Science 4 min read

Laplace's Demon Challenged by a New Interpretation of Physics

Weigelstein / Shutterstock.com

Weigelstein / Shutterstock.com

Since Sir Isaac Newton, physicists started viewing and describing the dynamics of bodies in nature differently.

They could use simple equations to predict what will happen accurately. If you know the location of an object and its momentum, you can determine its direction.

Newtonian physics does a pretty good job, but at the right scale, like the human scale. However, to describe too big or too small things, you have to call on relativity and quantum mechanics, respectively.

One of the scientists who built on Newtonian physics principles is French mathematician Pierre-Simon de Laplace (1749-1827). Laplace is credited with being the father of physical determinism.

What is Laplace’s Demon?

Pierre-Simon Laplace is known for the famous concept of “Laplace’s Demon,” which might have religious connotations, but could have similarities to Socrates’ daemon. Laplace’s demon is a thought experiment formulated by Laplace in 1814.

Laplace’s determinism rests on the laws of classical mechanics. It concerns the idea of causality that past events determine the future. Complete knowledge of the past leads to full knowledge of the future.

In this mental exercise, Laplace invites you to imagine an entity (the demon) that knows all the initial positions, mass, and velocity of all the atoms in the universe.

A superintelligence with such knowledge and power would be able to see the universe’s past, present, and future at any given time.

Writes Laplace in his Philosophical Essay on Probabilities:”

“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 any given moment knew all of the forces that animate nature and the mutual positions of the beings that compose it… For such an intellect nothing could be uncertain and the future just like the past would be present before its eyes.”

In Laplace’s Demon world, free will can’t exist. But we won’t rehash all the arguments around determinism vs. free will here.

Maybe Physics Isn’t That Deterministic After all

Laplace’s determinism gained popularity among scientists and philosophers until the late 1800s.

Physicists, astronomers, and mathematicians started encountering seemingly unsolvable problems that Newtonian mechanics couldn’t help with. The infamous math challenge called “three-body problem” is an illustration of the limits of determinism.

Notions like “chaos,” “non-linearity,” and “uncertainty” were getting their due place in physics. This is especially true for microscopic events: enter quantum theory.

Quantum events mesh well with “randomness” more than with determinism. A particle can be here and there at the same time.

But, has classical physics ever been entirely deterministic?

Flavio Del Santo, Vienna Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences and the University of Vienna, and Nicolas Gisin, the University of Geneva, answer this question.

The two researchers published an article in the journal Physical Review A, where they provide alternative interpretations of classical physics.

They criticize the use of additional tacit assumptions.

Say you’re playing a pinball game. Per classical physics, you can correctly determine the path of the ball hitting the pins provided you know its initial position and velocity.

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If you can’t make the ball roll down the board following the exact path once again, that’s only because you can’t set the initial conditions precisely.

According to the alternative model proposed by the researchers, after a certain number of pins, the ball follows a genuinely random path. And that’s not due to the limitations of our measurement instruments.

The ball’s propensity to bounce right or left at each hit is not determined a priori. We can determine the path with certainty for the first few hits, or that propensity is 100% for one side and 0% for the other.

But after a certain number of pins, there’s no predetermination left, and the propensity of the ball gradually shifts for the right and the left.

The authors wrote in their paper:

“The alleged determinism of classical physics relies on the tacit, metaphysical assumption that there exists an actual value of every physical quantity, with its infinite predetermined digits (which we name principle of infinite precision). Building on recent information-theoretic arguments showing that the principle of infinite precision (which translates into the attribution of a physical meaning to mathematical real numbers) leads to unphysical consequences.”

The pair’s work provides new insights on the potential relationship between classical and quantum mechanics.

They propose a model of classical indeterminism based on finite information quantities (FIQs). Explaining how indeterminate values become definite is a long-standing problem in quantum mechanics, known as the “quantum measurement problem.”

Read More: Quantum Darwinism, a New Theory on the Nature of Reality

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Zayan Guedim

Trilingual poet, investigative journalist, and novelist. Zed loves tackling the big existential questions and all-things quantum.

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