Newton’s work touched not only on math and physics, it also had philosophical ramifications. For any system of bodies interacting via the force of gravity, he showed that the present position and velocity (the present state) of each body actually determines what their positions and velocities will be at all times in the future (their future states). This idea is called the principle of causal determinism.
DEFINITION
Generally speaking, the state of an object boils down to all the information you need to explain how the laws of physics will affect it. For billiard balls rolling around a table or satellites orbiting Earth, about all you need is their position and velocity (that is, the speed and direction of travel).
If you really think about it, this means that all future states of the bodies in the system have already been determined by their present states. Much to the moon’s chagrin, Newton demonstrated that celestial bodies did not have free will to move about as they please. Newton’s laws of motion—also known as classical mechanics—forever prescribe their motion!
What’s more, these laws tell us that if we happen to know the present state of each body in a gravitational system, then we can predict their states at all moments in the future. This is the principle of predictive determinism.
DEFINITION
Causal determinism tells us that the present state of a system will determine all future states of the system. Predictive determinism tells us that if the present state of a system is known, then all future states can be predicted.
We use Newton’s laws, along with their power of predictive determinism, to forecast many natural phenomena observed each and every day. Take, for example, the tides. Tides come and go according to the position of the moon relative to Earth. Since the moon is a very massive body, its gravitational attraction is strong enough to noticeably influence the ocean. When the moon is directly above, it draws the ocean upward and we have high tide. Conversely, when the moon is low on the horizon, we have low tide. Since we can use classical mechanics to predict the position of the moon, we can use it to predict when we’ll have high and low tide.
These same laws can be used to predict the arrival of comets, the timing of eclipses, and even—in principle—tomorrow’s weather.
Inspired by Newton’s theories, the British watchmaker George Graham built the first modern orrery in the early 1700s. This mechanical device demonstrates the relative positions of the sun, its planets, and their moons as they all move in their predetermined orbits, like many hands on a giant clock. Newton’s universe was thus a clockwork universe; once wound up and set in motion, the position of all celestial bodies is forever known.
Newton’s laws of motion have been used to describe the motion of everyday objects like planets, billiard balls, and even grains of sand, without fail for centuries. About a hundred years ago, however, physicists began to toy around with things that were much smaller. And as we will see the concepts of determinism become a little less clear in the microscopic realm of quantum physics.
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