The Universe Is Always Looking | The Atlantic

Source: The Universe Is Always Looking | The Atlantic, by Philip Ball

[Schrödinger’s] cat is still hauled out today as if to imply that we’re as puzzled as ever by the mere fact that the quantum world at small scales turns into the world of classical physics at human scales. The fact is, however, that this so-called quantum-classical transition is now largely understood. … quantum physics is not replaced by another sort of physics at large scales. It actually gives rise to classical physics.

At the root of the distinction, though, lies the fact that quantum objects have a wave nature—which is to say, the equation Schrödinger devised in 1924 to quantify their behavior tells us that they should be described as if they were waves, albeit waves of a peculiar, abstract sort that are indicative only of probabilities. It is this waviness that gives rise to distinctly quantum phenomena like interference, superposition, and entanglement. These behaviors become possible when there is a well-defined relationship between the quantum “waves”: in effect, when they are in step. This coordination is called “coherence.”

Macroscopic, classical objects don’t display quantum interference or exist in superpositions of states because their wave functions are not coherent. … Every real system in the universe sits somewhere, surrounded by other stuff and interacting with it. … Quantum superpositions of states … are highly contagious and apt to spread out rapidly. And that is what seems to destroy them. … As time passes, the initial quantum system becomes more and more entangled with its environment. In effect, we then no longer have a well-defined quantum system embedded in an environment. Rather, system and environment have merged into a single superposition. … This spreading is the very thing that destroys the manifestation of a superposition in the original quantum system. Because the superposition is now a shared property of the system and its environment, we can no longer “see” the superposition just by looking at the little part of it. What we understand to be decoherence is not actually a loss of superposition but a loss of our ability to detect it in the original system.

And this has nothing to do with observation in the normal sense: We don’t need a conscious mind to “look” in order to “collapse the wave function.” All we need is for the environment to disperse the quantum coherence. We obtain classical uniqueness from quantum multiplicity when decoherence has taken its toll. … All of the photons of sunlight that bounce off the moon are agents of decoherence, and are more than adequate to fix its position in space and give it a sharp outline. The universe is always looking.