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

Certainly if there is any difference in length between lying down and standing up it must be quite small. After all, if it was one foot, you’d know it, wouldn’t you? You’d get out of bed in the morning, you’d stand up and go clunk —you’re one foot shorter. But if the difference was only 0.1 centimeters (1/25 of an inch) you might never know. That’s why I suspect that if my grandmother was right, then the difference is probably only a few centimeters, maybe as much as an inch.
    To conduct my experiment, I of course first need to convince them of the uncertainty in my measurements. So I begin by measuring an aluminum rod vertically—it comes to 150.0 centimeters—and I ask them to agree that I’m probably capable of measuring it with an uncertainty of plus or minus one-tenth of a centimeter. So that vertical measurement is 150.0 ± 0.1 centimeters. I then measure the bar when it’s horizontal and come up with 149.9 ± 0.1 centimeters, which is in agreement—within the uncertainty of the measurements—with the vertical measurement.
    What did I gain by measuring the aluminum rod in both positions? A lot! For one, the two measurements demonstrate that I was able tomeasure length to an accuracy of about 0.1 centimeter (1 millimeter). But at least as important for me is the fact that I want to prove to the students that I’m not playing games with them. Suppose, for example, that I have prepared a specially “cooked” meter stick for my horizontal measurements—that would be a terrible, very dishonest thing to do. By showing that the length of the aluminum rod is the same in the two measurements, I establish that my scientific integrity is beyond doubt.
    I then ask for a volunteer, measure him standing up, write that number on the blackboard—185.2 centimeters (or just over 6 feet), plus or minus 0.1 centimeter of course, to account for the uncertainty. Then I help him lie down on my desk in my measuring equipment, which looks like a giant Ritz Stick, the wooden shoe-store foot-measuring device, only his whole body is the foot. I joke back and forth with him about how comfortable he is and congratulate him on his sacrifice for the sake of science, which makes him just a wee bit uneasy. What have I got up my sleeve? I slide the triangular wooden block snug up against his head, and while he lies there, I write the new number on the board. So we now have two measurements, each uncertain by about 0.1 centimeters. What’s the result?
    Are you surprised to learn that the two measurements differ by 2.5 centimeters, plus or minus 0.2 centimeters of course? I have to conclude that he is in fact at least 2.3 centimeters (or about 0.9 inches) taller while lying down. I go back to my prone student, announce that he’s roughly an inch taller sleeping than standing up, and—this is the best part—declare, “My grandmother was right! She was always right!”
    Are you skeptical? Well, it turns out that my grandmother was a better scientist than most of us. When we are standing, the tug of gravity compresses the soft tissue between the vertebrae of our spines, and when we lie down, our spines expand. This may seem obvious once you know it, but would you have predicted it? In fact, not even the scientists at NASA anticipated this effect in planning the first space missions. The astronauts complained that their suits got tighter when they were in space. Studies done later, during the Skylab mission, showed that ofthe six astronauts who were measured, all six showed about 3 percent growth in height—a little over 2 inches if you’re 6 feet tall. Now astronauts’ suits are made with extra room to allow for this growth.
    See how revealing good measurements can be? In that same class where I prove my grandmother right, I have a lot of fun measuring some very odd items, all in order to test a suggestion of the great Galileo Galilei, the father of modern science and astronomy, who once asked himself the question, “Why are