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Article Excerpt
IN THE EARLY 19005 THE FRENCH POET and novelist, Guillaume Apollinaire, wrote a story entitled "Le Roi Lune" (the Moon King). In his tale a belt with fantastic powers allows its wearer to make love to all women of all centuries. If that isn't "getting around" I don't know what is.
For many decades science-fiction writers have been imagining the possibility of time travel. More recently, science writers, and even physicists, have been addressing the subject. Seriously. For his book and film Contact, Carl Sagan consulted Caltech physicist Kip Thorne, who subsequently developed the idea of using "wormholes" for time travel. Stephen Hawking has devoted many pages to time travel in his popular books. In 1999 the mathematical physicist Paul Davies published his book How to Build a Time Machine (see review this issue). In 2001 Princeton astrophysicist Richard Gott came out-with Time Travel In Einstein's Universe. And in 2002 Scientific American devoted a special issue to "The Matter of Time" that included a not-so-skeptical discussion on time travel by Davies.
From all the high-level interest in time travel you d think that by the year 3000 wormholes would be the standard means by which Pizza Hut delivers its pies. (Their future slogan: "If it doesn't arrive twenty minutes before you order it, it's free.")
Is the hoopla over time travel justified?
To answer that question it is necessary to understand what time travel is--at least the type of time travel physicists contemplate. And to do that--to understand the time travel permitted by the Theory of Relativity--we've got to go over some background, particularly the transition in thinking that occurred when Einstein profoundly amended Newton's model of the universe.
To be brief, we'll focus on the two most important elements of our topic: "speed" and "time." In Newton's universe, speed was understood as relative, but time was absolute. In Einstein's universe one speed, the speed of light, is absolute, while time is now relative. What led to this dramatic turn about?
Batman, Fast Goose, and the S.S. Roswell: Understanding Relativity
Let's begin with the relativity of speed. As a warm-up, consider this riddle:
Batman is driving the Batmobile at 100 mph, flames shooting out the back. He's pursuing his archenemy, the Penguin. The Penguin is somewhere on the streets of Gotham City, behind the wheel of a black oversized, gas-guzzling, diabolic SIN. Thanks to Alfred, the trusty butler, Batman learns of the Penguin's whereabouts. He sends the Batmobile careening around one corner, screeching around another. Batman sets his sight on the Penguin's vehicle, and while doing 80 mph, he hits it. Upon later inspection, we discover that neither vehicle has been visibly damaged. How is this possible?
If you don't know the answer, I'll give you a hint--human beings habitually consider the ground to be the primary, or the default, frame of reference. But there is more than one frame of reference.
Now have you figured it out?
If you answered something like, "Gad zooks, Batman, the Penguin was traveling in the same direction at 78 mph!" consider yourself a recipient of an honorary Robin award.
Motion, and therefore speed, is relative. If we want to know how fast something is traveling, we must remember to ask, "according to what frame of reference?" In the above case, in order to understand the physics of the collision, it doesn't matter what Batman's speed was relative to the ground (80mph). What matters was Batman's speed relative to the Penguin. And that speed (80-78 mph) was 2 mph--which is why no appreciable damage was done to either vehicle. And it is also why the Penguin lived to harass the Dynamic Duo another day.
What follows is another example of the relativity of speed. We'll use this example to illustrate the transition from Newton's to Einstein's model of the universe.
Imagine there is a goose flying north of an airport at 30 mph. This 30 mph is relative to the ground, and, consequently, to the airport control tower. Heading south at 300 mph, straight for the runway--and, unfortunately, the goose--is a Boeing 747. Painted in large, bold letters along the side of the aircraft are the words, "Cuisinart Airlines."
To the question, "At what speed is the goose traveling?" we have to add the question, "According to which frame of reference?" To the customary, default frame--meaning the earth and the control tower--the goose's speed is 30 mph. But according to the goose, the goose's speed is zero. Relative to the goose, the goose forever remains at x = 0, y = and z = 0. According to...
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