In Part Five of "Six Easy Pieces," Dr. Feynman tackles gravity, or a theory of gravitation. Perhaps except for high-level physicists, most people think of "gravity" as the
force that keeps objects connected to the earth, and that causes objects to return to earth once projected into the atmosphere.
Feynman's approach to gravity is much broader and more theoretical, as has been the case
with our other readings. This chapter might better be titled "why things move." For it is movement, and lack of movement, that concerns Feynman in Part Five.
Part Five opens with an extraordinarily astute observation:
gravity, or the movement of objects, is one of humankind's great "generalizations." That is, we rarely concern ourselves with the fact that things, and people, move. It is, to us, a given that we and the things around
us are able to move.
But it was not always so. Feynman calls our attention to "the ancients," specifically meaning the Greeks, Egyptians, Babylonians, and others, who were pioneers in the science of movement. Some of
their earliest observations concerned planetary movements, the sun's "movement" through the sky, and the "movement" of the stars.
Think, for a minute, just how much scientific and intellectual power it must have taken
to establish that the planets revolve around the sun. One of the subtexts (underlying, perhaps unintended, messages) of Feynman's discourse on gravity is the value of individual contributions over the very long term.
Feynman faithfully tracks the development of gravitational theory, which is certainly important to this week's reading. In the process, however, he also calls our attention to the importance of early science, and its
relative pace of development.
"The ancients" influenced Copernicus, who influenced Tycho Brahe, who influenced Johannes Kepler, who influenced Newton, who influenced Einstein. And all of them have influenced
generations and generations of great minds, helping us better understand the physical universe, slowly unlocking the keys to God's amazing creation.
There is a sense of "intellectual heritage" that often runs through
scientific—and perhaps social—discovery. We are, truly, as the saying goes, "ants, standing on the shoulders of giants." It is this process of discovery and contribution—the process of building on foundations set up in
earlier generations—that makes scientific progress possible. It is up to each generation, ours included, to make its contributions to the future of science.
All of that brings us back to the nature of gravity!
In
the process of explaining gravitational theory, Feynman articulates several foundational laws of physics. The first of them, the law of gravity, is perhaps the most fundamental:
"…every object in the universe attracts
every other object with a force which for any two bodies is proportional to the mass of each and varies inversely as the square of the distance between them. (p. 89)"
In English?
Every object in the universe is
fundamentally attracted toward every other object. In an earlier lesson (and later in this reading) Feynman points out that electricity is a much stronger force than gravity, because its properties include both
attraction and repulsion (opposites attract, likes repel). If not for electricity, we now understand, gravity would cause every object in the universe to simply collapse into one another.
Further, objects are
attracted to one another based on specific criteria: mass and distance. Put simply, more massive objects are most strongly attracted to one another, less massive objects are less attracted. And the greater the distance
between objects, the less the attraction.
This first principle defines the property of gravitation. It explains why objects remain attracted to the earth. It also uses mass to explain a related concept,
"weight." I am more "attracted" to the earth than is my 95 pound niece. I have greater mass, much greater, unfortunately.
The next law of gravitational theory helps us understand the other key question of Feynman's
lesson: why things move.
"…an object responds to a force by accelerating in the direction of the force by an amount that is inversely proportional to the mass of the object. (p. 89)"
Once again, in English?
Objects move in response to force. Every object at rest remains at rest until acted upon by an object of greater force (a rough paraphrase of Newton).
Specifically, objects move in the direction dictated by the force
brought to bear upon them, and with the velocity resulting from that force. Their actual speed (acceleration) is dictated by the relationship of their own mass to the force brought to bear upon them.
The greater their
mass in comparison to the force brought to bear upon them, the slower they move. The less their mass in comparison to the force brought to bear upon them, the faster they move.
While these laws may seem overly
simplistic, they contain the seeds of understanding some of the greatest of the physical science concepts. They also explain much of what we see in the world around us: why does an 80,000 pound diesel truck burn so much
more fuel than a compact car? Fuel (force) to weight (mass) ratio. The potential examples are almost numberless.
In its broadest application, gravitational theory explains how and why planets and stars move (see
Kepler's Laws and Cavendish's experiments). Feynman points out that, since gravity applies to other planets and the stars, it is indeed a "universal" principle. It applies everywhere in the universe.
On an equally
broad intellectual scale, gravity is, literally, a force that holds the world together. Feynman briefly—and breezily—walks us through a few of the theoretical relationships between gravity and electricity, indicating
that the force might (emphasize might) be inversely proportional at an astronomically sophisticated level.
Most important, though, Feynman, always honest with us, admits that there is yet no "machinery" to explain
gravitation. Even the advent of ever-smaller quantum mechanics has not explained the force behind gravity.
In acknowledging the limitations of science, Feynman again calls our attention to faith. A rhetorical question
emerges: Just what IS the force behind gravity?
