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:


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.

Speaking of time travel, are you a fan of time travel adventure stories?  The kinds of stories you might see on Doctor Who or The Twilight Zone?  Then please check out my new book, The Medusa Effect: A Tomorrow News Network Novella, featuring time traveling news reporter Talie Tappler and her cyborg cameraman, Mr. Cognis.

Sciency Words: Closed Timelike Curves

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


Austrian-born logician and mathematician Kurt Gödel was one of Albert Einstein’s closest friends.  At Princeton’s Institute for Advanced Study, the two were known to take long walks together, discussing all sorts of strange and wonderful things, no doubt.

As science historian James Gleick tells the story in his book Time Travel: A History, Gödel presented Einstein with a very special gift for Einstein’s 70th birthday.  It was the kind of gift only a person like Einstein would appreciate: a series of mathematical calculations.  Specifically, these were calculations based on Einstein’s own theory of general relativity which showed that yes, time travel is possible.

Gödel’s calculations were officially published in this 1949 paper.  Now I won’t try to explain Gödel’s math because a) I don’t really understand it and b) it’s not really important for the purposes of a Sciency Words post.  What is important for our purposes is that Gödel’s 1949 paper introduced a new concept called “closed timelike curves.”

Well, technically speaking, Gödel used the term “closed time-like lines,” not “closed timelike curves.”  But as Google ngrams shows us, the hyphen quickly dropped out of “time-like,” and by the 1990’s, “curves” beat out “lines.”  So today, closed timelike curves is the most broadly accepted way to say what Gödel was trying to say.  The term is also commonly abbreviated at C.T.C.

In short, a closed timelike curve is a path through space and time that circles back to its own beginning.  As I understand it, it would take a stupendous amount of force to twist space-time around itself in this way.  You’d need the extreme gravitational force of a black hole—or perhaps something even more extreme than that—in order to make a closed timelike curve happen.

But it could happen.  As Gödel demonstrated in 1949, general relativity would allow a closed timelike curve to exist, or at least relativity does not forbid such things from existing.

So time travel is possible.  It may not be anywhere near practical, but it is at least possible.

Speaking of time travel, are you a fan of time travel adventure stories?  The kinds of stories you might see on Doctor Who or The Twilight Zone?  Then please check out my new book, The Medusa Effect: A Tomorrow News Network Novella, featuring time traveling news reporter Talie Tappler and her cyborg cameraman, Mr. Cognis.

Sciency Words: Time’s Arrow

Hello, friends, and welcome back to Sciency Words!  Each week, we take a closer look at the definition and etymology of a science or science-related term.  Today’s Sciency Word is:


Which way is time going?  Prior to the 1890’s, no one would have asked such a silly question.  Time is time.  Everything about time is self-evident.  Why would anyone question it?

But then in 1895, H.G. Wells introduced the concept of time travel to the readers of adventure fiction.  And then in 1915, Albert Einstein started treating time as a variable, rather than a constant, as part of his general theory of relativity.  In his book Time Travel: A History, science historian James Gleick explains:

Millennia had gone by without scientists needing special shorthand like “time’s arrow” to state the obvious—the great thing about time is that it goes on.  Now, however, it was no longer obvious.  Physicists were writing laws of nature in a way that made time directionless, a mere change of sign separating +t from –t.

British astronomer Sir Arthur Eddington gets credit for introducing the “arrow of time” as a conceptual metaphor.  Eddington’s arrow points from the past toward the future.  Unless it doesn’t.  Depending on what sort of physics problem you’re trying to solve (or what sort of Sci-Fi story you’re trying to tell), it may be more convenient to imagine time’s arrow pointing from the future toward the past.

In 1927, in a series of lectures at the University of Edinburgh, and then later in a book titled The Nature of the Physical World, Eddington made three key points about time’s arrow, which I’ll paraphrase as:

  1. Gosh, time’s arrow sure does seem real to us humans.
  2. And common sense reasoning insists that time’s arrow must always point in the same direction.
  3. But when you do the math, you’ll find that none of the laws of physics actually require time’s arrow to exist, except one.

That one exception is the second law of thermodynamics, which tells us that the entropy of a closed thermodynamic system will inevitably increase with the passage of time.  So time’s arrow must always point in the direction of increasing entropy.

Of course a lot of people remain skeptical about time travel.  The Time Machine by H.G. Wells is a fine piece of fiction.  As for general relativity, treating time as a variable (rather than a constant) might help make the math work, but that doesn’t necessarily mean variable time is a real phenomenon.

Still, thanks in larger part to Arthur Eddington and his arrow metaphors, the question “which way is time going?” no longer sounds like total nonsense.

Next time on Planet Pailly: have we discovered a second planet orbiting Proxima Centauri?

Origin Stories: Who Invented Time Travel?

Welcome to Origin Stories, a monthly series here on Planet Pailly where we take a look at the origins of popular Sci-Fi concepts.  Today on Origin Stories, we’re looking at the origins of:


If I ever have a time machine—a real, working time machine—the first thing I’d do is go back in time and meet the person who invented time travel.  We do know who that person was.  His name was H.G. Wells, and he was the author of the classic science fiction novella The Time Machine.

Wells got the inspiration for The Time Machine from an unlikely source.  As science historian James Gleick explains in his book Time Travel: A History:

At some point [Wells] sees a printed advertisement for a contraption called Hacker’s Home Bicycle: a stationary stand with rubber wheels to let a person pedal for exercise without going anywhere.  Anywhere through space, that is.  The wheels go round and time goes by.

Of course there had been time travel-like stories before.  Remember the ghosts of Christmas Past, Present, and Future.  Remember the story of Rip Van Winkle, who found himself suddenly in the future after a really long nap.  Or remember Mark Twain’s A Connecticut Yankee in King Authur’s Court, in which a man from Connecticut gets bonked on the head and wakes up to find himself in the distant past.

But H.G. Wells was the first to take the idea of time travel semi-seriously.  He was the first to try to dress up the idea with scientific and technological jargon.  And in my opinion, no other author has handled time travel so clearly and concisely as Wells did.

The protagonist of The Time Machine, a man of science referred to only as “the Time Traveler,” first explains to a group of friends that we exist in a world of not three dimensions but four.  Everything that exists in this universe has the qualities of “Length, Breadth, Thickness, and—Duration.”  The Time Traveler’s friends then raise all the objections Wells’ readers might have had, and the Time Traveler explains all those objections away in exchanges like this:

“But,” said the Medical Man, staring hard at a coal in the fire, “if Time is really only a fourth dimension of Space, why is it, and why has it always been, regarded as something different?  And why cannot we move in Time as we move about in the other dimensions of Space?”

The Time Traveler smiled.  “Are you sure we can move freely in Space?  Right and left we can go, backward and forward freely enough, and men always have done so.  I admit we move freely in two dimensions.  But how about up and down?  Gravitation limits us there.”

“Not exactly,” said the Medical Man.  “There are balloons.”

“But before the balloons, save for spasmodic jumping and the inequalities of the surface, man had no freedom of vertical movement.”

In other words, we can only move freely in the third dimension thanks to technology—hot air balloons, airplanes, rockets….  Therefore technology may also give us the power to move freely through the fourth dimension of time.

Of course H.G. Wells didn’t actually believe in time travel.  As James Gleick goes on to say, all Wells was trying to do was “gin up a plausible-sounding plot device for a piece of fantastic storytelling.”  But as it would turn out a decade or so later, Wells was not too far off from the truth.  Physicists like Albert Einstein and Hermann Minkowski were soon treating time as variable, rather than a constant.  No, Einstein and Minkowski didn’t build any bicycle-like contraptions in their basements, but the notion of time as the fourth dimension—that soon became serious science.

Time travel has always been my favorite subgenre of science fiction.  It has been ever since my Dad first introduced me to Doctor Who.  I realize time travel isn’t everyone’s cup of tea, but personally I enjoy the kinds of brain-twisting puzzles that a good time travel adventure presents.  It’s the reason I still love Doctor Who, and it’s the reason time travel features so prominently in my own writing.

So if I ever have my own time machine, the first thing I’d do is go back in time to meet H.G. Wells.  I think I owe Mr. Wells a thank you.  

Special Announcement – The Tomorrow News Network

Ladies and gentlemen, friends and loyal readers, I am pleased to announce the beginning of my new writing project.

The Tomorrow News Network is a series of short stories featuring time traveler and journalist Talie Tappler.  She arrives at her stories before they begin, knowing in advance exactly what will happen, who will live, and who will die.  As a journalist, she can only observe and, because of her journalist’s code of ethics, cannot interfere.

I have spent most of my career working in TV News, and this series is partially based on my experiences.  At its heart, The Tomorrow News Network is about the real role of journalists.  Should they simply inform the public without commentary and let the people decide (as Fox News and MSNBC do not), or do they have a responsibility to fight against injustice?  To act as the voice of the people challenging those in power?

I hope you will come visit my new website,, to read these stories.  The first will be posted January 9th, 2012, and a new story will appear each month after that until October.