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In fact, quantum effects are not limited to the atomic scale alone. There are several examples of macroscopic quantum behavior. Quantum physics describes matter and energy as quantum wave functions, which sometimes act as waves and sometimes as particles, but are actually more complex entities than just waves or particles.

In fact, every object in the universe (from atoms to stars) acts according to quantum physics. In many situations, such as when throwing a ball, quantum physics leads to the same result as classical physics. In such situations, we use classical physics instead of quantum physics, because its mathematics is simpler and its principles are more intuitive.

The laws of quantum physics still apply in the ball, but their effect is not obvious, so we say that the system is non-quantum. A situation is described as quantum when its quantum behavior becomes apparent, although in fact it is always quantum. Thus, a "quantum effect" is an effect that is not properly predicted by classical physics, but is correctly predicted by quantum theory.

Classical physics describes matter as consisting of small solid particles. So whenever we make particles of matter act like waves, we show a quantum effect. (Classical waves, such as sound waves and sea waves, are not considered quantum, because motion is a wave, but the particles are still small solid balls. To be a quantum effect, the particle itself must act as a wave.)

Although quantum effects are not strictly limited to the atomic scale, they are certainly more common at the atomic scale. Why is this so? Let's look at matter. To be a quantum effect, we have to make matter act like waves. To be a macroscopic quantum effect, we have to make a lot of matter particles act like waves in an organized way.

If all the particles of matter act as waves in a random, disjointed way, then their waves interfere and are averaged to zero on a macroscopic scale. In physics, we call organized wave-like behavior " coherence." The more the undulating nature of the matter particles is aligned, the more coherent the object as a whole is. And the more coherent an object is, the more it acts as a wave as a whole.

As a rough analogy, consider a group of children splashing around in a pool. If all the children go about their business, the waves of water they create when they splash will be random. A lot of random water waves add up to about zero. This system is incoherent. Now, if the kids line up and all splash into the water at the same moment every two seconds, all their little waves add up to one big wave of water.

This system is coherent, and the water wave in the pool is obvious. The pool is just an analogy. Water waves act like waves of small solid particles and are therefore classical, not quantum. In order to act as quantum waves, matter particles must not just align their motions, they must also align their quantum-wave nature.

The key here is that a large-scale coherent state is unlikely as long as individual particles behave randomly. There are only a few possible ways to make the system act in a coordinated manner, while there are many more ways to make the system act in an uncoordinated manner. Therefore, coordinated behavior is less likely than inconsistent behavior, although not impossible.

For example, if you roll 5 traditional dice, there are six ways that all the numbers are the same in a single roll. On the contrary, there are thousands of ways to make sure that all the numbers are not the same. Making the dice show the same number is unlikely, but not impossible.

Similarly, quantum coherence at the macroscopic scale is unlikely, but not impossible. If the quantum-wave nature of individual matter particles can be aligned into a coherent state, then quantum effects will become apparent on the macroscopic scale. Below are some examples of macroscopic quantum effects.

Superconductivity. When a conducting material is sufficiently cooled, its electrons propagate into large-scale coherent wave states. These coherent wave states are able to pass by the atoms without perturbation, so that a material with zero electrical resistance is obtained. Superconductivity leads to interesting macroscopic effects, such as quantum levitation (the Meissner effect).

Superfluidity. When some materials are sufficiently cooled, their atoms can propagate into coherent wave states that resist surface tension, allowing the material to flow as a zero-viscosity liquid.

Bose-Einstein condensates. When some materials are sufficiently cooled, their atoms completely transition into a single giant coherent wave state. A macroscopic piece of matter that has condensed in this way acts as a wave and exhibits wave properties such as interference.

Note that laser light is often referred to as a macroscopic quantum effect. However, coherent light, such as laser light, is successfully explained by the classical Maxwell equations and is therefore not a quantum effect.

However, the way laser light is produced-through stimulated radiation and the transition between discrete energy levels-is a quantum effect. But stimulated radiation in lasers is an atomic-scale effect and is therefore not included in our list of macroscopic quantum effects.

Similarly, there are many atomic-scale quantum effects that lead to results observed at the macroscopic scale, such as the quantum effects that make modern computers possible. These effects don't actually occur on a macroscopic scale. Rather, the effects occur on an atomic scale, and then the results of the effect are amplified to a macroscopic level.

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