[SEQ RERUN] On Being Decoherent

MinibearRex

Today's post, On Being Decoherent was originally published on 27 April 2008. A summary (taken from the LW wiki):

When a sensor measures a particle whose amplitude distribution stretches over space - perhaps seeing if the particle is to the left or right of some dividing line - then the standard laws of quantum mechanics call for the sensor+particle system to evolve into a state of (particle left, sensor measures LEFT) + (particle right, sensor measures RIGHT). But when we humans look at the sensor, it only seems to say "LEFT" or "RIGHT", never a mixture like "LIGFT". This, of course, is because we ourselves are made of particles, and subject to the standard quantum laws that imply decoherence. Under standard quantum laws, the final state is (particle left, sensor measures LEFT, human sees "LEFT") + (particle right, sensor measures RIGHT, human sees "RIGHT").

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This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Where Experience Confuses Physicists, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.

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Today's post, On Being Decoherent was originally published on 27 April 2008. A summary (taken from the LW wiki):

When a sensor measures a particle whose amplitude distribution stretches over space - perhaps seeing if the particle is to the left or right of some dividing line - then the standard laws of quantum mechanics call for the sensor+particle system to evolve into a state of (particle left, sensor measures LEFT) + (particle right, sensor measures RIGHT). But when we humans look at the sensor, it only seems to say "LEFT" or "RIGHT", never a mixture like "LIGFT". This, of course, is because we ourselves are made of particles, and subject to the standard quantum laws that imply decoherence. Under standard quantum laws, the final state is (particle left, sensor measures LEFT, human sees "LEFT") + (particle right, sensor measures RIGHT, human sees "RIGHT").

'Handwaving' describes vagueness. Yet, just how much vagueness qualifies as 'handwaving' is not well-defined!

I don't disagree? I'm making essentially an aesthetic point.

I thought I qualified how much vagueness was acceptable -- there is vagueness that is pedagogically useful, and there is vagueness that is not pedagogically useful, and my accusation of handwaving is isomorphic to saying that the vagueness with Feynman paths here is not pedagogically useful.

This builds on the result of 'joint configurations', which is that for interference to occur, everything needs to line up. EVERYTHING. Otherwise, it's offset in some dimension or other, and not really in the same 'place' at all. With that in place, this is a short step to take.

I can't follow this explanation at all. Too many ambiguous pronouns. But this is okay; the goal isn't to explain it to me -- I have all the training in quantum mechanics that I care to have.

"Everything needs to line up" is the key point, and it once you understand it it's really quite simple. It just means that there is more than one way to get to the same configuration state. Think about history seeming to branch out in a tree-like way, as most people tend to imagine. But if two branching paths are not far apart (e.g. differing by just a single photon) then it is easy for then to come back together. History changes from a tree to a graph. Being a graph means that some point has two history paths (actually every point has an infinit... (read more)