Mechanics, Planetary Motion and the Modern Science Revolution 65
two masses for all bodies, these two effects always cancel so that all
masses fall at the same rate.
The equality of the gravitational and inertial masses could be a
coincidence. However, it most likely reflects a deeper relationship
between the two masses. Ernest Mach in the nineteenth century
suggested that the inertial mass of a body arises as a result of the
gravitational pull of all the other bodies in the universe. According to this
hypothesis if a body found itself alone in the universe it would not have
any inertial mass. The properties of a body depend on all the other
bodies. All things are interconnected. This idea is incorporated in
Einstein’s General Theory of Relativity to be discussed later. But even
Newton’s formulation of gravity and mechanics contains this concept.
Every particle of matter in the universe is gravitationally attracted to
every other particle of matter. This means that you and I are at every
moment in contact with every piece of matter in the universe. We are
pulling every element of the universe towards us and in turn are being
pulled toward each particle of the cosmos. Of course the matter
composing the Earth has the greatest influence upon us because it is
closer to us than the other masses in the universe.
Consideration of tides reveals how a body 400,000 kilometers
away (the Moon), can still exert an influence upon us. It is almost like
magic — the gravitational attraction of matter. Let us consider the Earth
and the Sun separated by 150 million kilometers of empty space and yet
exerting an action upon each other. The reason I describe this interaction
as magic is that if you want to exert a force on a friend, push them in
some direction for example, you would consider some way of making
contact with them. Either you would touch them directly with your
hands, or with some object in your hands such as a pole, or perhaps you
would throw something at them to force them to move in some direction.
If you could physically make your friend move by merely looking at
them without making some type of physical contact, you would be
considered a magician. And yet, we accept the idea that the Sun can
affect the motion of the planets without touching them, i.e. produce an
action at a distance. There is a magic there alight.
There is no way we can explain this action at a distance property of
the gravitational interaction other than to say this is the way matter
behaves. The only serious criticism made of Newton’s theory of motion
and gravitation was of this principle of action at a distance. Newton,
however, never claimed to have explained gravity. He only described