Advanced Concepts: Decoherence

Matt Leifer doesn't blog all that often, but what he posts is very good. It tends to be extremely high-level stuff about foundational problems in quantum theory, mind, so it's not for the faint of heart, but if you get into that sort of thing, it's fascinating. Wednesday's post on dechoerence is no exception:

[L]et me start by defining two problems that I take to be at the heart of understanding quantum theory:

1) The Emergence of Classicality: Our most fundamental theories of the world are quantum mechanical, but the world appears classical to us at the everyday level. Explain why we do not find ourselves making mistakes in using classical theories to make predictions about the everyday world of experience. By this I mean not only classical dynamics, but also classical probability theory, information theory, computer science, etc.

2) The ontology problem: The mathematical formalism of quantum theory provides an algorithm for computing the probabilities of outcomes of measurements made in experiments. Explain what things exist in reality and what laws they obey in such a way as to account for the correctness of the predictions of the theory.

If that's where you're starting, you know it's going to get deep...

I'm not going to attempt to explain decoherence this morning, or quite possibly ever. It's a slippery and abstract topic, and it's hard to imagine a world in which it would fit into a Basic Concepts sort of category. This post does sort of make me think that a "Basic Concepts" post on "Measurement" might be interesting, though.


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Thank you for the heads-up on the Quantum Quandaries blog - it is excellent (and nary a mention of "quantum consciousness" or "quantum healing" or Deepak Chopra).

By Eric Johnson (not verified) on 26 Jan 2007 #permalink

Thanks for the plug. I can tell when I get mentioned on a "mainstream" physics blog due to a huge spike in numbers.

You are right about my attitude to blogging. Most bloggers think of their blog like a newspaper column, i.e. you have to post interesting articles very regularly in order to maintain readership. There's nothing wrong with that, but what I find interesting is that you don't have to think of a blog like that at all. Since it's not being published anywhere where they care about readership numbers, you can essentially do whatever you like. I am partially paid to think about the foundations of QM, and occasionally have thoughts going round my head that I think are interesting but inappropriate for a paper, so it is nice to have a place to do a brain-dump. The comments are then very useful because I get an idea of how other physicists are thinking about the issues and what I need to do to explain myself better. If I had to write such articles on a daily basis then I would definitely get no work done, but I don't plan to do that so I don't need to worry. The fact that there is room for such a diversity of writing styles and publishing schedules in the blogsphere is one of its greatest strenghts in my opinion.