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.

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:

CLOSED TIMELIKE CURVES

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: Gravity Waves vs. Gravitational Waves

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today, we’re looking at two terms that have almost nothing to do with each other:

GRAVITY WAVES

and

GRAVITATIONAL WAVES

What happens when you combine a 29 solar mass black hole with a 36 solar mass back hole?

Fb08 Black Hole

In this not-so-hypothetical scenario, 29 solar masses plus 36 solar masses equals 62 solar masses. The remaining 3 solar masses are converted into energy in the form of gravity waves. I mean gravitational waves.

I’ve been making this mistake a lot lately, ever since LIGO announced that it had detected gravitational waves for the first time. It’s just easier to say gravity waves. It’s two syllables shorter. Unfortunately, gravity waves and gravitational waves are completely different concepts.

What are Gravitational Waves?

Gravitational waves are part of relativistic physics. According to Einstein’s general theory of relativity, gravity bends space-time. Among other things, this bending causes everything from spaceships to planets to even light itself to follow curved trajectories in the presence of a gravitational field.

Extremely massive objects moving rapidly together, such as a pair of co-orbiting neutron stars or, in the case of the recent LIGO discovery, a pair of merging black holes, bend space one way then the other so violently that they produce a rippling effect in the fabric of space-time. We call these ripples gravitational waves.

What are Gravity Waves?

Gravity waves are part of a different field of physics called fluid dynamics. A few years ago, gravity waves were observed in the atmosphere of Venus, most likely due to air masses rising over mountainous terrain and falling down the other side. After these air masses return to their original altitude (return to a “state of equilibrium,” to use the technical lingo), they tend to bob up and down a bit, producing characteristic ripples in the atmosphere around them. We call these ripples gravity waves (specifically, they’re atmospheric gravity waves).

While this phenomenon has been observed on Venus and Titan, it is best understood here on Earth. Gravity waves are known to appear in Earth’s atmosphere, in lakes and oceans, at the interface between the atmosphere and the ocean… basically anywhere you find a fluid or fluid-like medium. Whenever a fluid returns to a state of equilibrium, either due to gravity or buoyancy, you can expect to see gravity waves.

One Wave is Not Like the Other…

Of course if you’re having a casual conversation about the LIGO experiment (who doesn’t have casual conversations about experiments in relativistic physics?) and you mistakenly say gravity wave instead of gravitational wave, I doubt anyone will be confused. Nine times out of ten, context will make it clear which kind of wave you meant. Just so long as somewhere in the back of your mind, you know there is a difference.