As for measuring decoherence rates in specific cases, this has been done.
I thought we were discussing the (hypothetical) decoherence propagation rate, not the time it takes for a quantum system to decohere in the lab, which has nothing to do with relativity. It is even measured in different units (inverse seconds vs. meters/second). So no, this experiment is not what I was asking about.
The claim I made was that world-splitting is not instantaneous. The fact that systems decohere gradually establishes this (if you buy that "world-splitting" is just a consequence of decoherence). I'm not sure I see your worry. Perhaps you can indicate what, specifically, you think is wrong with my initial claim?
Today's post, Spooky Action at a Distance: The No-Communication Theorem was originally published on 05 May 2008. A summary (taken from the LW wiki):
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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 Bell's Theorem: No EPR "Reality", and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
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