About Time Travel
Is time-travel possible? That depends (as Mr. Clinton might say) on what you mean by "time-travel." In a sense, we're all "time-travelers," since we experience time flowing while we travel from our past to our future. That's the normal, boring sense of time-travel.
But can we zip forward hundreds or thousands of years into our future? Thanks to Einstein, all physicists agree that the answer is yes. All you need is a very fast rocket-ship that can get you up to almost lightspeed. With that, you can go as far into the future as you want. That's an exciting prospect, but . . .
Can you travel backward in time? In theory, you could, via a wormhole. In Einstein's classical theory of general relativity, that is. But quantum mechanics might destabilize it, preventing you from actually going through the wormhole. As of now, the question is not quite decided, but it looks like quantum mechanics may be a show-stopper. So maybe nature has really made the universe "safe for historians," as Stephen Hawking put it. Still, it's a lot of fun to think about, isn't it?
After all, if you could go back in time, maybe you could change something. If so, what would you do? Take advantage of yesterday's stock fluctuations? Bet on last year's Super Bowl? Kill the infant Hitler?
There are a ton of books out there that explain the ideas of modern physics in more or less simple terms. I'd like to say I've read 'em all, but I haven't; there are just too many of the varmints. Here is a list of those I've read that deal most directly with time-travel. The following links will take you down the page to the appropriate review.
Timeline, by Michael Crichton
Hyperspace, by Michio Kaku
Black Holes & Time Warps, by Kip Thorne
Time Machines, Second Edition, by Paul J. Nahin
Timeline
Michael Crichton's technothriller Timeline is an engaging and entertaining read. I expect it'll do for time-travel what Jurassic Park did for chaos theory. Quantum foam and multiverses just escaped from the lab into the cocktail circuit. This will be a great year to be a physicist.
In Timeline, an egomaniac physicist, Robert Doniger, has created the quantum teleportation technology needed to disassemble people in this universe and reassemble them in a parallel universe. One of Doniger's hired archaeologists, Professor Robert Johnston, gets himself captured while on a brief foray into 1357. Doniger sends three of Johnston's grad students and a couple of armed security goons after him. The students are supposed to provide the brains, while the goons provide the brawn. They have 37 hours to get in, find the prof, and get out.
Of course, the goons get killed right away, the return machinery gets wrecked, and the students are temporarily stranded in the middle of a war zone in which civilians are getting raped, beheaded, and worse with reckless speed. (What's worse than getting beheaded? You don't want to know, but it involves a red hot poker and lots of screaming. And it cures hemorhoids FOREVER.) Did I mention that bad-guy Doniger has withheld vital information from the students? He has, but that's a surprise, so I'll say no more.
Crichton's strength has always been his ability to explain weird science in a way that normal people like to read. In Timeline, oddly enough, his science is a bit weak, but his historical depiction of medieval France is superb.
You'll learn how to dress in a doublet, overshirt, hose, goofy shoes, and the all-important dagger, which doubles as a weapon and an eating knife. You'll learn to say, "Thousay trewe," when agreeing with someone. You'll be jammed into a suit of armor, hoisted on a horse, and pushed into a terrifying to-the-death joust with a knight who hates your guts and intends to view them. You'll hear old English, Latin, Occitan (that's medieval French, but you knew that), and a bazillion other languages.
As for the science, well, Crichton adds a note at the end of the book pointing out that time-travel is very definitely a fantasy. This is probably true. The best hope for time-travel is via wormholes, and there are serious problems with stability of wormholes at the quantum level. But if we're going to suspend our disbelief and have a time-travel novel (and why not? -- they're fun) the science should still be right. Here Crichton doesn't manage very well. Oh, he explains some of the hard stuff really well -- quantum interference, multiple universes, teleportation.
Then he botches an easy one, on the issue of whether you can change the past. This is handled very well in Paul Nahin's book Time Machines, which Crichton lists in his bibliography. The answer is no, you can't change the past, though you can influence the past. For the difference, see Nahin's book (or just read my time-travel novel, Transgression, where all is explained). For starters, think about the parallel question: Can you change the future? Of course not; it hasn't happened yet. But you can influence the future.
Crichton gets this question wrong. Totally wrong. Oh, he gets the right answer, but then flubs the explanation. He argues, in essence, that one person can't have much effect on history, using an extended analogy with a baseball game in which you're an observer. Can you change the outcome of the game? He says you can't. He's wrong. Remember the kid in New York a few years ago who grabbed the deep fly ball before the outfielder could catch it? And has Crichton forgotten the butterfly effect from Jurassic Park? If you go to 14th century France and sneeze, you better believe the weather in 20th century New York will be different than it would have been if you hadn't gone.
Besides that, the book has a serious logic error. The professor, Robert Johnston, trapped in the 14th century, sends a crucial message to his 20th century students by leaving a handwritten note to them to be found on the site of the archaeological dig where they're working. OK, so 600 years later, they're working there, find the note, and that convinces them that their prof has really gone back. But no, this can't work. According to Crichton, his physicists are not traveling back in time in our own universe (which he insists is impossible). Rather, they're teleporting to a parallel universe. OK, that's a possibility. But if so, then the note the prof left would be found by grad students 600 years later in that parallel universe. In this universe, there's no note. How can there be? There wasn't any professor in the 14th century in this universe.
OK, enough on technical quibbles. This is an enjoyable book. The plot is stronger than the characters, of whom there are too many. The book starts with a tourist couple in New Mexico, switches to a local doctor and cop, then finally takes us to the headquarters of the evil physicist. Eventually, we meet our protagonists, three of them. Which is two too many. There's rarely time to get to know any of them, because the pace continues relentlessly. The average scene lasts less than two pages. At the end of the scene, one or more characters are left cliff-hanging in mortal danger and the action switches to somebody else. Then a page or two later, that poor SOB is left in mortal danger, and somebody else is on the hot seat. It goes on like that for more than 400 pages. Imagine if John Grisham had Attention Deficit Disorder and you get the picture.
Crichton is good, or this wouldn't work. In the hands of a lesser writer, the machine-gun pace of the plot would drive you nuts. Crichton pulls it off, but this is not his best work.
But fergit all that. If you like Michael Crichton's other books (and who doesn't?), you'll like this one. Buy it. Read it. Enjoy it. Then at the next cocktail party, when the other guy is spouting about quantum foam and multiverses, you can drop his jaw by pointing out that Crichton got the hard parts right but screwed up the easy logic. If you get any guff from your quantum-foaming foe, tell 'im that "causality is not compromised on closed time-like loops in spacetime." Hee, hee! No doubt hordes of attractive members of your preferred gender will come swarming to you when you spout that line.
Browse "Timeline" at Amazon.com.
Hyperspace
Michio Kaku is a well-known theoretical physicist who's made a number of important contributions to elementary particle theory and superstring theory. He also writes very well, and has a knack for storytelling.
In Hyperspace, Kaku dives into the mysteries of the universe. Modern superstring theories are embedded in a ten-dimensional hyperspace. Theorists hope to incorporate all of physics in these theories. Kaku breaks the book into two parts.
In Part I, he breaks the reader into the notion of higher dimensions by discussing a five-dimensional universe. (The usual three spatial dimensions, plus time as the fourth dimension, plus a strangely-curled up fifth dimension that accounts for light.) He tells some of the history of mathematics and physics, with stories of Gauss, Riemann, Einstein, and other great mathematicians and physicists.
In Part II, Kaku begins with an explanation of quantum mechanics, then plunges into the arcane world of supergravity and superstrings. Without a deep background in math, it's impossible to really understand these theories, but Kaku makes a valiant effort, and he certainly captures the main ideas. Next, he moves on to cosmology, black holes, and wormholes. Check out his sketches of a wormhole on pages 229 and 230! He has some fun discussing the paradoxes of backwards time-travel. He also gets into the recent developments in wormhole theory pioneered by Kip Thorne and his students at Caltech (and inspired by Carl Sagan's novel Contact).
Hyperspace is an excellent book. Expect to have your mind stretched. Don't expect to finish the book an expert in modern physics. You'll know the buzzwords, but unless you also spend ten years learning the math, you won't grasp it all. And even if you know the math, you still won't ever fully grasp it all. That's the charm of physics -- there's more to it than you can ever learn.
Browse "Hyperspace" at Amazon.com.
Black Holes & Time Warps
Kip Thorne is a Caltech professor and one of the world's leading experts on Einstein's theory of general relativity. In the late 1980s, he and his students created a new cottage industry studying wormholes -- portals through space and time. It wasn't even really Thorne's idea; Carl Sagan prompted it by asking for a "scientific" way to travel quickly through the universe. Sagan was working on his novel Contact at the time. Thorne wound up discovering many new ideas in wormhole theory. This book is his highly personal account of those discoveries.
It's an amusing book, full of interesting anecdotes about many physicists. Thorne spends the early chapters talking about gravity, relativity, warped spacetime, black holes, white dwarfs -- the usual suspects in modern astronomy and cosmology. There's quite a bit of overlap with Michio Kaku's book Hyperspace, reviewed above. And that's just as well, because most people need to hear this stuff several times before it starts sinking in. And that includes most physicists.
For our time-travel interests, chapter 14 is a jewel. There, Thorne gets into the details of wormholes (the logical details, not the mathematical ones) and shows that backward time-travel through wormholes causes some unexpected logical problems. And not just the usual "can I go back in time and kill my own grandfather" paradoxes. In the presence of wormholes, it becomes impossible to predict the trajectory of even simple things like billiard balls. An infinite number of possible trajectories are allowed! Poor Isaac Newton would turn over in his grave. You'll be amused to see Thorne's picture of a wormhole on page 500 and even more amused to see that it looks very different from the picture in Kaku's book. Thorne's is more accurate; it shows the light-diffusing properties of a wormhole.
Black Holes & Time Warps is a very good book. While it's not quite as readable as Kaku's book Hyperspace, you'll find it complementary in many ways.
Browse "Black Holes & Time Warps" at Amazon.com.
Time Machines
Paul J. Nahin is a professor in the Electrical and Computer Engineering Department of the University of New Hampshire. You might imagine that an electrical engineer wouldn't know much about Einstein's general relativity. You would be wrong. Professor Nahin knows more about the subject than most physicists. He very much would like to see a time machine built. This book is about that dream.
The book is organized into four exceedingly long chapters. The chapters are a very weird and delightful mix of hard physics, philosophy, and summaries of science fiction stories. Nahin seems to have read practically every time-travel story ever written, and he can illustrate practically any good (or bad) idea with half a dozen stories using that idea.
Chapter 1 gives an overview of time travel, and it's a very broad overview. Nahin is most interested in the possibility of backwards time-travel, and so he needs to resolve the paradoxes. Is it logically possible to go back in time? If so, then can one change the past? And if time-travel will eventually be discovered, then where are all the "time-tourists" from the future? Nahin doesn't solve these issues in this chapter, but he brings them all up and promises to wrestle with them later in the book.
Chapter 2 discusses the nature of time, spacetime, and the universe. Is time real? Does the past exist somewhere? What about the future? Is the universe like a giant "block" that God (or some other person "outside" of spacetime) could observe all at once? Can God be omniscient? What does that do to free will? These are interesting questions, and philosophers have given an amazingly wide range of answers. Nahin has his own opinions about who's right and who's wrong and he examines some of these questions in detail.
Chapter 3 moves on to the question of the direction of time. Why does time seem to flow in only one direction? Can there be backward causation? What does "now" mean? Could you imagine a universe in which time flowed backward? What does that mean? What about multidimensional time?
Chapter 4 gets to the really fun stuff, the paradoxes of time-travel. Can we change the past? Nahin doesn't believe we can. Since he hopes that time-travel is possible, this leads him to conclude that time-travelers to the past can affect the past but not change it. That is, their actions may certainly influence the past, but only in such a way as to bring about the past that actually occurred. In other words, the past must be consistent.
Nahin goes on to discuss the grandfather paradox in this light. Why can't you go back in time and kill old Grandad? You could, but you won't, because you didn't. If you had, then you wouldn't be here in the first place.
Causal loops are even more intriguing. Suppose you read a book on how to build a time machine. You build the machine, go back in time, and publish the book -- which later gets read by a younger version of you living in the future. OK, fine, this is all self-consistent. But where did the knowledge of how to build the time-machine come from? Apparently from nowhere. Nahin sees no problem with this kind of paradox, in which something happens without a cause. Big-bangers believe in a Big Bang without a cause, he argues. Quantum mechanics tells us that radioactive decay has no cause. Theologians argue that God has no cause. So where's the paradox with a little book explaining how to travel back in time? You may not buy his argument, but have fun trying to refute it.
Then there are sexual paradoxes. The most elegant of these is given in Robert Heinlein's short story, "All You Zombies --". The story is too long to summarize here, but the upshot is that "Jane" is her own mother, father, lover, and daughter. Bizzare! Where did Jane come from?
If you're technical, you'll enjoy the appendices in Nahin's book. They start out easy and get progressively more difficult until only specialists will follow it all. Enjoy yer meal!
It's all good fun and it stretches your mind, so where's the harm? The ultimate proof of time-travel will come when somebody builds a time machine. Until then, we'll wonder.
Browse "Time Machines" at Amazon.com.
