Isaac Newton realized that the laws of motion he had discovered, including the Law of Universal Gravitation, could explain, at least in principle, the behavior of all physical objects in the universe known at the time. These physical laws were so powerful they allowed scientists of the time to calculate with accuracy the movements of bodies in the solar system.
In Newton’s mind, the universe resembled a clockwork mechanism. Once built and wound up by God, it wouldn’t need God’s intervention at all, since it would tick along with its gears governed only by the laws of physics. The Clockwork Universe Theory leads to the inescapable philosophical conclusion that in such a mechanistic universe, all things have already been set in motion and are just parts of a predictable machine. Furthermore, since our conscious thoughts are simply the result of physical and chemical interactions between the particles that make the matter in our brains, free will is deemed to be an illusion in a clockwork universe. We are simply observers of events that must happen—our choices are simply preprogrammed events meant to happen from the time the clock was wound.
French mathematician and astronomer Pierre-Simon Laplace stated these ideas as the philosophical concept of Determinism. In his words:
We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.
Laplace was imagining what we could now think of as some awesomely powerful computer capable of tracking the position and momentum of every single particle in the universe. Such a computer (known as “Laplace’s Demon”) would be able to predict the state of the universe at any given time in the future based on simply knowing the position and momentum of all particles at a certain time. Laplace understood that measuring and tracking the position and momentum of every particle in the universe would always be beyond the grasp of human ability, but he saw this merely as a technical limitation, not as a theoretical constraint that would allow humans to have free will.
Let’s explore this concept a bit further. Imagine that you have a pool table, and that two balls sit on it. You use a precise laser scanner to measure the position of the balls very exactly. You then hit one of the balls with a pool cue that has been fitted with very accurate force sensors. You also know about the friction between the balls and the pool table’s cloth, as well as about the springiness of the table’s edges. You would surely agree that a computer fed with all this accurate information can use Newton’s laws to calculate the exact position of the balls at any time in the future. Adding one or more balls, or adding forces (e.g., air resistance) complicates the calculations, but does not prevent us—at least in principle—from predicting the future position of all the balls on the table. Divine intervention and consciousness will have no place in these calculations.§
Today, space travel owes its tremendous success to the power of these ideas. Indeed, NASA can calculate in advance the exact trajectories its spacecraft must take to rendezvous with distant planets decades after the spacecraft are launched. It is no surprise then that physicists embrace the concept of Determinism and find it very hard to let go of the idea that each particle in the universe must follow a path determined solely by physical laws that govern its motion.
Yet, in 1927, Determinism would be challenged. Heisenberg and Bohr believed that the Uncertainty Principle is a deeply rooted property of nature, limiting in principle what can be known of particles in the universe. As such, Bohr, and his school of quantum physics centered in Copenhagen, believed that even with the most unrealistically accurate measurement instruments it is impossible—even in principle—to determine the state of a system, and that all that one could ever hope to do is to calculate the probability of observing an outcome from an experiment. That is, in a quantum pool table, it is impossible to calculate the exact position of the balls at any time in the future. Even with the most accurate measurement systems and computing power in the universe, all we could ever hope for is to calculate the probability of possible future positions for the quantum balls. By arguing that large objects are made of quantum particles, one can see that the Copenhagen Interpretation deals a death blow to Determinism.
On the opposite end, Einstein could not accept the idea that reality at its deepest level would not follow a set of simple physical laws. He believed that the uncertainty posed by Heisenberg is a reflection of our ignorance of some fundamental property of reality, but not a property of reality itself. This is summarized in a famous quote by Einstein that is paraphrased as “God doesn’t play dice with the Universe.¶ Einstein wasn’t talking about belief in the will of a Creator,|| but rather about nature not following a deterministic path, regardless of its level of complexity.
Einstein was not alone. De Broglie interpreted his matter waves as somehow guiding the motion of particles. This can be compared to the way a surfer rides an ocean wave—we can’t see the wave, but the particle indeed feels its effects. This view, espoused by Einstein, de Broglie, and many other scientists of the time is known as the Pilot Wave Theory. It was the first type of what is known today as a Hidden-Variables theory, which attempts to interpret quantum mechanics as a deterministic theory, avoiding the troublesome paradoxes (such as “Schrödinger’s Cat”).
Modern experiments have shown that the universe is indeed a very strange place, and it seems Einstein was wrong about Bohr’s interpretation of quantum mechanics. It is very likely that the Uncertainty Principle and wave–particle duality are inherent features of nature, and that there is no way around them. Not for us mortals, and not for any superior intelligence. God does seem to play dice with the universe. Please note that the Copenhagen Interpretation demolishes classical Determinism by introducing true randomness into the machine. However, it does not restore a place for free will, which remains an elusive question best left for philosophers to ponder.
Nevertheless, quantum physics does make some exact predictions. For example, it predicts precisely the energy levels of atomic electrons in absolute agreement with experimental measurements. What quantum mechanics can’t predict is the precise path and behavior of individual particles, only the probability that they will be found in a certain place at a certain time.
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