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David Darling's Newsletter #14


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August 11, 2003


Contents

1. Meanderings
2. Time Travel

3. Bookends


1. Meanderings

Both our offspring – son and daughter – and my parents live in the UK, so we often make the trek back from Minnesota for family get-togethers. This newsletter is coming to you from a blisteringly hot Cumbria (NW England) two days after a temperature record was set for this green and pleasant land – 100 degrees F in the shade at Heathrow Airport. Anyone who doubts the reality of global warming should look at what has happened to the weather in western Europe over the past few years and, in particular, the past few months in places like France, Italy, and Spain. Not that I'm complaining – anything is better than perpetual grey skies and drizzle!

One of the things I miss about being in the American midwest is the lack of visible human history: buildings and monuments going back hundreds and even thousands of years. So it's always a pleasure to see the old houses, churches, and pubs, the ancient stone circles and earthworks, the castles and manor houses, and Hadrian's Wall marching across the wild, windswept ridges of Northumberland. Treks in the fells (hills and mountains) of the Lake District are another high spot (literally!). On this occasion, together with my daughter and son-in-law, an assault was made on the Langdale Pikes (go here) and we plan a gentler stroll up Silver How near Grasmere (where Wordsworth is buried) over the next few days. I hope to post some pictures of the holiday on the personal pages of my website when we get home.

Being in England at the moment is particularly apt since the main topic for this issue is time travel and, of course, the first great story of a leap into the future was penned by Herbert George Wells of London. Could a device like the Time Machine ever be built? And, if so, what strange consequences might it have?


2. Time Travel

Time travel is easy. We do it all the . . . time. Relentlessly, we ride the wavecrest of now from an ever-growing past into an uncertain (and possibly shrinking) future. The trouble is, we don't seem to be able to control the rate or the direction of that movement as we can the rate or direction of our movement through space. In space, you're free to go back and forth, from side to side, or up and down (though with less freedom, because of gravity) and as far as you like. The same doesn't seem to be true of the fourth dimension of time. But in part that restriction is illusory. Where, in practice, you can get to in space depends on how fast you can move. Put another way, your freedom of movement in space is restricted by speed. This is exactly true of time. Moving around on Earth at comparatively low speeds, we're confined to move through time at the same rate. But step aboard a spaceship that can accelerate to significant fractions of the speed of light and you can progress into the future arbitrarily faster than folks who stay back home. This could be both a boon and a disaster. Zoom up to 99.99999% of light-speed and (ignoring speed-up and slow-down times) you could get to the center of the Galaxy and back to Earth in just a few years, but all your friends and relatives, and even their great-great-great grandchildren, who stayed behind, would be long dead upon your return. More than 50,000 years would have gone by on Earth during your galactic jaunt and you'd be as hopelessly cut off from the world you knew as was the famous Time Traveller (whose name we never learn) in the novel by H. G. Wells. I call this problem "time dislocation" and talk a bit about it on my encyclopedia page here.

You might say this is cheating – using Einstein's theory of relativity to make one-way trips into the future. It is, though, a genuine, if limited, way of moving out of the normal time stream and leaping to any point in the future. Of course, you first have to solve the non-trivial problem of how to get a manned spacecraft up to high percentages of the speed of light. But the physics of relativistic time dilation is well understood and, in the case of fast-moving subatomic particles, has been thoroughly tested and verified.

I don't want to get too much into the possible ways and means of time travel (this may come in a future newsletter when I've perfected my time machine design!), but instead say something about the nature of time and, specifically, the kinds of logical conundrums that arise if certain types of time travel are attempted. Traveling into the future is fairly free from paradoxes because, if you assume that the future has yet to be decided (which is by no means necessarily the case), any journeys into it simply become incorporated into it without affecting what has already happened. Trips into the past, by contrast, can throw up horrendous problems. The most obvious and well-known of these is the "Grandfather Paradox" in which you go back, say 50 years, and accidentally kill your grandfather, thus rendering your birth an impossibility. One way around this mind-bender is to insist categorically that you just can't do this sort of thing – that there's no provision to let it happen or that nature will always conspire to prevent it happening. Stephen Hawking has proposed something along these lines called the "Chronology Protection Conjecture," which argues that quantum mechanical effects will either prevent loops back in time ("closed temporal loops," or CTLs) from forming or will destroy any would-be time traveller who approaches one. Another way of avoiding a logical catastrophe in the face of the Grandfather Paradox is to assume that any attempt to change the past causes the universe to split irrevocably into two distinct branches. Along one of these, the original time-line, you are born and eventually become a time traveller; along the other, one of your direct ancestors is killed (by you!) prematurely and so you immediately disappear and never come into existence. Notice that in neither branch is there a "you" later than the point at which you start your time journey.

Some kinds of trips into the past circumvent the perils of the Grandfather Paradox but may still lead to strange and unexpected outcomes. Most of these weird possibilities have been explored by science fiction writers. In one short story, who's author and title I've forgotten (help anyone?), a would-be time tourist is given strict instructions not to do or touch anything that could affect future events. (In Star Trek this is called the "temporal prime directive".) He then hops back in time to the era of the dinosaurs but accidentally, if memory serves, steps on and kills a small creature. Upon his return to the present, he notices that things are not quite as he remembered them – the entire future history of Earth has been subtly altered. In chaos theory, this is known as the butterfly effect – the idea that the beating of a butterfly's wing somewhere and somewhen will influence future weather on a global scale. Do tiny events always get amplified in this way so that their effects ripple out to change the course of history on a macroscopic level? I doubt it. Small stuff, I'd guess, almost always gets damped out very quickly and very locally so that it has no effect whatever beyond a certain small "radius" of space and time. I'd be willing to place a pretty hefty bet that if you went back a few hundred million years and squashed a bug that otherwise might have lived you'd come back to a present indistinguishable from the one you knew. But obviously the bigger the change you made in the past the greater its future ripple effect would be. Go back and assassinate Hitler in his youth and who knows what the world of 2003 would be like?

Another type of excursion into the past doesn't exactly alter the course of events but, in a curious fashion, becomes an essential part of them. One of the most dramatic and controversial examples in science fiction of this is Michael Moorcock's "Behold the Man". A disturbed individual, obsessed with ideas of suicide and the nature of Jesus, travels back to about A.D. 27 in search of Christ. He subsequently discovers the identity of Jesus and has his death-wish granted in the most dramatic manner imaginable. Although this kind of loop back in time can lead to serious headaches it doesn't plunge us into a genuine paradox because it can be seen as being built into the structure of the spacetime continuum from the outset. I should mention that in the Einsteinian view of the universe (and in the classical, Newtonian view, for that matter), there are no privileged moments in time, such as what we call "now" (a mere artifact of the conscious mind), but instead just an unchanging and unchangeable block universe that stretches out across all of space and time. Each object and person has their own individual wormlike "wordline" that threads through a tiny part of this predetermined expanse of spacetime, beginning at birth and ending at death (theological modifications aside!) George Gamow's entertaining autobiography is called "My Wordline".

I mentioned one monstrosity in time travel – the Grandfather Paradox – that could only be allowed while preserving self-consistency by way of multiple universes. Another such case is the "Free Lunch Paradox". Suppose you go back in time and meet Shakespeare before he's had chance to write any of his famous plays. You hand him the manuscripts to Macbeth, Hamlet, Midsummer Night's Dream, and all the other works of the Great Bard, and encourage him to publish them as if they were his own. He does so and they eventually become accepted as literary masterpieces. You see the problem? Who supplied the creative genius? How did the plays actually arise? The problem only goes away if you allow a division of the time stream into two courses, along one of which Shakespeare really does compose his own material.

The idea of spawning a whole new universe simply through the act of time travel seems preposterous. But the so-called "many worlds" interpretation of quantum mechanics, in which *every* act of observation causes every possible outcome of that observation to be played out in some alternative universe, raises very few eyebrows in modern physics. In fact it is routinely accepted as being true in quantum cosmology and underpins the operation of quantum computers, which are slated to be the next great revolution in information technology. I've only scratched the surface of the many bizarre possibilities that time travel can conjure up. For those who don't mind having their brains tied in knots, I recommend Robert Heinlein's 1941 story "By His Bootstraps" and the 1980 movie "Somewhere in Time", starring Christopher Reeve and Jane Seymour. The latter, in turns out, can only be explained by a violation of the second law of thermodynamics or a dizzying, infinite spiral of parallel realities.


3. Bookends

Some years ago I wrote a children's book about time and time travel as part of my "Could You Ever...?" series, called "Could You Ever Build a Time Machine?" Several of the entries in the third of my encyclopedias, to be published next year by John Wiley and devoted to recreational mathematics, will be on this same subject. Meanwhile, following the first book in this series, The Complete Book of Spaceflight, published at the end of last year, comes the companion volume, "The Universal Book of Astronomy," which will be in print in just a few weeks. Amazon, Barnes & Noble, Powells Books, and others, are already taking on-line orders and the book will be on shelves everywhere by October. My next task is to finish work on the illustrations for the math encyclopedia and then segue in to the newest project – the first comprehensive guide to the story and science behind teleportation. As always, you can find out more about my books, and what else is keeping me busy, on my main page.


Until next time,
Best wishes,

David Darling