com Secrets of the Universe b y Pa u l F l e i s h e r s L e r n e r P u b l i c a t i o n s Co m p a n y • M i n n e a p o l i s www.com For India Copyright © 2002 by Paul Fleisher All rights reserved. International copyright secured. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means—electronic, mechanical, photocopying, recording, or otherwise—without the prior written permission of Lerner Publications Company, except for the inclusion of brief quotations in an acknowledged review. The text for this book has been adapted from a single-volume work entitled Secrets of the Universe: Discovering the Universal Laws of Science, by Paul Fleisher, originally published by Atheneum in 1987.
Illustrations by Tim Seeley were commissioned by Lerner Publications Company. New back matter was developed by Lerner Publications Company. Lerner Publications Company A division of Lerner Publishing Group 241 First Avenue North Minneapolis, MN 55401 U. Website address: www.com Library of Congress Cataloging-in-Publication Data Fleisher, Paul.
Relativity and quantum mechanics : principles of modern physics / by Paul Fleisher. — (Secrets of the universe) Includes bibliographical references and index. Relativity (Physics)—Juvenile literature. Quantum theory—Juvenile literature.11–dc21 00-012110 Manufactured in the United States of America 1 2 3 4 5 6 – JR – 07 06 05 04 03 02 www.com Contents Introduction: What Is a Natural Law?.
Conservation of Mass/Energy. The Uncertainty Principle .48 Biographies of Scientists .50 For Further Reading .62 About the Author .com I NTRODUCTION Everyone knows what a law is. It’s a What Is rule that tells people what they must or must not do. Laws tell us that we a Natural shouldn’t drive faster than the legal Law? speed limit, that we must not take someone else’s property, that we must pay taxes on our income each year.
Where do these laws come from? In the United States and other democratic countries, laws are created by elected representatives. These men and women discuss what ideas they think would be fair and useful. Then they vote to decide which ones will actually become laws. But there is another kind of law, a scientific law.
You probably have heard about Albert Einstein’s law of relativ- ity, for example. Among other things, it tells us that nothing in our universe can go faster than the speed of light. Where did that law come from, and what could we do if we decid- ed to change it? The law of relativity is very different from a traffic www.com speed limit or a law that says you must pay your taxes. Speed limits are different in different places.
On many interstate highways drivers can travel 105 kilometers (65 miles) per hour. On crowded city streets they must drive more slowly. But relativity tells us that light travels at exact- ly the same speed no matter where it is or where it came from. In the country or the city, in France, Brazil, the United States, or even in interstellar space, light travels at 300,000 kilometers per second (186,000 miles per second).
Sometimes people break laws. When the speed limit is 88 kph (55 mph), people often drive 97 kph (60 mph) or even faster. But what happens when you try to break the law of relativity? You can’t. Here on Earth, if you accurately meas- ure the speed of light a thousand times, it will always travel at the same rate.
It will never be faster or slower. The law of relativity doesn’t apply just when people are around, either. We know that the law stays in effect whether people are watching or not. The law of relativity is a natu- ral law, or a rule of nature.
Scientists and philosophers have studied events in our world for a long time. They have 7 made careful observations and done many experiments. Introduction And they have found that certain events happen over and over again in a regular, predictable way. You have probably noticed some of these patterns in our world yourself.
A scientific law is a statement that tells how things work in the universe. It describes the way things are, not the way we want them to be. That means a scientific law is not something that can be changed whenever we choose. We can change the speed limit or the tax rate if we think they’re too high or too low.
But no matter how much we want to make light go faster or slower, its speed remains the same. We cannot change it; we can only describe it. A scientist’s job is to describe the laws of nature as accurately and exact- ly as possible. The laws you will read about in this book are universal laws.
That means they are true not only here on Earth, but www.com elsewhere throughout the universe too. The universe includes everything we know to exist: our planet, our solar system, our galaxy, all the other billions of stars and galax- ies, and all the vast empty space in between. All the evi- dence that scientists have gathered about the other planets Relativity and Quantum Mechanics and stars in our universe tells us that the scientific laws that apply here on Earth also apply everywhere else. In the history of science, some laws have been found through the brilliant discoveries of a single person.
The law of relativity, for example, is the result of Albert Einstein’s great flash of individual understanding. But ordinarily, sci- entific laws are discovered through the efforts of many sci- entists, each one building on what others did earlier. When one scientist receives credit for discovering a law, it’s important to remember that many other people also con- tributed to that discovery. Even Einstein’s discovery was based on problems and questions that many other scientists had been working on for years.
Scientific laws do change, on rare occasions. They don’t change because we tell the universe to behave differently. 8 Scientific laws change only if we have new information or more accurate observations. The law changes when scien- tists make new discoveries that show the old law doesn’t describe the universe as well as it should.
Whenever scien- tists agree to a change in the laws of nature, the new law describes events more completely, or more simply and clearly. Relativity is good example of this. In the 1900s, scien- tists had believed that they should be able to measure dif- ferences in the speed of light, depending on whether the light source—a star for example—was moving rapidly toward us or away from us. They kept trying more and more accurate experiments.
But better measurements still didn’t show any difference. The speed of light always measured the same 300,000 kilometers per second. Einstein finally realized that there was nothing wrong with the www. Instead, the speed of light was always the same no matter where or when it was measured.
This idea meant that scientists had to look at many of the laws of the universe in a completely new way that seemed very differ- ent from everyday experience. Natural laws are often written in the language of math- ematics. This allows scientists to be more exact in their descriptions of how things work. For example, you’ve probably heard of Einstein’s equation E = mc2.
It’s one of the most famous equations in science. But don’t let the math fool you. It’s simply a mathematical way of saying that mass (m), or matter, can be changed into energy (E). Writing it this way lets scientists compute the amount of energy contained in a certain amount of matter.
The science of matter and energy and how they behave is called physics. In the hundreds of years that physicists have been studying our universe, they have discovered many natural laws. In this book, you’ll read about several of these great discoveries. There will be some simple exper- iments you can do to see the laws in action.
Read on, and 9 share the fascinating stories of the laws that reveal the Introduction secrets of our universe.com C HAPTER Picture yourself riding down the road 1 Relativity in your family’s car. The speedometer says that you are traveling 80 kilometers (50 miles) per hour. But how fast are you really going? If you look out the window, you’ll see the countryside moving past you at 80 kilometers per hour. But if you look at the person sitting next to you in the car, it looks as if he or she isn’t moving at all.
You’re both sitting perfectly still. Are you really moving or not? If you think about the situation further, it gets even more puzzling. Your car is traveling on the surface of Earth. Earth is rotating on its axis at about 1,700 kilometers (1,000 miles) per hour, and so is everything on it.
Perhaps you are really moving that fast. Earth is traveling around the Sun at a speed of 30 kilometers (about 20 miles) per second. And the solar system is moving through our galaxy at a speed of about 240 kilometers (150 miles) per second. Which is the correct speed for your car? The answer is: It depends on www.com what you’re comparing your speed to.
You can’t measure speed unless you choose something to measure it against. Your car ’s speedometer measures your speed by comparing it to the road, which it considers to be stand- ing still. Suppose you toss a ball up and down as you sit in your car riding down the road. You would see the ball going straight up and down.
But someone standing by the road- side would see something completely different. He or she would see the ball moving forward as it goes up and down. Both of you would be correct, from your own points of view. What the ball is really doing depends on how it is being seen.
11 Relativity The ball appears to move differently, depending on whether you are viewing it from inside or outside the car.com The name for this idea is relativity. Relativity means that what you observe and measure about an event depends on your own point of view as well as the event itself. Observations are relative to the frame of reference, or view- point, of the observer. Relativity also applies to larger Relativity and Quantum Mechanics events in the universe.
For example, we can tell how fast our planet is moving only if we compare it to something else. Imagine a single planet in a completely empty uni- verse. How fast is it moving? In what direction is it going? Unless we can compare it to some other object, those ques- tions are meaningless. Around 1900, a young German physicist named Albert Einstein wondered about relativity.
How does it affect objects traveling at very high speeds? Since light travels very fast—300,000 kilometers per second in a vacuum—Einstein wondered what light waves would look like to a person traveling at the speed of light. He realized that one possible answer might be that the light would seem to be standing still, just as the person sit- ting next to you in the moving car seems to be sitting 12 still. Einstein also realized that answer didn’t make sense. Light is made of waves, and waves must move to exist.
So he decided to explore another possibility. He saw that the speed of light must always be 300,000 kilometers per sec- ond, no matter how fast someone is moving when he or she observes it. Einstein’s law that the speed of light is always con- stant doesn’t seem odd at first. But it doesn’t fit our every- day, commonsense view of nature.