Laplace's Demon

Are not two sparrows sold for a farthing? and one of them shall not fall on the ground without your Father. But the very hairs of your head are all numbered. (Matthew 10:29-30)

A friend was scheduled to attend a business seminar on the morning of 9/11 at the Windows on the World, a restaurant on the 106th floor of the World Trade Center. At the last minute he decided to stop at his office around the corner to check his e-mail. He was still at his desk when his assistant rushed in to tell him a plane had struck the North Tower, where the restaurant was located. My friend has been haunted ever since about what almost happened that morning. But if the physicist and mathematician Pierre Simon de Laplace is to be believed, there was not the remotest likelihood that my friend would have died in the World Trade Center on 9/11, based on a thought experiment known as Laplace’s Demon.

Laplace was one of the greatest scientists of his day, solving problems in celestial mechanics that had stumped Isaac Newton a century earlier. Newton, of course, had formulated the laws of motion, as well as the law of gravitation, which he used to calculate the gross movements of the planets in the solar system. However, the complex gravitational effects of one planet upon another were beyond him, and he concluded that divine intervention was required from time to time to keep things running on an even keel. Laplace set out to prove that no divine intervention was needed to account for the movement of the heavenly bodies, and he was able to do so using differential calculus, which Newton had invented. Laplace presented his findings to his patron Napolean Bonaparte, who asked why his work made no mention of the Creator’s role. Laplace is said to have replied, “I had no need of that hypothesis.”

Laplace’s Demon is an outgrowth of his belief that natural phenomena are entirely explainable by scientific laws. If you knew the position, direction and velocity of every particle in the universe, he reasoned, you could determine their state at any other time, past or future, using the laws of classical physics alone. Every action, including our thoughts and intentions, would be precisely determined from the beginning of time until its end. This is why my friend had no cause to be alarmed about his close call on 9/11, since he was destined from the beginning of time to stop off at his office to catch up on his e-mail that morning and would always be at his desk when the plane struck the North Tower.

The scientific community today lends little credence to Laplace’s theory of absolute determinism. The second law of thermodynamics, which deals with entropy, put an end to the notion that physical processes were reversible, since entropy increases over time, and information is lost. The result is that you cannot reconstruct past conditions by knowing the position, direction and velocity of every particle in the universe. Quantum physicists later found that elementary particles cannot have a fixed position and velocity, at least not at the same time, which meant that future conditions were a matter of probabilities rather than precise determination. So it turns out that knowing everything there is to know about current conditions tells you nothing about either the past or the future.

Laplace’s theory was predicated on the assumption that an intelligence of infinite capacity could know everything about the current state of the universe. This became the “demon” of his thought experiment, although Laplace himself never called it such. He had set out to prove that the universe could function solely according to the laws of nature, without any divine hocus-pocus. The trouble was that an intelligent agent capable of knowing all there was to know about the position, direction and velocity of every particle in the universe would require more computing power than existed in the universe. Any intelligent agent operating in the universe would be subject to entropy, meaning that information would be lost. But what if an omniscient intelligence operated outside the physical universe? We would not normally call such an entity a demon; we would call it God. And Laplace had once boasted that he had no need of such a hypothesis. Perhaps he might want to reconsider.

Pierre Simon Laplace, A Philosophical Essay on Probabilities

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