[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").

Discuss the post here (rather than in the comments to the original post).

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.

Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.

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").

Does anybody else not like the general phrasing "The system is in the superposition STATE1 + STATE2" ?

The way I'm thinking of it there is no such thing as a superposition. There is simply more than one configuration in the (very recent) past that contributes a significant amount of amplitude to the "current" configuration.

Have I got this wrong?

I think it is a good way to map what people have commonly called "superposition," but the sentence should probably be "The system is in the superposition STATE1 + STATE2, relative to STATE 3, where STATE 3 roughly factors out". STATE 3 in this case is usually an observer. I mean, if I flip a "quantum coin" and I have not told you if it is heads or tails, then the coin (and I) is in a superposition of "HEADS + TAILS" relative to you, but due to decoherence on my end, it is not in a superposition relative to me. For me this was an important concept to learn, as it helped me see that "many worlds" is a local and non-discrete phenomenon.