# Laplace’s Demon Meets Quantum Mechanics

In 1814, Pierre-Simon Laplace published A Philosophical Essay on Probabilities. He said that given complete knowledge of the present, one could perfectly predict the future:

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 … nothing would be uncertain and the future just like the past would be present before its eyes.

Makes sense, right? If atoms are nothing more than tiny billiard balls bouncing around then, in principle, our lives are as predictable and determined as the activity on a pool table.

A century later, along came quantum mechanics, which showed that the activity of the universe at the smallest scales is probabilistic. Laplace’s Demon (as the all-knowing intellect in the quotation above has come to be known) can’t be so sure after all!

What a relief! There is wiggle room for free will after all! By inserting the lever of our wills at this fulcrum of indeterminacy, we can move the world.

Or, as argued in books such as Modern Physics and Ancient Faith, God can influence what happens without violating the laws of physics.

Except that’s not how it works. Both of these views misunderstand what the indeterminacy of quantum mechanics is.

Quantum mechanics centers on a mathematical function, the wave function, that gives the probability of all possible outcomes of a particle or system of particles over time from any given starting point. And here’s the thing: the unfolding of this wave over time is completely deterministic.

Under one interpretation of quantum mechanics, the actual outcome results from the “collapse of the wave function” when an observer interacts with the system. Think of a quarter spinning perfectly on a table. With your eyes closed, you slap it down, lift your hand and open your eyes. The coin will show heads or tails. Your act of observation (slap and peek) has collapsed the 50/50 probability of heads or tails to a single outcome, but no amount of wishing will make heads more likely. If you disagree, you should stop reading this blog right now, get yourself to a slot-machine parlor, get rich, and come back to tell me how wrong I was.

Under the Everett, or Many Worlds, Interpretation, the probability wave does not collapse. Instead, if you observe a system that has two possible outcomes, your wave function becomes “entangled” with that of the system at two points: the portion of the system’s wave that we might call Outcome A becomes entangled with a portion of your wave function that we might call Observer A, and Outcome B becomes entangled with Observer B. Entanglement means that Observer A is only aware of Outcome A, and the same for Observer B and Outcome B. In a sense, there are now two of you, but that’s not really true. There is still just one wave function, but the entanglement state has changed. Although this is fantastical in the extreme, it is an interpretation favored by many professional physicists, who are perhaps more accustomed than the rest of us to the fact that the universe consists of mathematical abstractions.

In either case, the observer cannot possibly influence the outcome.

What are we to make of this?

It certainly feels like we have free will, but what if we don’t? I would say this: sometimes feelings that exceed reality can be just as enjoyable as those that are more Earth-bound. (Have you ever felt like you could fly?)

### One response to “Laplace’s Demon Meets Quantum Mechanics”

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