

So, I maintain that time travel to the future is both physically and logically possible, while time travel to the past is at least logically possible, which is illustrated quite nicely in the original Terminator movie.  But what’s going on in Terminator Genisys?  See Part Two: Genisys is Not About Time Travel.


  1. Yeah, I'd agree that the biggest impediment to time travel to the past concerns logical, not physical problems. Our best theories in physics certainly allow for it, but because of the logic of things like the grandfather paradox, physicists simply throw up their hands and say, "My suspicion is that time travel to the past is prevented because of some currently unknown physical law." Of course, that doesn't count as evidence against the idea at all.

  2. Replies
    1. Hi Jordan! Thanks for your comment. If you ever get a chance to read the David Lewis article "The Paradoxes of Time Travel," I think you'd really enjoy it. I have been surprised recently to hear that some physicists say backwards time travel may be physically possible. We'll see what happens in the future (or the past).