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Monday, January 08, 3798 2018

The Problem With Time Travel

Science fiction movies often depict time travel. Just how realistic is it?

T ime travel is, as we all know, a popular motif in sci-fi. The challenge that writers face is that nobody knows what would happen if you changed the past. Would the future change instantaneously? Would time heal itself, canceling out anything that we did? Or would the universe split into multiverses, making each possibility happen in a different world, as one particularly dreadful movie *cough*Star Trek 2009*cough* supposed?

Photo of my back yard as seen through a time machine

The immutability of the past is what makes the world real. If we could change the past at will, the present would whip back and forth constantly, without our knowledge, and we would be constantly edited in and out of existence as others tried to fix their social faux pas, undid their dumb tweets, and changed how they voted again and again. We would fight over the past until the universe became a fairyland where nothing made sense.

It might, for instance, turn our world into a hellish place where politicians deceive the voters instead of being scrupulously honest, where personal injury lawyers advertise on TV, and people post naked pictures of each other on the Internet and call each other nasty names instead of having erudite intellectual discussions like they do now.


The TV show Continuum depicted how this scenario might occur. A group of libertarians go back in time to prevent a government-corporate tyranny in the North American Republic. Rachel Nichols played a lady cop who gets sucked back in time and tries to stop them. It was a struggle for survival, and their leader, a guy named Kagame (Tony Amendola), set the tone by blowing up a skyscraper in Vancouver, BC. In the end they prevent tyranny, but in its place a militaristic tribe of genetically enhanced supersoldiers is created. The supersoldiers come back in time to try to stop the libertarians.

Although interesting, it was a bit confusing. Not only did events and histories keep changing—at one point the genius kid's girlfriend dies, so he goes back in time to save her and gets in a gun battle with himself—but the theory of how time travel could affect the future kept changing as well. Eventually it became evident that the writers had created a narrative black hole and the producers pulled the plug.

On the other hand, Lexa Doig was in it!

The chronology protection conjecture

Hawking and others conjectured that quantum effects on curved spacetime conspire to make time machines forbidden. This is the chronology protection conjecture, not to be confused with Penrose's cosmic censorship hypothesis, which has to do with black holes.

Fictional scenarios like that TV show have convinced many people that even if time travel were possible, people would keep altering the past until, by chance or design, it would change in such a way that a time machine is never invented. Thus, history would mysteriously weave this way and that, conspiring to ensure that no one would ever invent a time machine.

It's not just that information would get censored. The person who invents it might mysteriously never be born. Or a civilization that is destined to discover time travel could get wiped out by a sudden plague, an influx of really annoying immigrants, or the sudden, mysterious appearance of a truly crappy educational system.

Or the history of science could change just enough to give those scientists who claim that time travel is impossible a Nobel prize, leaving those who could have discovered it to sweep up the lab cursing the cruel world that stomped on their brilliant idea.

People often fantasize about going back in time and repairing the catastrophes of history—shooting Hitler, perhaps, or sticking their foot out in front of Karl Marx, causing his notes for Das Kapital to blow away in the wind, and thereby saving 100 million lives. But if the historical role of Marx and Hitler was to screw up history just enough to prevent us from inventing time travel, you would never get the chance.

Either way, whether time travel is possible or not, it means our world is already fatally flawed. In either case we will never know the truth.

If that theory is true, then every book and every web page you read that discusses the subject, including this one, must be one of two things: either it will disappear with no trace, or it will be a false trail that misleads you about time travel.

Quantum Mechanics

All this serves as an introduction to what I really want to discuss: a new book titled The Problem of Time (Fundamental Theories of Physics, vol. 190; Springer, 2017), where physicist Edward Anderson spends 920 pages reviewing Newtonian mechanics, quantum mechanics, and general relativity to contrast how they depict time. (See his Arxiv article here). There are, he says, nine facets or aspects to the problem of time: temporal relationalism, configurational relationalism, constraint closure, assignment of beables, spacetime relationalism, foliation independence, spacetime constructability, global validity, and no unexplained multiplicities.

From this, we can be certain that, no matter how the problem of time is finally solved, it will be very, very complicated. So far, I'm only up to the chapter on quantum field theory. Anderson ends the book by saying this:

This generalization is based on Grothendieckian mathematics having an even broader scope of structural concepts than even its Topos Theory portion does, by which maybe that another part of this mathematics is required for the foundations of QG.

The book is also packed with strange mathematical equations like this one on page 537:
Schrodinger equation for rectified time
I'll update this page if I ever figure out what he's talking about.

Would time travel cause a Big Bang?

But there's an even bigger problem. If even a single photon goes back in time and re-enters your time machine in the past, you have created an unstoppable feedback loop. Unlike a regular feedback loop, a time warp is a physical gateway that's not limited by any electrical device. So there's nothing to stop matter and energy from piling up indefinitely in the same spot, being created over and over, instantaneously producing a Big Bang.

Those of us who played with tape recorders know that no matter how long you delay the feedback, you still get a feedback squeal. A time machine would do this instantaneously, and it would collapse spacetime, creating a colossal explosion. If this happened it would be very, very, very, very bad indeed. So a time machine with this flaw would only be built once.

Intuition tells us, however, that before we can say whether we can travel back in time, we would need to know what time is. Physicists say creating closed timelike curves is theoretically possible according to relativity, but would require infinite amounts of energy. Arefeva et al. suggest that a new theory called AdS/CFT holography may provide a way out. But if the time censorship hypothesis is true, it's quite possible that they're all lying.

jan 08, 2018, 5:11 am

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