Our goal in this book is to describe Einstein's theory of space and time in the simplest way possible, while simultaneously revealing its profound beauty. Ultimately, this will allow us to arrive at his famous equation E = mc² using mathematics no more complicated than the Pythagorean theorem. And you don't need to worry if you can't remember the Pythagorean theorem, because we will explain that as well. An equally important aim of ours is for every reader of this small book to understand how modern physicists think about nature and how they build theories that turn out to be astonishingly useful and ultimately change our lives. By constructing a model of space and time, Einstein paved the way for us to understand how stars shine, revealed the deeper reason why electric motors and generators work, and ultimately laid the foundation upon which all modern physics rests. This book also intends to be challenging and demanding. The subject is not physics itself: as we will discover throughout the book, Einstein's theories are very well documented and supported by countless experimental data. We should emphasize, however, that in due course, Einstein’s theory might necessarily give way to an even more accurate picture of nature. In the natural sciences, there are no universal truths, only worldviews that have not yet been proven wrong. The only thing we can say with certainty is that, for now, Einstein’s theory works. The challenge mentioned above lies in how science calls us to think about the world around us. Each of us, whether we have studied a natural science or not, has some intuitive understanding, and we all draw conclusions about the world from our everyday experiences. But if we subject our observations to the cold and precise light of the scientific method, we often discover that nature overturns our intuition. Over the course of this book, we will discover that when things move at high speeds, the common-sense notions of space and time collapse and are replaced by something entirely new, unexpected, and elegant. The lesson that emerges is beneficial and enlightening, leaving many scientists with a sense of awe: The universe is much richer than our everyday experiences might make us believe. Perhaps the most wonderful thing of all is the fact that the new physics, despite its richness, is governed by a magical mathematical elegance.
No matter how difficult it sometimes seems, the science of physics is fundamentally not a complicated subject. One might even say it is an effort to eliminate our innate prejudices in order to observe the world as objectively as possible. Although it may be more or less successful in this aim, few can dispute its success in teaching us how the universe “works.” The truly difficult part is learning not to trust what we might prefer to consider common sense. By teaching us to accept nature as it is, and not as our prejudice might suggest it should be, the scientific method has shaped the modern technological world. In short, it works.
In the first half of the book, we will derive the equation E = mc². By saying we will “derive,” we mean that we will show how Einstein reached the conclusion that energy equals mass times the speed of light squared, which is what the equation tells us. If you think about this relationship for a moment, it will seem very strange. Perhaps the most familiar kind of energy is kinetic energy; if someone throws a cricket ball at your face, it will hurt when it hits you. A physicist would say the reason is that the thrower gave energy to the cricket ball, and this energy is transferred to your face when it stops the ball. Mass is a measure of how much matter an object contains. A cricket ball has more mass than a ping pong ball but less than a planet. What the relationship E = mc² says is that energy and mass can be converted into each other, much like dollars can be exchanged for euros, and that the “exchange rate” factor is the speed of light squared. How on earth could Einstein arrive at this conclusion, and how can the speed of light appear in an equation about the relationship between energy and mass? On our way to this equation, we assume no prior scientific knowledge and avoid mathematics as much as possible. However, our goal is to offer the reader a genuine explanation (and not just a description) of science. From this perspective especially, we hope to offer something new.
In the final part of the book, we will see how the equation E = mc² forms the basis of our understanding of the mechanisms of the universe. Why do stars shine? Why is nuclear energy so much more efficient than coal or oil? What is mass? This question will lead us to the world of modern particle physics, to the Large Hadron Collider at CERN in Geneva, and to the hunt for the Higgs particle which is likely to lead to an explanation for the very origin of mass [see Editor’s Note on p. 162]. The book ends with Einstein’s astonishing discovery that the structure of space and time is ultimately responsible for the force of gravity, and the strange idea that the Earth falls “in a straight line” around the Sun.
