Knocking on Heaven's Door - Lisa Randall [20]
I better understood the key role Galileo played in developing this way of thinking as I explored Padua and its historical landmarks. The Scrovegni Chapel is one of its most famous sites, housing Giotto’s frescoes from the early fourteenth century. These paintings are notable for many reasons, but to scientists the extremely realistic image of the 1301 passing of Halley’s comet (over the Adoration of the Magi) is a marvel. (See Figure 6.) The comet had been clearly visible to the naked eye at the time the painting was made.
But the images weren’t yet scientific. My tour guide pointed to an astral image in the Palazzo della Ragione that she had initially been told was the Milky Way. She remarked that a more expert guide had afterward explained to her the anachronistic nature of the interpretation. At the time the painting was made, people were just illustrating what they saw. It might have been a starry sky, but it was not anything so well defined as our galaxy. Science, as we understand it today, was yet to arrive.
[ FIGURE 6 ] Giotto painted this scene, which appears in the Scrovegni Chapel, in the early fourteenth century when Halley’s comet was visible to the naked eye.
Before Galileo, science relied on unmediated observations and pure thought. Aristotelian science was the model for the way people had tried to understand the world. Math could be used to make deductions, but the underlying assumptions were taken on faith or in accordance with direct observations.
Galileo explicitly refused to base his research on a “mondo di carta” (a world of paper)—he wanted to read and study the “libro della natura” (the book of nature). In achieving this goal, he changed the methodology of observation and, furthermore, recognized the power of experiments. Galileo understood how to construct and use these artificial situations to make deductions about the nature of physical law. With experiments, Galileo could test hypotheses about the laws of nature that he could prove—and, as importantly, disprove.
Some of his experiments involved inclined planes: the tilted flat surfaces that feature so prominently—and somewhat annoyingly—in every introductory physics text. For Galileo, inclined planes weren’t just some made-up classroom problem, as they sometimes appear to introductory physics students. They were a way to study the velocity of falling bodies by spreading out the descent of objects over a horizontal distance so that he could make careful measurements of how they “fell.” He measured time with a water chronometer, but he also cleverly added bells at specific points so that he could use his gifted musical ear to listen and establish speed as a ball rolled down, as illustrated in Figure 7. Through these and other experiments dealing with motion and gravity, Galileo, along with Johannes Kepler and René Descartes, laid the foundation for the classical mechanical laws that Isaac Newton so famously developed.
Bells Per Unit of Time
[ FIGURE 7 ] Galileo measured how quickly a ball went down an inclined plane, using bells to register their passage.
Galileo’s science also went beyond what he could observe. He created thought experiments—abstractions based on what he did see—in order to make predictions that would apply to experiments no one at the time could actually perform. Perhaps most famous is his prediction that objects—in the absence of resistance—all fall at the same rate. Even though he couldn’t set up the idealized situation, he predicted what would occur. Galileo understood gravity’s role in objects falling toward the Earth, but he also knew that air resistance slows them down. Good science involves understanding all the factors that might enter into a measurement. Thought experiments and actual physical experiments helped him to better understand the nature of gravity.
In an interesting historical coincidence, Newton, one of the greatest physicists to continue this scientific tradition, was born the year that Galileo died. (At a talk Stephen