Sciency Words: The Chronological Protection Conjecture

Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we talk about all that weird terminology scientists like to use.  Today on Sciency Words, we’re talking about:

THE CHRONOLOGICAL PROTECTION CONJECTURE

English theoretical physicist Stephen Hawking had a lot to say about time travel.  There are plenty of Hawking quotes out there that seem to suggest that time travel is possible, or at least that it’s not totally impossible.  This seems odd to me, because when you read Hawking’s actual research, he is about as anti-time travel as a physicist can get.

As we discussed in last week’s episode of Sciency Words, Einstein’s theory of general relativity would apparently allow time travel to occur.  Relativity permits space-time to twist around itself into something called a “closed timelike curve.”  Hawking could not allow that to stand, and in 1991 he published this paper introducing something he named the “chronological protection conjecture.”

Hawking summarized his conjecture as follows: “The laws of physics do not allow the appearance of closed timelike curves.”  If a closed timelike curve ever did start to form, Hawking goes on to explain, then some other physical law—vacuum polarization, repulsive gravity, quantum effects—would get in the way, causing the closed timelike curve to die before it was ever truly born.

Based on my read of Hawking’s paper, it sounds like a closed timelike curve might (might!) still be possible inside a black hole.  But if you’re a time traveler trapped inside a black hole, you can’t do much to interfere with the course of history, can you?  Thus, regardless of what may or may not be happening inside black holes, the rest of the universe is still safe from time travel paradoxes.

So if Hawking’s physics is so adamantly against closed timelike curves, why did Hawking make so many public statements teasing us with the possibility of time travel?  Well, Hawking was a big fan of science fiction, and he seems to have loved many of the usual Sci-Fi tropes, including time travel.  The laws of physics may not allow for time travel, according to Hawking, but stories about time travel are still fun.  Maybe Hawking didn’t want to take that fun away from us.