From Costanza's original thread (entire text):
This is for anyone in the LessWrong community who has made at least some effort to read the sequences and follow along, but is still confused on some point, and is perhaps feeling a bit embarrassed. Here, newbies and not-so-newbies are free to ask very basic but still relevant questions with the understanding that the answers are probably somewhere in the sequences. Similarly, LessWrong tends to presume a rather high threshold for understanding science and technology. Relevant questions in those areas are welcome as well. Anyone who chooses to respond should respectfully guide the questioner to a helpful resource, and questioners should be appropriately grateful. Good faith should be presumed on both sides, unless and until it is shown to be absent. If a questioner is not sure whether a question is relevant, ask it, and also ask if it's relevant.
Meta:
- How often should these be made? I think one every three months is the correct frequency.
- Costanza made the original thread, but I am OpenThreadGuy. I am therefore not only entitled but required to post this in his stead. But I got his permission anyway.
To expand on what pragmatist said: The wavefunction started off concentrated in a tiny corner of a ridiculously high-dimensional space (configuration space has several dimensions for every particle), and then spread out in a very non-uniform way as time passed.
In many cases, the wavefunction's rule for spreading out (the Schrödinger equation) allows for two "blobs" to "separate" and then "collide again" (thus the two-split experiment, Feynman paths and all sorts of wavelike behavior). The quote marks around these are because it's not ever like perfect physical separation, more like the way that the pointwise sum of two Gaussian functions with very different means looks like two "separated" blobs.
But certain kinds of interactions (especially those that lead to a cascade of other interactions) correspond to those blobs "losing" each other. And if they do so, then it's highly unlikely they'll accidentally "collide" again later. (A random walk in a high-dimensional space never finds its way back, heuristically speaking.)
So, as long as the universe has relatively low entropy (as it will until what we would call the end of the universe), significant interference with "blobs" of wavefunction that "split off of our blob" in macroscopic ways a long time ago would be fantastically unlikely. Not impossible, just "a whale and a petunia spontaneously appear out of quantum noise" degree of improbability.