Legacy

Galileo took the metaphysics out of physics, and so begins the story that will
unfold in the remaining chapters of this book. As Stephen Hawking writes, “Galileo,
perhaps more than any single person, was responsible for the birth of modern
science. . . . Galileo was one of the first to argue that man could hope to
understand how the world works, and, moreover, that he could do this by observing
the real world.” No practicing physicist, or any other scientist for that
matter, can do his or her work without following this Galilean advice.
I have already mentioned many of Galileo’s specific achievements. His work
in mechanics is worth sketching again, however, because it paved the way for
his greatest successor. (Galileo died in January 1642. On Christmas Day of that
same year, Isaac Newton was born.) Galileo’s mechanics is largely concerned with
bodies moving at constant velocity or under constant acceleration, usually that
of gravity. In our view, the theorems that define his mechanics are based on the
equations v gt and , but Galileo did not write these, or any other, algt2
s
2
gebraic equations; for his numerical calculations he invoked ratios and proportionality.
He saw that projectile motion was a resultant of a vertical component
governed by the acceleration of gravity and a constant horizontal component
given to the projectile when it was launched. This was an early recognition that
physical quantities with direction, now called “vectors,” could be resolved into
rectangular components.
I have mentioned, but not emphasized, another building block of Galileo’s
mechanics, what is now called the “inertia principle.” In one version, Galileo
put it this way: “Imagine any particle projected along a horizontal plane without
friction; then we know . . . that this particle will move along this plane with a
motion which is uniform and perpetual, provided the plane has no limits.” This
statement reflects Galileo’s genius for abstracting a fundamental idealization from
real behavior. If you give a real ball a push on a real horizontal plane, it will not
continue its motion perpetually, because neither the ball nor the plane is perfectly
smooth, and sooner or later the ball will stop because of frictional effects.
Galileo neglected all the complexities of friction and obtained a useful postulate
for his mechanics. He then applied the postulate in his treatment of projectile
motion. When a projectile is launched, its horizontal component of motion is
constant in the absence of air resistance, and remains that way, while the vertical
component is influenced by gravity.
Galileo’s mechanics did not include definitions of the concepts of force or
energy, both of which became important in the mechanics of his successors. He
had no way to measure these quantities, so he included them only in a qualitative
way. Galileo’s science of motion contains most of the ingredients of what we now
call “kinematics.” It shows us how motion occurs without defining the forces
that control the motion. With the forces included, as in Newton’s mechanics,
kinematics becomes “dynamics.”
All of these specific Galilean contributions to the science of mechanics were
essential to Newton and his successors. But transcending all his other contributions
was Galileo’s unrelenting insistence that the success or failure of a scientific
theory depends on observations and measurements. Stillman Drake leaves us
with this trenchant synopsis of Galileo’s scientific contributions: “When Galileowas born, two thousand years of physics had not resulted in even rough measurements
of actual motions. It is a striking fact that the history of each science
shows continuity back to its first use of measurement, before which it exhibits
no ancestry but metaphysics. That explains why Galileo’s science was stoutly
opposed by nearly every philosopher of his time, he having made it as nearly
free from metaphysics as he could. That was achieved by measurements, made
as precisely as possible with means available to Galileo or that he managed to
devise.”