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.”

The concept comes from the science of ordinary waves. Here, too, orderly wave interference (like that from double slits) happens only if there’s coherence in the oscillations of the interfering waves. If there is not, there can be no systematic coincidence of peaks and troughs and no regular interference pattern, but just random, featureless variations in the resulting wave amplitude.

Read: An experimental boost for quantum weirdness

Likewise, if the quantum wave functions of two states are not coherent, they cannot interfere, nor can they maintain a superposition. A loss of coherence (decoherence) therefore destroys these fundamentally quantum properties, and the states behave more like distinct classical systems. Macroscopic, classical objects don’t display quantum interference or exist in superpositions of states because their wave functions are not coherent.

Notice how I phrased that. It remains meaningful to think of these objects as having wave functions. They are, after all, made of quantum objects and so can be expressed as a combination of the corresponding wave functions. It’s just that the wave functions of distinct states of macroscopic objects, such as a coffee cup being in this place and that place, are not coherent. Quantum coherence is essentially what permits “quantumness.”

There is no reason (that we yet know of) why, in principle, objects cannot remain in coherent quantum states no matter how big they are—provided that no measurement is made on them. But it seems that measurement somehow does destroy quantum coherence, forcing us to speak of the wave function as having “collapsed.” If we can understand how measurement unravels coherence, then we would be able to bring measurement itself within the scope of quantum theory, rather than making it a boundary where the theory stops.

The crucial factor in understanding quantum decoherence is that ubiquitous entity present but largely ignored in all scientific studies: the surrounding environment. Every real system in the universe sits somewhere, surrounded by other stuff and interacting with it. Schrödinger’s cat might be placed inside a sealed box, but there must be air in there for the cat to have any chance of staying alive. And the cat is resting on a surface of some kind, exchanging heat with it.

In quantum mechanics, the environment has a central role in how things happen. It turns out to be precisely what conjures the illusion of classical physics out of the quantum soup.

It’s often suggested that quantum states such as superpositions are delicate and fragile. Put them in a noisy environment (the story goes), and all that jiggling and shaking by the surroundings destroys these frail quantum states, collapsing wave functions and shattering superpositions. But this isn’t quite right. Indeed, why should quantum states be fragile if quantum mechanics supplies the most fundamental description of the universe? What kinds of laws are these, if they give up the ghost so easily?