Dr. Christopher S. Baird

# Why do quantum effects only happen on the atomic scale?

Category: Physics      Published: April 22, 2014

Superconductivity, such as is used to levitate magnets, is a macroscopic quantum effect. Public Domain Image, source: NIST.

Quantum effects are not only confined to the atomic scale. There are several examples of macroscopic quantum behavior. Quantum physics describes matter and energy as quantum wavefunctions, which sometimes act like waves and sometimes act like particles, but are actually more complicated entities than just waves or particles. In reality, every object in the universe (from atoms to stars) operates according to quantum physics. In many situations, such as when throwing a baseball, quantum physics leads to the same result as classical physics. In such situations, we use classical physics instead of quantum physics because the mathematics is easier and the principles are more intuitive. The laws of quantum physics are still operating in a baseball thrown across the field, but their operation is not obvious, so we say the system is non-quantum. A situation is described as quantum when its quantum behavior becomes obvious, even though it is really always quantum. A "quantum effect" is therefore an effect that is not properly predicted by classical physics, but is properly predicted by quantum theory. Classical physics describes matter as composed of little, solid particles. Therefore, anytime we get the pieces of matter to act like waves, we are demonstrating a quantum effect. (Classical waves such as sound and sea waves don't count as quantum because the motion is a wave, but the pieces are still little solid balls. In order to be a quantum effect, the particle itself must be acting like a wave.)

While quantum effects are not strictly confined to the atomic scale, they certainly are more common at the atomic scale. Why is this? Let's look at matter. To be a quantum effect, we have to get matter to act like waves. To be a macroscopic quantum effect, we have to get many bits of matter to act like waves in an organized fashion. If all the bits of matter are acting like waves in a random, disjointed manner, then their waves interfere and average away to zero on the macroscopic scale. In physics, we refer to an organized wave-like behavior as "coherence". The more the wave-like natures of the bits of matter are aligned, the more coherent is the object overall. And the more coherent an object, the more it acts like a wave overall. As a rough analogy, consider a group of kids splashing about in a swimming pool. If the kids are all doing their own thing, then the water waves they create when they splash will be random. A bunch of random water waves adds up to approximately zero. This system is non-coherent and the water waves are not obvious unless you look closely. Now, if the kids line up and all splash the water at the same moment every two seconds, all of their little waves add up to one giant wave of water. This system is coherent, and the water wave in the pool is obvious. The swimming pool is only an analogy. Water waves act like waves of little solid particles, and are therefore classical and not quantum. In order to act like quantum waves, bits of matter must not just have their motions aligned, the bits of matter must also have their quantum wave natures aligned.

The key here is that a large-scale coherent state is improbable as long as the individual parts are behaving randomly. There are only a handful of possible ways to have a system of pieces act in a coordinated fashion, while there are far more ways to have the system act in an uncoordinated fashion. Therefore, coordinated behavior is less likely than uncoordinated behavior, although not impossible. For example, if you roll 5 traditional dice, there are six ways ways to get all the numbers to be the same in one roll. In contrast, there are thousands of ways to get all the numbers to not be the same. Getting the dice to show the same number is improbable but not impossible. In a similar way, quantum coherence on the macroscopic scale is improbable, but not impossible. If the quantum wave natures of the individual bits of matter can be aligned into a coherent state, then quantum effects will become evident on the macroscopic scale. Below are some examples of macroscopic quantum effects.

• Superconductivity. When a conducting material is cooled enough, its conduction electrons spread out into large-scale coherent wave states. These coherent wave states are able to flow past impurities and atoms without being perturbed, so that a material with zero electrical resistance results. Superconductivity leads to interesting macroscopic effects such as quantum levitation (the Meissner effect).
• Superfluidity. When certain materials are cooled enough, their atoms can spread out into coherent wave states that resist surface tension, allowing the material to flow like a liquid with zero viscosity.
• Bose Einstein Condensates. When certain materials are cooled enough, their atoms spread out completely into a single, giant, coherent wave state. A macroscopic chunk of matter that has condensed in this way acts like a wave and exhibits wave properties such as interference.

Note that laser light is often mentioned as a macroscopic quantum effect. However, coherent light such as laser light is successfully explained by the classical Maxwell equations and therefore is not a quantum effect. However, the way laser light is produced; through stimulated emission and a transition between discrete energy levels; is a quantum effect. But, stimulated emission in lasers is an atomic-scale effect and therefore does not make our list of macroscopic quantum effects. Similarly, there are many atomic-scale quantum effects that lead to results that are observable on the macroscopic scale, like the quantum effects that make modern computers possible. These effects are not really happening on the macroscopic scale. Rather, the effects happen on an atomic scale, and then the results of the effect are amplified to a macroscopic level.

Christopher S. Baird is a physics professor at West Texas A&M University and author of the book, The Top 50 Science Questions with Surprising Answers.