If the laws of physics are reversible, why can we change the future - but not the past? Why is it virtually impossible to unscramble an egg, or unstir the cream in our cup of coffee? Why does the arrow of time move in only one direction?
In the world of "Lost," the arrow of time gets tied up in knots: A mother shoots her time-traveling son before he was born ... A plane-crash survivor nearly kills a kid who grows up to enlist him as an assassin. Virtually everyone on the island gets zapped from the present to the past and the future when somebody pushes a creaky old wheel through a glowing slice of weird electromagnetic energy.
So far, however, the show hasn't run into the kinds of paradoxes that plague science-fiction tales ranging from "The Twilight Zone" to the latest "Star Trek" movie. Nobody goes back and kills Hitler to head off the Holocaust. Nobody kills his mother before he's born. As one character says, "Whatever happened, happened."
That's a refreshing perspective, says Caltech theoretical physicist Sean Carroll, the author of "From Eternity to Here."
"I'm surprised at how well time travel is treated in 'Lost,'" Carroll told me today. "It acts as a narrative illustration of what I say in the book. ... At a deeper level, they get the message: The reason why time travel affects us on a visceral level is because it touches on this idea of destiny versus choice."
It's all about the entropy
The way Carroll sees it, the mysteries of time - including the permanence of the past, as well as predestination versus free will in the future - focus on the cosmic concept known as entropy. Entropy is generally defined as the progression from order to disorder, from useful energy to useless energy equilibrium, from an ice cube to a puddle of lukewarm water.
Our universe appears to moving irrevocably from a state of low entropy (the big bang) to high entropy (the Big Chill). That means the past has less entropy than the future ... and that, in turn, means you can't go back and change things. "The way the laws of physics work, we're never going to experience anything going backward in time from our own point of view," Carroll said.
Sure, you can refreeze the water into an ice cube. Heck, it's theoretically possible to reprocess a scrambled egg, molecule by molecule, so that it looks like a raw egg again. But that requires adding more energy into the system. The total entropy of the system still goes up. It's the law - specifically, the Second Law of Thermodynamics.
Carroll's book isn't so much about the impossibility of time travel as it is about that bigger mystery of entropy. Why does the arrow of time fly in just one direction, and where did that arrow come from, anyway? Does it make sense to talk about what happened before the arrow flew?
Some physicists would say such questions can't be discussed in scientific terms. But Carroll isn't one of those physicists. After explaining in detail what we know about entropy and its relation to the arrow of time, he moves on to the more speculative side of the issue.
"Why do we ever find an egg?" Carroll asked me during our interview. "It's because it's not an isolated system in the universe. ... We should think about the universe like we think about the egg, which is that it came from something else."
Bubble universes ... again!
In the book, Carroll explains why the idea that our universe is just one isolated corner or "bubble" in a larger multiverse makes scientific sense, even though it's based totally on speculation.
The bubble-universe idea has popped up in other contexts as well - for example, to explain why our particular universe seems to work as well as it does. "It's eerily similar to what cosmologists are talking about," Carroll acknowledged. He said the bubble-universe concept is "certainly very far from established" but helps explain some features of the cosmos that scientists have long puzzled over, including the arrow of time.
Carroll suggests that the big bang could have been a cosmological blow-up that started pushing our universe from low to high entropy, and set the arrow of time on its flight path. Other bubble universes might have gone in different directions. "These universes would be created both 'forward' and 'backward' in time," Carroll said.
If we could see a high-to-low-entropy universe in action, it might look to us as if the world was running in reverse. But that's a completely theoretical "if," because every universe is completely cut off from the others.
Or are they?
"There are models out there that make predictions that haven't yet been strongly tested," Carroll said. "Pockets of bubble universes might have bumped into each other and left a spot."
Some physicists think the marks left behind by those bumps just might show up in a detailed map of cosmic microwave background radiation - perhaps even the one currently being made by the European Space Agency's Planck satellite. (However, the latest analysis of data from the Wilkinson Microwave Anisotropy Probe has not found any anomalies, as reported by Columbia mathematical physicist Peter Woit on Not Even Wrong.)
Carroll is also looking forward to seeing results from Europe's Large Hadron Collider, which is due to begin its science campaign in earnest this month. Those results may not immediately shed new light directly on the arrow-of-time question, but they will provide more grist for Carroll and his fellow theorists to chew over.
"One of the best things that could come out of the Large Hadron Collider is a better understanding of space-time," Carroll said. "Finding new particles is great, but particles are interesting because they teach us something deeper than that."
Another wrinkle in time
While we're on the subject of depth, I should mention that Carroll's 438-page book delves deeply into the details of quantum theory, thermodynamics and thought experiments such as the Boltzmann brain paradox. If you're looking for something a bit less scientifically rigorous than "From Eternity to Here" but more rigorous than "Lost," you might want to page through "In Search of Time," Canadian science writer Dan Falk's survey of the various concepts of time through the centuries.
Falk's book touches not only on the arrow(s) of time, but also on the history of timekeeping, the neuroscience of time perception and much more. Sir Martin Rees, Britain's Astronomer Royal, says it's "almost unputdownable." Because the book came out a couple of years ago, it qualifies as this month's selection for the Cosmic Log Used Book Club.
For almost eight years, the CLUB Club has been highlighting books with cosmic themes that have been around long enough to turn up at your local library or used-book shop. Send in your CLUB Club recommendation, and if it's picked up as a future monthly selection I'll send you a signed copy of my own book, "The Case for Pluto."