For the Love of Physics by Walter Lewin Read Free Book Online

(force equals mass times acceleration), perhaps the most important single equation in physics, or Einstein’s E = mc 2 (energy equals mass times the square of the speed of light), the most renowned equation in physics. How else do physicists express relationships except through mathematical equations about measurable quantities such as density, weight, length, charge, gravitational attraction, temperature, or velocity?
    I will admit that I may be a bit biased here, since my PhD research consisted of measuring different kinds of nuclear decay to a high degree of accuracy, and that my contributions in the early years of X-ray astronomy came from my measurements of high-energy X-rays from tens ofthousands of light-years away. But there simply is no physics without measurements. And just as important, there are no meaningful measurements without their uncertainties.
    You count on reasonable amounts of uncertainty all the time, without realizing it. When your bank reports how much money you have in your account, you expect an uncertainty of less than half a penny. When you buy a piece of clothing online, you expect its fit not to vary more than a very small fraction of a size. A pair of size 34 pants that varies just 3 percent changes a full inch in waist size; it could end up a 35 and hang on your hips, or a 33 and make you wonder how you gained all that weight.
    It’s also vital that measurements are expressed in the right units. Take the case of an eleven-year-long mission costing $125 million—the Mars Climate Orbiter—which came to a catastrophic conclusion because of a confusion in units. One engineering team used metric units while another used English ones, and as a result in September 1999 the spacecraft entered the Martian atmosphere instead of reaching a stable orbit.
    In this book I use metric units most of the time because most scientists use them. From time to time, however, I’ll use English units—inches, feet, miles, and pounds—when it seems appropriate for a U.S. audience. For temperature, I’ll use the Celsius or Kelvin (Celsius plus 273.15) scales but sometimes Fahrenheit, even though no physicist works in degrees Fahrenheit.
    My appreciation of the crucial role of measurements in physics is one reason I’m skeptical of theories that can’t be verified by means of measurements. Take string theory, or its souped-up cousin superstring theory, the latest effort of theoreticians to come up with a “theory of everything.” Theoretical physicists, and there are some brilliant ones doing string theory, have yet to come up with a single experiment, a single prediction that could test any of string theory’s propositions. Nothing in string theory can be experimentally verified—at least so far. This means that string theory has no predictive power, which is why some physicists, such as Sheldon Glashow at Harvard, question whether it’s even physics at all.
    However, string theory has some brilliant and eloquent proponents. Brian Greene is one, and his book and PBS program The Elegant Universe (I’m interviewed briefly on it) are charming and beautiful. Edward Witten’s M-theory, which unified five different string theories and posits that there are eleven dimensions of space, of which we lower-order beings see only three, is pretty wild stuff and is intriguing to contemplate.
    But when theory gets way out there, I am reminded of my grandmother, my mother’s mother, a very great lady who had some wonderful sayings and habits that showed her to be quite an intuitive scientist. She used to tell me, for instance, that you are shorter when standing up than when lying down. I love to teach my students about this. On the first day of class I announce to them that in honor of my grandmother, I’m going to bring this outlandish notion to a test. They, of course, are completely bewildered. I can almost see them thinking, “Shorter standing up than lying down? Impossible!”
    Their disbelief is understandable.