Update 03/09: No grandfathers were harmed in the writing of this post!
Update 01/09: It's time to celebrate my first blogoversary!! That's right, today marks a whole year since The Life of Psi appeared on the blogosphere, and I penned my first post on Schrödinger's Cat and the Measurement Problem! I'm really happy and proud I didn't stop after one post, but stayed active all year long (although I still hope to increase my writing output). I've loved sharing my thoughts on quantum entanglement, quantum immortality, quantum teleportation and time travel with you, dear reader. It's been great fun, and on top of all that, it opened new doors for me into the world of science communication, such as my participation in FameLab, and my first Pecha Kucha ever.
Thank you so much for reading along and for all the comments you have given me throughout the year. It means the world to me, and I hope you'll all continue to follow The Life of Psi for the new year to come! Please feel free to subscribe to my blog by entering your email address on the About page (or at the bottom of the column on your right) in order to receive notifications of future posts by email.
Update 01/09: Time to switch gears. Now that we know how to build a time machine to fast-forward to the future (see the first part of my time travel trilogy), let’s have a look at time travel to the past and the many paradoxes it entails.
Everyone who has seen Back to the Future (1985) or the highly acclaimed Hot Tub Time Machine (2010) knows that when you start meddling with the past, things can become pretty murky. That is, whereas time travel to the past is considered theoretically possible (we’ll come to that in another post), it certainly seems logically impossible.
Which came first: the chicken or the egg?
For one thing, time travel to the past would violate one of the most fundamental rules governing the universe: that causes always precede their effects. To see this, imagine traveling back to the Jurassic period when the dinosaurs were still roaming the Earth, roughly 145 million years before you were actually born. In the flow of time, the effect (you) would therefore exist before the cause (your birth), a process referred to as causality violation.
The grandfather paradox
But that’s not all! Anyone using a shortcut to the past would bump against a bunch of other paradoxes as well, most of which are so deliciously mind-boggling (and fun) it’s worth exploring them in some more detail.
Without doubt, the most notable of time travel paradoxes is the classic grandfather paradox — brainchild of the French science fiction writer René Barjavel who first posited it in his 1943 book Le Voyageur Imprudent (literally The Imprudent Traveller, although the English translation is titled Future Times Three).
The story goes something like this: a serial-killing time traveler (or chrononaut), let’s call him Jack, goes back in time with the intent to kill his biological grandfather when the latter was still a little boy.
STOP! Before you continue reading, grab yourself another cup of coffee, and give it a moment of thought. What do you think will happen?
Jack, who happens to be an expert sniper, arrives in his TARDIS or DeLorean, grabs a shotgun and pulls the trigger. A moment later, Jack’s grandfather lies dead on the ground. The poor man no longer meets his future wife, and as a result, one of Jack’s parents (and by extension Jack himself) is never conceived.
Since Jack ceases to exist, he can’t go back in time to kill his father’s father! So Jack’s grandfather inevitably meets Jack’s grandmother. They mary, have children, and some years later, little Jack is born, allowing him to travel back in time to shoot his grandfather. Go back to step 1.
Clearly, each possibility implies its own negation: if Jack's grandfather dies, he lives; if he lives, he dies?! In philosophical discourse, this is known as an inconsistent causal loop. Physicists, on the other hand, prefer the term closed timelike curve, usually abbreviated as CTC. We'll have plenty of opportunity to talk more about CTCs in future posts.
Variations on a theme
Over the years, numerous variations have appeared on grandfather’s theme. Stephen Hawking recently proposed a simpler version which he called the mad scientist paradox. Here a scientist goes back in time to kill his younger self, which in philosophical circles is also known as autoinfanticide. Check out the movie below to see what would happen.
Another permutation of the grandfather paradox is the Marty McFly conundrum from Back to the Future, where Marty gets into trouble by going back into the past, and preventing his parents from falling in love with one another, thus putting his own existence into danger.
Here’s a fun experiment you might try for yourself: Open a new tab in your internet browser and Google the phrase “go back in time and”. Funnily enough, Google’s search engine will suggest completing the phrase in one of the following five ways:
- kill Hitler
- change the past
- kill your grandfather
- stop 9/11
- step on a butterfly
I especially like the last one, where smashing a butterfly might prevent the famous butterfly effect from happening, thereby dramatically changing all future events. This was also the inspiration for the similarly named movie The Butterfly Effect (2004) with Ashton Kutcher and Amy Smart.
The Hitler paradox
The Hitler paradox is another well-known variation on grandfather’s theme. Consider traveling back in time in an attempt to murder Adolf Hitler before he instigates the Second World War. And suppose you actually succeeded in killing Hitler, say in the year 1938, thus preventing the Holocaust from happening. The problem here is that this action also removes any reason to go back in time and murder Hitler in the first case. After all, if World War II didn’t happen, you wouldn’t know about the war and all the horrors caused by Hitler.
Solving the paradox
When scientists in the 1980s realized backwards time travel wasn’t forbidden by the Laws of Nature, they naturally wondered whether the grandfather paradox presented a real barrier to time travel, or if there was any way out of this conundrum?
Some scientists, like Stephen Hawking, were convinced the paradox was too perplexing to be solved in any simple way, and that this was proof enough that backwards time travel was impossible. In a much-quoted paper of 1991, Hawking proposed his chronology protection conjecture which said that “the laws of physics [will always] prevent the appearance of closed timelike curves.” Hawking argued that the universe would always find a way to protect itself against causality violation. He ended his paper jokingly: “There is also strong experimental evidence in favor of the conjecture from the fact that we have not been invaded by hordes of tourists from the future.”
But Hawking’s conjecture was only an educated guess, a gut feeling, and not everyone agreed. Scientists like Kip Thorne, Richard Gott, Igor Novikov and David Deutsch believed backwards time travel was possible and did not have to lead to any paradoxes. As to the grandfather paradox, the question was simple: “Can Jack or can he not kill his grandfather?” Thorne and Novikov answered in the negative, whereas Deutsch believed he could. But as we will see in one of the next posts, either answer created its own problems.
Solution 1: Novikov’s self-consistency conjecture
The first way of answering the grandfather paradox is to say that inconsistent causal loops cannot occur in our Universe. You're free to travel back in time, but you won't be able to alter history since in a sense you have always been part of that history. “What has happened, has happened” says Novikov. “It cannot be repeated twice in different ways.” Any action in the past preventing the time travel trip itself from happening is thus strictly forbidden. The entire story has to remain self-consistent. In the scientific literature, this is known as Novikov’s self-consistency principle (after the Russian physicist and time travel pioneer, Igor Novikov).
On this account, Jack will fail to kill his biological grandfather — his own existence, after all, proves that his grandfather lived for many years after the attempted murder. So no matter how skilled Jack might be with a shotgun, something will always occur to prevent the paradoxical situation from taking place. “The particular how [a bird pooping at just the right moment, a distracting noise, a quantum fluctuation, or some other strange conspiracy of events] is unimportant,” says David Toomey. “The point is that [Jack] must fail, or to put it most succinctly, he will fail because he did fail.”
Here’s my favourite way of illustrating the point. Consider the scenario where Jack decides to commit autoinfanticide. He finds a portal to the past, meets his younger self, and fires. But Jack misses! Instead of hitting his younger self in the hearth or in the brain, he shoots right in the shoulder. Why? Because Jack has suffered from a bad shoulder and trembling arm ever since he was a child and someone pumped his shoulder full of lead!
If you’re a fan of time travel stories on the silver screen, you’ll enjoy testing them on their self-consistency. Most movies will fail the test as they contain multiple plot holes; Looper (2012) is a case in point. But those that pass the test usually involve ingenious ways of keeping everything self-consistent, seamlessly interweaving the actions of the time traveller into the entire story. In Harry Potter and the Prisoner of Azkaban, for instance, Harry uses a time-turner to travel back in time and safe himself from a Dementor attack, in a perfectly self-consistent way.
Solution 2: Parallel Universes
The other solution to the grandfather paradox is perhaps a bit more extravagant, so brace yourself! In contrast with Novikov’s solution, where only one (self-consistent) timeline is assumed, this response appeals to the idea of multiple parallel universes, each with a different timeline. One of its main defenders is David Deutsch, a well-known professor of physics at Oxford University.
On this account, when Jack goes back in time, murdering his grandfather, he enters another world, one where his grandfather dies as a little boy and never has any kids (situation 1 above). By trying to change the course of history, our time traveller has landed in a different parallel universe with an alternate timeline as compared to the one he left where his grandfather didn't die and happily married (situation 2 above). Notice that upon murdering the old man, Jack himself doesn't die. He merely disappears from his own universe to get stuck in another one.
So, whose side are you on? Do you agree with Stephen Hawking that the grandfather paradox rules out time travel? Or do you think some mysterious force of Nature will prevent the paradox from happening, keeping everything self-consistent, as Igor Novikov proposed? Or, perhaps, you're a fan of the many-worlds theory, like David Deutsch? Let me know in the comments and stay tuned for a possible answer in my next post!
Deutsch, D. and Lockwood, M. "The Quantum Physics of Time Travel." Scientific American (March 1994): 68-74.
Bonsor, K. and Lamb, R. "How Time Travel Works: Time Travel Paradoxes." How Stuff Works (20 October 2000)
Lewis, D. "The Paradoxes of Time Travel." American Philosophical Quarterly (April 1976): 145-152. [This is a classic paper by the philosopher David Lewis on the grandfather paradox. His position is the same a Novikov's, namely that pastward time travel does not have to present any paradoxes because everything can remain self-consistent.]
Hawking, S. W. "Chronology Protection Conjecture." Physical Review D 46, no. 2 (1992): 603-611.
Friedman, J., Morris, M S., Novikov, I. D., Echeverria, F., Klinkhammer, G., Thorne, K. S. and Yurtsever U. "Cauchy Problem in Spacetimes with Closed Timelike Curves." Physical Review D 42, no. 6 (1990): 1915-1930. [This is Novikov's well-known paper on the self-consistency principle.]
Deutsch, D. "Quantum Mechanics near Closed Timelike Lines." Physical Review D 44, no. 10 (1991): 3197-3217. [In this paper, Deutsch argues that the grandfather paradox can be solved by adhering to the many-worlds interpretation of quantum mechanics.]