June 3, 2012
I still argue with the voice on my GSP. Don’t look at me that way: You do it, too! The voice has a British accent; we call her Petula. And I still argue when I want to stay on I-95 and she tries to send me onto US-1. But I do have a little more respect for her — or, at least, for the device — now that I’ve read Chad Orzel’s book “How to Teach Relativity to Your Dog.”
Orzel, a professor who also wrote “How to Teach Physics to your Dog,” explains in this more recent book that the technology of global positioning systems relies on a principle of the theories of relativity first formulated by Albert Einstein. A principle of the special theory of relativity holds that a clock that is in motion will “tick” increasingly more slowly as the speed of its motion increases. The general theory of relativity, on the other hand, holds that a clock runs faster the higher it is — that is, the greater its altitude in the gravitational field.
The atomic clocks aboard the GPS satellites speed up because of their altitude and slow down because of the speed at which the satellites orbit the earth. At the altitude of those satellites, the clocks are quickened more than they are slowed, but they are still keeping time at a different rate than clocks on the surface of the earth. Each satellite emits a radio signal with the time on its atomic clock. The GPS unit picks up two or three of those signals, calculates the difference between the time on the satellite clock and the time on the surface of the earth, and uses that information to determine the distance to each satellite and, from that, the location of the unit on the ground.
Pretty cool, huh?
Still, I don’t read about physics because I’m interested in the practical applications so much as because I’m interested in the theories and principles. It can be mind-bending stuff, but if given enough concentration and persistence, it can lead to some moments of enlightenment about how the universe works. And studying the theories of relativity, in particular, can be an eye-opening series of reminders that things are not always what they appear to be.
Orzel’s technique in this book is to explore the special and general theories of relativity, and some other matters, as though he were discussing them with his real-life dog, Emmy. This is the writer’s way of making the material more accessible to people like me, but frankly, it gets tiresome. The dog’s constant references to Orzel as “Dude” and the overworked jokes about Emmy’s appetite, disdain for cats, and fixation with chasing rabbits, grow old pretty quickly. And the premise crumbles as the dog begins to talk about physics as if she were a graduate student at Princeton.
I think anyone who picked up this book thinking that the dialogue with the dog, and the use of dog-world examples, would make physics easier to understand would be disappointed. Orzel’s explanations are clear, but he could have been just as clear without the input from the dog. More important, with or without the dog, a reader won’t get much out of this book without focusing attention on it, frequently stopping to think hard about what Orzel has just written, frequently re-reading paragraphs or whole sections and consulting the glossary at the back of the book.
Both the special and general theories of relativity depend on the idea that the laws of physics work the same for observers who are in motion and observers who are stationary, even though an event — such as a person dropping a ball from above his head to the floor at his feet on a moving train — will appear differently to the person dropping the ball and a person observing the event while standing still on the station platform.
An interesting thing that comes up again and again in Orzel’s book is the fact that researchers are still discovering implications of these theories that Einstein expounded at the beginning of the 20th century. Already Einstein’s work has led to the understanding that the mass of an object is a measure of its energy and the two properties are connected by the constant e=mc²; that time and space are expressions of the same thing; that gravity bends light; that large objects bend space; that a moving object shrinks in length in the direction in which it is moving — the faster it moves, the more it shrinks.
Orzel’s also discusses black holes, those concentrations of mass so dense that even light can’t escape their gravity; the principles behind nuclear energy — both the relatively weak energy that holds atoms together and the enormous energy that can power cities or destroy them; the discovery that the universe is expanding at a constantly increasing rate; and the likelihood that this expanding universe began as a single point that exploded in what we know call the “big bang.”
The dog? I can take or or leave her. But reading this book — some of it two or three times — was worth the energy (which, by the way, equals mass times the speed of light squared)