Does Uncertainty Make Room for Choice?
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Ever since the late 17th century, when a now-famous, possibly apocryphal apple landed on Sir Isaac Newton’s head, modern science and “free will” philosophy have been at odds. Newton sought to describe the motion of the world around him and came up with sets of equations that did just that. For all of the best measurements available at the time, it appeared that science was over. Given a single initial condition, Newton could predict the evolution of a system of moving objects arbitrarily far into the future. The world was astounded. Soon after, people began to realize that, as far as anyone could prove, they too were just systems of moving objects--which implied that all of their thoughts, words, and actions could be deterministically predicted by Newton’s equations. And thus, free will philosophy was pitted against Newtonian physics.
Variations in Newton’s theory came and went in the next 200 years, each as deterministic as the last. Finally, in the year 1900, theoretical physicist Max Planck published a new theory that would deliver the fatal blow to determinism: quantum mechanics. Planck’s theory held that systems of very small particles evolve according to calculable probabilistic equations but that the final state of the system when measured is actually random. Measuring the position of a small particle is like rolling a pair of dice--you can know what outcomes are more and less likely, but you can never be absolutely sure what you’ll end up with. Experimental measurements in the following years showed time and time again that quantum mechanics was making correct predictions where no other theory could. And so the opponents of determinism rejoiced.
Large systems are composed of small systems, so a fundamental inability to predict the evolution of the smallest systems leads to a failure of determinism in all larger systems. Newton’s laws only seem to hold in daily life due to enormous sample sizes. A tennis ball is composed of about 10 25 small particles, all of which, when hit with a tennis racket, are most likely to go across the court. The chance of the whole tennis ball’s path diverging from Newton’s theory is like the chance of flipping a coin and getting tails 10 25 times in a row. Unbelievably miniscule. But, more as a matter of principle, it is still nonzero. And so the philosophical dragon of determinism was vanquished, but randomness took its place.
We can then finally pose the question: Does this quantum randomness open up the door to free will? The answer, as most do, depends on how we define our terms. A relatively common definition for free will is “the power to control our own actions, without the constraint of outside forces.” In turn, we also inspect our definition of randomness. A key distinction arises between the example of flipping a coin and true quantum randomness. If we tried our very hardest, with the best instruments that money can buy, we could predict with 100% certainty how a flipped coin will land (barring the <10 data-preserve-html-node="true" -25 % chance of quantum theory getting involved 1 ). The information predicting the outcome is out there. It is just difficult to find. Quantum mechanics makes a much stricter statement: that information does not exist 2.
With these definitions, we return to the question. Does quantum randomness give us any more possibility of free will than Newtonian physics? Free will seems to require two major pieces. First, there can be no outside force that determines your decisions. Then further, you must actually have the power to determine your decisions. Newtonian physics negates this first piece. If Newton’s equations could fully predict motion, then our actions, the products of motion, are fully predictable and thus determined. Quantum randomness, on the other hand, is unpredictable and thus satisfies the first requirement of free will. But can we say that we have any power over our own decisions? Choosing to make a decision requires at some point that we know what decision we are going to make. If not, then any “choice” we have is like picking marbles from a bag blindfolded--totally out of our control! Quantum randomness then negates the second requirement for free will. If the world is composed of particles that obey quantum mechanics, then we also are composed of those particles, and so our actions are the product of many instances of quantum randomness. If this randomness fundamentally prohibits predictability, then surely we cannot know what decision we are going to make, and so we cannot choose them.
In the end, then, quantum theory is not the god-sent solution that free will philosophers have hoped for. The concept of free will, at least according to the definition above, remains diametrically opposed to all modern science. But fear not ー science is always changing, and new theories may crop up that carve out space for free will. And if they don’t, then we may not be able to choose whether we fear or not anyway.
1 This percentage is more of a callback than anything. The actual value is much lower. With some back-of-the-envelope calculations, assuming there are about 10 24 particles in a coin, and each particle has a 90% chance (incredibly liberal) of deviating significantly from classical predictions, the chance that all of the particles do (or even some appreciable percentage) is on the order of 0.9 (1024) , which is approximately 10 (-1022) .
2 If you find this distinction strange and difficult to prove, you are not alone: see these articles on Bell’s Inequality to be convinced.
https://doi.org/10.1119/1.4823600
https://www.sciencedirect.com/topics/mathematics/bell-inequality