Why aren't macroscopic objects—or really the entire universe—coherent? From what I understand, when quantum particles interact with the environment, they get entangled with it and the entanglement rapidly spreads until it's dispersed into the environment. So wouldn't this mean that a particle that has undergone decoherence is actually just entangled with the environment so you can't really observe its "quantumness" within the environment? And since entanglement "spreads" the coherence (if I understand correctly), wouldn't this mean that the entire environment is technically coherent but we just can't observe it? How do things become decoherent (i.e. how do the wave functions become no longer aligned with each other)?
Any insight would be greatly appreciated!
 A: The comment thread looks a bit like it's going in circles so I will just make the bold claim. To the best of our knowledge, macroscopic objects, including the entire universe, are coherent. The reason is exactly the one you mention. Statements about a system decohering after a certain time only make sense in a context where you specify what "the environment" is from the outset. If you could keep track of everything, e.g. a joint wavefunction of $10^{24}$ particles, then the Schroedinger equation would show that entanglement just builds up forever.
From the birth of "modern physics", there have been heated arguments about the idea that a quantum system never becomes classical. But 100 years later, there is still no direct evidence that unitary evolution is an approximation or that more than one wavefunction exists at a fundamental level. Quantum mechanics does not contain the seeds of its own destruction.
What I've just described is usually called the many worlds interpretation to reflect the wavefunction of the universe being something other than a delta function which has "collapsed" to just one possibility. However, the word "interpretation" above is a bit misleading. The theory that everyone uses to explain experiments has the lack of wavefunction collapse as an unambiguous prediction. Attempts to have collapse be something fundamental rather than an approximation / teaching tool are governed not by data or internal consistency but simply by taste. People who are uncomfortable with the fact that QM has to be made more complicated before it will no longer imply many worlds accept this tradeoff and make QM more complicated.
