How does quantum theory allow a rock to turn suddenly into a duck?
Category: Physics Published: August 23, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Quantum theory does not allow a rock to turn suddenly into a duck. It does not allow any other bizarre transformation to happen either. This idea is a myth perpetuated by people who misunderstand quantum theory. Foundational to quantum theory is the concept of particle uncertainty. It is impossible to know a particle's exact location and exact momentum at the same time. Furthermore, a quantum particle's exact location and exact momentum does not even exist at the same time. The uncertainty in quantum theory is not a result of measurement limitations but is inherent to each quantum object itself. The more you try to trap a single electron and therefore force it to have a more defined position, the less defined becomes its momentum. This uncertainty means that quantum particles can end up in unexpected places. The more unexpected its destination, the less likely it is to end up there, but the probability is never zero.
The line or reasoning goes that because a rock is made out of quantum particles (electrons, protons, and neutrons), and because quantum particles can do unexpected things on account of the uncertainty principle, then a rock can do unexpected things like turn suddenly into a duck. According to this line of reasoning, the probability of a rock turning into a duck is very low, so that it almost never happens, but it is physically possible and so it will happen if we wait long enough. This line of reasoning is wrong. It goes wrong in applying the uncertainty principle to large-scale objects. The uncertainty principle applies only to individual quantum objects and small collections of quantum objects. When trillions of quantum objects are involved, the uncertainty goes completely away. The behavior of a trillion atoms is not just the behavior of one atom times a trillion. The reason for this is that the atoms are interacting with each other. As a loose analogy, the behavior of 10 kids at one birthday party is very different from the summed behavior of 10 kids, each alone at 10 different birthday parties. Even the smallest objects visible to human eyes, such as a grain of sand, have trillions upon trillions of atoms. Human-sized objects therefore have no uncertainty and will never do anything bizarre no matter how long you wait. Quantum theory is fundamentally statistical. This means that there is always uncertainty for a single particle, but for a statistically-significant collection of particles, the system predictably obeys the equations of quantum theory.
Quantum uncertainty is not a magic ticket that allows you to break all other laws of physics. The conservation laws are fundamental, universal laws of physics that hold everywhere and cannot be broken. The conservation of mass/energy states that matter/energy cannot be created or destroyed but only transformed from one state into another. The conservation of momentum states that the total momentum of a system is constant. Momentum can not be created or destroyed. If a tiny pebble suddenly transformed into a large duck, it would definitely break the law of conservation of mass/energy as matter would have been created out of nothing. If a rock sitting motionless suddenly shot in the air with no external force applied, it would break the law of conservation of momentum as momentum would have been created out of nothing. The conservation laws of physics and the statistical nature of quantum theory keep the universe ticking forward in a very predictable way despite the uncertainty present in single particles. A rock will never turn into a duck no matter how long you wait.