Physics and Reality

Why the quantum world looks classical

Quantum physics is the description we currently have of nature at its most fundamental level. And yet the world we actually experience does not usually look quantum.

Tables do not appear in several places at once. Cats are not visibly spread across incompatible possibilities. Our surroundings do not present themselves as delicate superpositions. They present themselves as definite objects with definite properties, evolving in a way that is, to a very good approximation, classical.

This raises a natural question. If the underlying language of the universe is quantum, why does the world appear otherwise?

The answer is not that classical physics was wrong and should have been discarded. Nor is it that the quantum world somehow stops being quantum when things become large. The answer is that classical behaviour is an emergent approximation. Under suitable conditions, the richer quantum description organizes itself in a way that is extremely well captured by classical language.

For example, in classical physics, an electric field is described by assigning one definite value to it at every point in space. In quantum physics, the state of that same system is a more complicated object. In general, it contains much more structure than one classical field profile does. But there are special quantum states for which all that extra structure is organized in a particularly simple way. In such cases, the system behaves, to a very good approximation, as if only one classical field configuration were really there.

In other words, classicality is not fundamental, but it can arise as a very good approximation to certain quantum states. Furthermore, this approximation becomes much more robust when large numbers of particles are involved.

A microscopic system can maintain delicate quantum relations between different alternatives. A macroscopic system usually cannot. Once many particles interact with one another, and with their wider environment, the different alternatives cease to interfere in any appreciable way. In such cases, the underlying quantum state may still be a superposition of different classical-looking realities, but these evolve almost independently, as if the superposition were not there at all.

That is why a detector records one outcome in a stable way. It is also why a chair has a location, why a notebook contains one written result rather than a visible superposition of different records, and why our everyday surroundings do not display the full quantum structure that lies underneath.

The point is not that quantum physics ceases to apply to large systems. It is that large systems are exactly where classicality emerges most strongly.

This also explains why our own experience of reality is classical in character. We do not observe the world as elementary quantum systems. We observe it as macroscopic beings, equipped with macroscopic senses, embedded in a macroscopic environment. The information available to us is already coarse-grained and already organized in a way that suppresses the delicate quantum features present in the deeper description.

In that sense, the classical world is not separate from the quantum one. It is the form the quantum world takes when viewed at the scale of large objects, stable records, and ordinary human experience.

This is why quantum physics can be true without making the everyday world look bizarre. The strangeness does not disappear. It is simply hidden beneath a level of description in which the quantum state has settled into patterns that classical language can describe extremely well.

The world therefore looks classical not because it is fundamentally classical, but because classicality is what quantum physics naturally gives rise to under the conditions in which we live.

What this means for our perception of reality is one of the reasons the question is so interesting.