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The Puzzles of Time Travel

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  • The Puzzles of Time Travel

    THE FABRIC of the COSMOS, Brian Greene, 2004
    ```(annotated and with added bold highlights by Epsilon=One)
    Chapter 15 - Teleporters and Time Machines
    The Puzzles of Time Travel
    There's no denying that life would be different if teleporting macroscopic objects were as easy as calling FedEx or hopping on a subway. Impractical or impossible journeys would become available, and the concept of travel through space would be revolutionized to that rare degree at which a leap in convenience and practicality marks a fundamental shift in worldview.

    Even so, teleportation's impact on our sense of the universe would pale in comparison to the upheaval wrought by achieving volitional travel through time. Everyone knows that with enough effort and dedication we can, at least in principle, get from here to there. Although there are technological limitations on our travels through space, within those constraints our travels are guided by choice and whim. But to get from now to then? Our experiences overwhelmingly attest to there being at most one route: we must wait it out — second must follow second as tick by tock now methodically gives way to then. And this assumes that "then" is later than "now." If then precedes now, experience dictates that there is no route at all; traveling to the past seems not to be an option. Unlike travels through space, travels through time appear to be anything but a matter of choice and whim. When it comes to time, we get dragged along in one direction, whether we like it or not.

    Were we able to navigate time as easily as we navigate space, our worldview would not just change, it would undergo the single most dramatic shift in the history of our species. In light of such undeniable impact, I am often struck by how few people realize that the theoretical underpinnings for one kind of time travel — time travel to the future — have been in place since early last century.

    When Einstein discovered the nature of special relativistic spacetime, he laid out a blueprint for fast-forwarding to the future. If you want to see what's happening on planet earth 1,000, or 10,000, or 10 million years in the future, the laws of Einsteinian physics tell you how to go about it. You build a vehicle whose speed can reach, say 99.9999999996 percent of light speed. At full throttle, you head off into deep space for a day, or ten days, or a little over twenty-seven years according to your ship's clock, then abruptly turn around and head back to earth, again at full throttle. On your return, 1,000, or 10,000, or 10 million years of earth time will have elapsed. This is an undisputed and experimentally verified prediction of special relativity; it is an example of the slowing of time with the increasing of speed described in Chapter 3. 8 Of course, since vehicles of such speed are beyond what we can build, no one has tested these predictions literally. But as we discussed earlier, researchers have confirmed the predicted slowing of time for a commercial airliner, traveling at a small fraction of light speed, as well as that of elementary particles like muons racing through accelerators at very nearly the speed of light (stationary muons decay into other particles in about two millionths of a second, but the faster they travel the slower their internal clock's tick, and so the longer the muons appear to live). There is every reason to believe, and no reason not to believe, that special relativity is correct, and its strategy for reaching the future would work as predicted. Technology, not physics, keeps each of us tethered to this epoch. *

    Thornier issues arise, though, when we think about the other kind of time travel, travel to the past. No doubt you are familiar with some of these. For example, there's the standard scenario in which you travel to the past and prevent your own birth. In many fictional descriptions this is achieved with violence; however, any less drastic but equally effective intervention — such as preventing your parents from meeting — would do just as well. The paradox is clear: if you were never born, how did you come to be, and, in particular, how did you travel to the past and keep your parents from meeting? To travel to the past and keep your parents apart, you had to have been born; but if you were born, traveled to past, and kept your parents apart, you wouldn't have been born. We run headlong into a logical impasse.

    A similar paradox, suggested by the Oxford philosopher Michael Dummett and highlighted by his colleague David Deutsch, teases the brain in a slightly different, perhaps even more baffling way. Here's one version. Imagine I build a time machine and travel ten years into the future. After a quick lunch at Tofu-4-U (the chain that overtook McDonald's after the great mad-cow pandemic put a dent in the public enthusiasm for cheeseburgers), I find the nearest Internet café and get online to see what advances have been made in string theory. And do I get a splendid surprise. I read that all open issues in string theory have been resolved. The theory has been completely worked out and successfully used to explain all known particle properties. Incontrovertible evidence for the extra dimensions has been found, and the theory's predictions of super-symmetric partner particles — their masses, electric charges, and so on — have just been confirmed, spot on, by the Large Hadron Collider. There is no longer any doubt: string theory is the unified theory of the universe.

    When I dig a little deeper to see who is responsible for these great advances, I get an even bigger surprise. The breakthrough paper was written a year earlier by none other than Rita Greene. My mother. I'm shocked. No disrespect intended: my mother is a wonderful person, but she's not a scientist, can't understand why anybody would be a scientist, and, for example, read only a few pages of The Elegant Universe before putting it down, saying it gave her a headache. So how in the world could she have written the key paper in string theory? Well, I read her paper online, am blown away by the simple yet deeply insightful reasoning, and see at the end that she's thanked me for years of intense instruction in mathematics and physics after a Tony Robbins seminar persuaded her to overcome her fears and pursue her inner physicist. Yikes, I think. She'd just enrolled in that seminar when I embarked on my trip to the future. I'd better head back to my own time to begin the instruction.

    Well, I go back in time and begin to tutor my mother in string theory. But it's not going well. A year goes by. Then two. And although she's trying hard, she's just not getting it. I'm starting to worry. We stay at it for another couple of years, but progress is minimal. Now I'm really worried. There is not much time left before her paper is supposed to appear. How is she going to write it? Finally, I make the big decision. When I read her paper in the future, it left such an impression on me that I remember it clear as day. And so, instead of having her discover it on her own —something that's looking less and less likely — I tell her what to write, making sure she includes everything exactly as I remember reading it. She releases the paper, and in short order it sets the physics world on fire. All that I read about during my time in the future comes to pass.

    Now here's the puzzling issue. Who should get the credit for my mother's groundbreaking paper? I certainly shouldn't. I learned of the results by reading them in her paper. Yet how can my mother take credit, when she wrote only what I told her to? Of course, the issue here is not really one of credit — it's the issue of where the new knowledge, new insights, and new understanding presented in my mother's paper came from. To what can I point and say, "This person or this computer came up with the new results"? I didn't have the insights, nor did my mother, there wasn't anyone else involved, and we didn't use a computer. Nevertheless, somehow these brilliant results are all in her paper. Apparently, in a world that allows time travel both to the future and to the past, knowledge can materialize out of thin air. Although not quite as paradoxical as preventing your own birth, this is positively weird.

    What should we make of such paradox and weirdness? Should we conclude that while time travel to the future is allowed by the laws of physics, any attempt to return to the past must fail? Some have certainly thought so. But, as we'll now see, there are ways around the tricky issues we've come upon. This doesn't mean that travel to the past is possible — that's a separate issue we'll consider shortly — but it does show that travel back in time can't be ruled out merely by invoking the puzzles we've just discussed.
    * The fragility of the human body is another practical limitation: the acceleration required to reach such high speeds in a reasonable length of time is well beyond what the body can withstand. Note, too, that the slowing of time gives a strategy, in principle, for reaching distant locations in space. If a rocket were to leave earth and head for the the Andromeda galaxy, traveling at 99.999999999999999999 percent of light speed, we'd have to wait nearly 6 million years for it to return. But at that speed, time on the rocket slows down relative to time on earth so dramatically that upon returning the astronaut would have aged only eight hours (setting aside the fact that he or she couldn't have survived the acceleration to get up to speed, turn back, and finally stop).
    Last edited by Reviewer; 10-14-2012, 10:17 PM.